Specific Expectations
In this chapter, you will learn how to . . .
•
D1.1 analy ze , on the basis of research,
some of the social and ethical
impl ications of research i n genetics and
genomics
•
(6.3)
D1.2 evaluate , on the basis of research,
the importance of some recent
contributions to the knowledge,
tech niq ues, and technologies related to
genetic processes
•
•
(6.3)
D 2.1 use a ppropriate terminology
related to genetic processes (6.1, 6.2, 6.3)
D2.3 use the Pun nett square method to
solve basic genetics problems involving
monohybrid crosses, incomplete
domina nce, codominance, dihybrid
crosses, and sex-linked genes
•
(6.1, 6.2)
D3.3 explain the concepts of genotype,
phenotype, dominance, incom plete
dominance, codomina nce, recessiveness,
and sex l i nkage according to Mendelian
Jaws of i n heritance
•
(6.1, 6.2)
D3.4 describe some genetic disorders
caused by chromosomal abnormalities
or other genetic m utations in terms of
chromosomes affected, physical effects,
and treatments
(6.1, 6.2)
T
he inherited traits of an individual are the result of a complex
array of genetic interactions. As genetics research continues to
advance, we have a better understanding of these complexities.
A significant advancement is the Human Genome Project. In 2003,
a team of over 2000 researchers, working in laboratory groups around
the world, completed the Human Genome Project. For this project,
numerous images like the one shown here were analyzed. This photo
shows the products of chemical reactions that are used to identify
the nucleotide sequence of a piece of DNA. Scientists used these to
determine, base by base, the DNA sequence of the human genome.
Other goals of the Human Genome Project included identifying
all of the human genes and making them available for study. Because
such scientific goals have consequences for society, there are also
groups of researchers that explore and monitor the ethical and social
impacts of these scientific achievements.
240
MHR · Unit 2 Genetic Processes
Assembling a Mini-Genome
The 46 chromosomes that make up our genome contain over
3 000 000 000 base pairs. Each chemical reaction that is used to determine
the sequence of DNA can only provide the sequence of a few hundred
bases at a time. Therefore, to determine the DNA sequence of the human
genome, scientists all over the world worked together to analyze millions
of DNA sequencing reactions. They then assembled the DNA sequence
of the human genome by piecing together the much smaller fragments of
sequences. In this activity, you will model how scientists did this.
=I
==== ==,
output
v
gel
gel
detector
computer
lanes
T ATAAAACAT T TT AAAAG:. TAGTI'C C CAGTN:.CTT GAGTTCCAA A G:. CCAATGTTGrT CPCTATGGTTCPCAATr::a:K. CA
140
150
160
170
180
190
200
210
220
The products of a DNA sequencing reaction are modified so they are visible
under u ltraviolet light. They are then separated in each lane of a gel-like material.
The information i n each lane is sent to a computer, which provides output
in the form of a printout of the sequence of bases i n a piece of DNA. Recall
that n u cleotides are often identified by thei r bases. For these data, red bands
represent thymines, green bands represent adenines, blue bands represent
cytosines, and black bands represent guanines.
Materials
•
•
paper DNA fragments
tape
Proced ure
1. Obtain the sequence of DNA that you are to work with from
your teacher.
2.
With your classmates, construct one continuous segment of
sequenced DNA from your individual fragments by matching
overlapping sections and taping them into place.
Q u estions
1. How did you decide how to match and link the fragments together?
2.
How important was it to collaborate and discuss your results with
other class members in order to obtain the full sequence?
3. How important do you think it was for scientists to develop a
systematic and organized approach to sequencing the human genome?
How do you think computers played a role?
Chapter 6 Complex Patterns of I n heritance ·
MHR
241
Beyond Mendel's Observations of Inheritance
Key Terms
incom plete dominance
codominance
heterozygous advantage
conti nuous variation
polygenic trait
Much of to day's genetics research uses sophisticated technologies to study cellular
processes at the level of individual molecules and atoms. In addition, international
research collaborations and multi-million-dollar budgets are now common. Think of
what a stark contrast this is to Mendel's experiments. It is astounding that Mendel's
basic and, at times, simple observations led him to infer patterns of inheritance that
still form the basis of our current understanding of heredity.
As more sophisticated experimental technologies became available, scientists realized
that patterns of inheritance are more complicated than what Mendel proposed. Some
patterns result in phenotypes that are between dominant and recessive phenotypes.
Other patterns result in phenotypes that are created when both alleles for a trait are
equally expressed.
Incomplete Domina nce
incomplete dominance
a condition in which
neither allele for a gene
com pletely conceals the
presence of the other; it
results in intermediate
expression of a trait
Incomplete dominance describes
a condition in which neither of the two alleles for-the
same gene can completely conceal the presence of the other. As a result, a heterozygote
exhibits a phenotype that is somewhere between a dominant phenotype and a recessive
phenotype. One example is the flower colour of snapdragons (Antirrhinum majus).
As you can see in Figure 6.1, a cross between a true-breeding red-flowered plant and
a true-breeding white-flowered plant produces offspring with pink flowers in the
F 1 generation. If the F 1 plants are allowed to self-fertilize, the F2 generation will
include offspring with all three phenotypes-red, pink, and white. The Punnett square in
predicts that all three phenotypes will be observed in the F2 generation in a ratio
(red:pink:white), which is what is observed experimentally. In true Mendelian
Figure 6.1
of 1:2:1
inheritance, we would have predicted a phenotypic ratio of 3:1. Nevertheless, the alleles
for flower colour do segregate according to Mendel's law of independent assortment.
When representing incomplete dominance, upper-case and lower-case letters are
not usually used to represent the alleles, since neither allele is dominant over the other.
One way to represent incomplete dominance is by using superscripts. In the example of
snapdragon flower colour, both alleles affect the colour of the flower, C. The two alleles
are represented as superscripts, R for red (CR), and W for white (Cw). Lower-case
letters are only used to represent a recessive allele.
Figure 6.1 When red
(CRCR) flowers and wh ite
(Cwcw) flowers of the
P generation
g ametes
snapdragon are crossed,
the resulting offspring have
self-fertilization
of F1 offspring
an intermediate phenotype,
pink flowers
F 1 generation
(CRew). I n
the F 2 generation, a l l three
phenotypes are observed.
red
X
pink
white
242
MHR Unit 2 Genetic Processes
•
F2 generation
Incomplete Dominance and Human Disease
There are many examples of genetic disorders in humans that exhibit incomplete
dominance. For example, there is a genetic disorder, called familial hyper­
cholesterolemia, that prevents tissues from removing low-density lipoproteins (LDL)
from the blood and causes very high levels of cholesterol in the bloodstream. In the
majority of cases, the disorder is due to a mutation in the LDLR gene. LDL p a ticles
transport molecules like cholesterol throughout the body. The mutated version of the
LDLR gene no longer produces a protein that interacts with LDL particles and removes
them from the bloodstream. This disorder has an autosomal dominant inheritance
p attern. So, an individual only requires one allele of the mutated form of the gene to
show symptoms of the disorder. However, if the allele for the normal form of the gene
is present, symptoms of the disease will not be as severe. People who are homozygous
dominant for the trait have six times the normal amount of LDL in their blood and
may have a heart attack by the age of 2. Heterozygotes have about twice as much
cholesterol in their blood and may have a heart attack by the age of 35.
Scientists are now finding that identifying the p atterns of inheritance for many
traits is not as straightforward as first thought. Today's more accurate techniques
are showing that, in some .cases, what had been identified as a dominant inheritance
p attern may actually be incomplete dominance. As a result, an individual who is
heterozygous for a trait is not exactly the same as an individual who is homozygous
dominant for the trait.
Codominance
is a situation in which both alleles are fully expressed. A roan animal is
an excellent, visible example of codominance. A roan animal is a heterozygote in which
both the base colour and white are fully expressed. If you look closely at the individual
Codominance
h airs on a roan animal, such as the cow in Figure 6.2, you will see a mixture of red
hairs and white hairs. One allele is expressed in the white hairs, and the other allele is
expressed in the red hairs.
·
codominance the
condition i n which
both a l leles for a trait
are equa l ly expressed
in a heterozygote; both
al leles are dominant
Figure 6.2 A roan cow is the product of a mati ng between a red cow a n d a white cow. The red
and white hairs may be present in patches, as shown here, or be completely i ntermingled.
Chapter 6 Complex Patterns of I n heritance ·
MHR
243
Sickle Cell Anemia
hetero zygous
advantage a survival
benefit for individuals
who in herit two
d ifferent alleles for the
same trait
Sickle cell anemia is one of the most thoroughly studied genetic disorders. Although it
is often described as being the result of autosomal recessive inheritance, it is actually
an example of codominance. Sickle cell anemia is caused by a specific form of the gene
that directs the synthesis of hemoglobin. Hemoglobin carries oxygen in the blood.
The hemoglobin molecule that is made in individuals with the sickle cell allele leads
to a C-shaped (or sickled) red blood cell. These misshaped red blood cells, like the
one shown in Figure 6.3, do not transport oxygen effectively because they cannot pass
through small blood vessels. This leads to blockages and tissue damage.
The allele for normal hemoglobin is represented as HbA, and the allele for sickle
cell hemoglobin is represented as Hb5. As shown in Figure 6.4, individuals who are
homozygous (Hb5Hb5) have sickle cell anemia. I ndividuals who are heterozygous
(HbAHb5) h ave some normal and some sickled red blood cells. These individuals are
said to have the sickle cell trait, but they rarely experience any symptoms. In fact,
having the sickle cell trait can be an advantage, because these heterozygotes are more
Figure 6.3 Normal red
blood cells are flat and
disk-shaped. Sickle-shaped
ce lls are elongated and
"C" shaped.
resistant to malaria. Malaria is a life-threatening disease caused by a parasite that
is transmitted to humans through mosquito bites. The parasite infects the liver and
eventually the red blood cells. The sickling of red blood cells is thought to prevent the
parasites from infecting the cells. Resistance to malaria is very beneficial in certain
parts of Africa, where deadly epidemics can occur. The sickle cell trait is an example
of the principle of heterozygous advantage, which describes a situation in which
heterozygous individuals h ave an advantage over both homozygous dominant and
homozygous recessive individuals.
sickle cell
trait
sickle cell
trait
normal
sickle cell
trait
sickle cell
trait
sickle cell
anemia
Figure 6.4 When a man and a woman are both heterozygous for the sickle cell
gene, there is a one i n four chance that they will have a child with sickle cell anemia.
Learning Check
1. Distinguish between incomplete dominance and
4.
The frequency of the appearance of the sickle cell
allele in human populations is much higher in
Africa than in most other areas of the world. What
has been proposed to explain this observation?
5.
Provide two pieces of evidence that support the idea
that some inheritance patterns are more complex
than those originally proposed by Mendel.
6.
Scientists first thought that sickle cell anemia was
inherited as an autosomal recessive allele. What led
them to identify the true inheritance pattern of
the disease?
codominance.
2.
Why do geneticists use notations like cw and cR to
describe incomplete or codominant alleles instead of
using Wand w orR and r?
3. A plant that produces white flowers is crossed with
a plant that produces purple flowers. Describe the
phenotype of the offspring if the inheritance pattern
for flower colour is
a.
incomplete dominance
b. codominance
244
MHR Unit 2 Genetic Processes
·
Multiple Alleles
The traits you have studied so far have all been controlled by one gene with two alleles,
such as the flower colour in pea plants. Many traits in humans and other species are
the result of the interaction of more than two alleles for one gene. A gene with more
than two alleles is said to have multiple alleles. As you know, any individual has only
two alleles for each gene-one allele on each homologous chromosome. However, many
different alleles for a gene can exist within the population as a whole.
Possible alleles from female
Human Blood Groups
Do you know what blood type you are? In humans, a single gene
determines a person's ABO blood type. This gene determines what type
of an antigen protein, if any, is attached to the cell membrane of red blood
cells. An antigen protein is a molecule that stimulates the body's immune
system. The gene is designated I, and it has three common alleles: JA, !8,
and i. As shown in Figure 6.5, the different combinations of the three alleles
produce four different phenotypes, which are commonly called blood
types A (IAJA homozygotes or JAi heterozygotes), B (I8I8 homozygotes or
I8i heterozygotes), AB (IAJB heterozygotes), and 0 (ii homozygotes). The
JA allele is responsible for the presence of an A antigen on the red blood
cells. The !8 allele is responsible for the presence of the B antigen, and
"'
E
E
"'
.g
-;;
fAJB
Q)
::0
·;;;
"'
Q)
or
0
c..
(i)
the i allele results in no antigen. Of the three alleles that determine blood
type, one (i) is recessive to the other two, and the other two (IA and !8)
are codominant.
JBJB
ii
fAi
blood types
B
0
of the three I a l leles result i n four
Figure 6.5 Different combinations
type B, type AB, and type 0.
d ifferent blood types: type A,
Rabbit Coat Colour
Another example of multiple alleles involves coat colour in rabbits, as shown in
Figure 6.6. The gene that controls coat colour in rabbits has four alleles: agouti
(C), chinchilla (c"h ), Himalayan (d'), and albino (c). In that order, each allele
is dominant to all the alleles that follow. The order of dominance sequence can
be written as C > c"h > d' > c, where the symbol > means is dominant to.
Himalayan
agouti
chinchilla
albino
Figure 6.6 Rabbits have m u ltiple alleles for coat colour, with four possible phenotypes.
Predict the possible genotypes for each rabbit.
Chapter 6 Complex Patterns of I n heritance ·
MHR
245
Clover Leaf Patterns
The pattern on the leaves of the clover plant is also controlled by multiple alleles. While
a single gene is responsible for clover leaf pattern, there are seven different alleles for
the pattern. Varying combinations of these result in 22 different p atterns that can be
expressed in clover leaves. Patterns for the seven h omozygous combinations of alleles
are shown in Figure 6.7.
Figure 6.7 There are seven
different a l leles for clover
leaf pattern.
A
v/v
v'lv'
Sample Problem
Using a Pun nett Square to Analyze I n heritance of Mu ltiple A l leles
Problem
If a man has type 0 blood and a woman has type B blood, what possible blood types
could their children have? If this couple has six children, all with type B blood, what
could you infer about the woman's genotype?
What Is Required?
You are asked to determine all possible blood types of the children and the possible
genotype of the mother based on all the children having type B blood.
What Is Given?
The man has blood type 0, the woman has blood type B.
Plan Your Strategy
Determine the possible genotypes of
the man and the woman.
Act on Your Strategy
Since the man has blood type 0, his genotype must be ii.
The woman has blood type B, so her genotype could be
either JBJB or JBi.
Make Punnett squares for all the
possible combinations of genotypes.
i
father
i
i
father
List all the possible genotypes and
phenotypes of the children.
What could be the mother's genotype
based on the children being type B?
Check Your Solution
i
JB
mother
J Bi
JB i
JB i
J Bi
JB
[Bj
J Bi
JB
mother
i
ii
ii
The children could have genotype JBi, resulting in type B
blood, or genotype ii, resulting in type 0 blood.
The mother's genotype is most likely JBJB.
The only genotype that produces type 0 blood is ii. To have type B blood, the woman must
have at least one JB allele. Her second allele could be either JB or i. Since all of the children
had to receive an i allele from their father, they must have inherited an JB allele from their
mother. Since all of the children have type B blood, the mother is most likely JBJB.
246
MHR · Unit 2 Genetic Processes
Practice Problems
1. If a man has type AB blood and a woman has type
8.
A blood, what possible blood types could their
children have?
a. Use Punnett squares to show how two different
sets of parent genotypes could produce this result.
b. Which of the two sets of potential parents in your
2. A baby has blood type AB. If the baby's mother
has blood type B, what blood type(s) could the
father have?
3. A couple just brought home their new baby from
the hospital. They begin to suspect that the hospital
switched babies, and the baby they brought home is
not theirs. They check the hospital records, and find
that the man's blood type is B, the woman's blood
type is AB, and the baby's blood type is 0. Explain
why the parents are or are not justified in their
concern about this baby.
4.
5.
6.
7.
Four children have the following blood types: A, B,
AB, and 0. Could these children have the same two
biological parents? Explain.
In one family, all three siblings have type B blood.
answer to (a) is more likely to be the parents of
these siblings? Explain why.
9.
In dogs, coat colour is determined by the interaction
between three alleles. The allele A5 produces a dark
coloured dog, aY produces a sandy coloured dog, and
at produces a spotted dog. The order of dominance
is A5 > aY > a1. Determine the following from the
pedigree below.
a.
the genotypes of the parents (I -1 and I-2)
b. the probability of an offspring from the mating
between individuals II-2 and II-3 having spots
c. the possible genotypes of individual II - 1
A
W
Some of the offspring of a chinchilla rabbit and a
Himalayan rabbit are albino. What are the genotypes
of the parents?
A chinchilla rabbit with genotype c:hd' is crossed
with a Himalayan rabbit with genotype d'c. What
is the expected ratio of phenotypes among the
offspring of this cross?
Could a mating between a chinchilla rabbit and
an albino rabbit produce a Himalayan rabbit?
Explain your reasoning.Your answer should include
reference to the genotypes and phenotypes of the
parents and the offspring.
II
2
3
D
Q
Key
dark
coloured
sandy
coloured
spotted
10. A dark coloured dog is mated with a sandy coloured
dog. The litter of puppies includes a dark puppy, a
sandy puppy, and a spotted puppy. Use a Punnett
square to determine the possible genotypes of the
offspring and the parents. Note: Use the information
about dog coat colour inheritance from question 9
to answer this question.
Environmental Effects on Complex Patterns of Inheritance
Environmental conditions often affect the expression of traits. For example,
some genes are influenced by temperature. The dark colour in Himalayan
rabbits, shown in Figure 6.8, is on the cooler parts of their bodies: the
face, ears, tails, and feet. In these animals, dark colouring is the
result of a gene that is only active below a certain temperature.
One way to study the effect of the environment on expression
of traits is to study genetically identical organisms placed
in different surroundings. For example, identical twins are
genetically identical. Differences in the activity of their
genes can be due to environmental effects.
Suggested Investigation
Plan You r Own Investigation
6-A, Environmental
I nfluences on the
Production of Chl orophyll
Figure 6.8 The dark ears, nose, feet, and ta ils
of Hi malayan rabbits are thought to be caused
by lower body temperature i n these areas.
Chapter 6 Complex Patterns of I n heritance ·
MHR 247
Polygenic I nheritance
continuous variation
a range of variation in
one trait resulting from
the activity of many
genes
polygenic trait a trait
that is controlled by
more than one gene
Mendel carefully selected plants that had very different heights so there would be
no question about phenotypes. However, there are traits that exhibit continuous
variation. These are traits for which the phenotypes vary gradually from one extreme
to another.
Some examples of traits that show continuous variation include height and skin
colour in humans, ear length in corn, and kernel colour in wheat. Continuous traits
cannot be placed into discrete categories because they vary over a continuum. For
example, height in humans varies over a wide range of values. People cannot be
categorized as only short or tall.
Traits that exhibit continuous variation are usually controlled by more than one
gene. For some traits this can involve several genes. Traits that are controlled by many
genes are called polygenic traits. A group of genes that all contribute to the same trait
is called a polygene. Each dominant allele contributes to the trait. Recessive alleles do
not contribute to the trait. For skin colour, the more dominant alleles a person has,
the darker their skin. The graph in Figure 6.9 shows that there are more intermediate
phenotypes than extreme phenotypes.
Skin Colour
>.
v
c
Cll
:I
CT
...
0
aaBbCc
AAbbcc
aaBBcc
aabbCC
Aabbcc
aaBbcc
aabbCc
2
aaBbCC
AAbbCc
AabbCC
AABbcc
aaBBCc
AaBBcc
3
AABBcc
AaBbCC
AaBBCc
AABbCc
4
Number of Dominant Alleles
5
6
Figure 6.9 This graph shows possible shades of skin colour from three of the sets of a l leles that
determine this trait.
Predict the effect of more gene pairs on the possible phenotypes.
Activity
6.1
Identifying a Polygenic Trait
A polygenic trait is one that is controlled by more than
one gene and shows continuous variation. In this activity,
you will choose one h u man trait that you hypothesize is
controlled by more than one gene and shows continuous
variation. You will then collect data from you r cla ssmates to
test you r hypothesis.
Materials
•
ruler or measuring tape (if necessary)
•
graph paper
2. Construct a data table to organize your data. Keep in
mind that you will be measuring a particular trait and
recording the nu mber of times that measurement
of the trait occurs.
3. Collect you r data from you r classmates.
4. Create a line graph of you r data. You r graph should
reflect the actual measurements you took and the
frequency of the va lues that you measured.
Questions
Procedure
1. In you r group, choose one human trait that you think
is polygenic. Make sure you r choice is one for which
data can be easily and respectfu lly col lected from you r
classmates.
248
MHR Unit 2 Genetic Processes
·
1. Do you r data support your hypothesis that the trait
you selected i s polygenic? Explain.
2. How could this activity be improved to provide a
clearer picture of the inheritance pattern of the trait
you selected?
THIS WEEK ON QUIRKS & QUARKS
Selecting for Genetic Defects
Human molecular geneticists
Related Career
Most scientists agree that certa i n i n herited
Curiously, it does seem to. I ndividuals with
study genetic processes in
traits are favoured when they i m prove
the deafness mutation also had skin that
humans, particularly how genes
chances for survival. But what if improved
was margi nally thicker than the skin of
function in human disease.
chances for survival are due to a mutation
people who do not have the mutation.
These scientists are referred to as
associated with hereditary deafness? Bob
Tests were conducted on the m utated
McDona ld interviewed Dr. David Kelsell,
skin cells to see whether the deafness
Professor of Human Molecular Genetics at
m utation helped skin form a better barrier
Queen Mary College, U n iversity of London,
agai nst bacterial invasions, and whether the
to discuss this question.
affected skin cells healed differently. Results
Good News and Bad News
Scientists have known which gene is
associated with most cases of hereditary
i n gene Cx26 (Connexin 26) is the culprit.
deafness since 1996. A specific mutation
People carrying one copy of the gene with
the deafness mutation have normal hearing,
Because the deafness mutation in Cx26 is
while people with two copies are deaf.
found i n many human populations around
the world, Dr. Kelsell's team suspected it must
convey some kind of survival advantage.
Outer Ear
Middle Ear
showed that the thicker skin could offer
"molecular" geneticists because
they look at the structure
and function of genes at the
molecular level. For example,
they look for the effects of a
genetic mutation by studying
the mutant protein that is
better protection and that healing could
formed from it and how it affects
occur much more qu ickly.
processes in the body.
Therefore, while there is a risk of deafness
if two copies of the mutation are inherited,
agai nst skin diseases. As the Quirks host said,
one copy seems to provide better protection
Go to scienceontario
to find out more
"How is it that one gene can affect two such
different and seemingly u n related things­
deafness and thi ckness of skin ?""This;' sa id
Dr. Kelsell, "is one of the great mysteries:'
Inner Ear
�------�'�--�--A'--�
The protein product of the Cx26 gene is needed
for movement of potassium ions between cells
in the cochlea of the inner ear. This movement
of ions is needed for proper hearing.
QUESTIONS
1. Is deafness due to a Cx26 m utation i n herited by
an autosomal recessive or autosomal dominant
pattern? Expla i n your answer.
cochlea
2. Explain why the deafness mutation of Cx26 is an
example of heterozygous advantage.
3. Use the I nternet or print resources to find out more
about the work of h u man molecular geneticists.
What essential skills wo uld you need in order to
work in this field?
Chapter 6 Complex Patterns of I n heritance ·
MHR
249
Section Summary
•
•
Incomplete dominance leads to the expression of an
intermediate phenotype. In the case of codominance,
both alleles are fully expressed.
Although an individual has only two alleles for any
gene, multiple alleles for a gene may exist within the
population.
Review Questions
1.
CfJ!I A white-flowered plant is crossed with a
•
•
Environmental conditions can influence the expression of
certain traits.
Polygenic traits are controlled by more than one gene
and can usually be identified by continuous variation in
phenotype.
6. tillll The following pedigree shows the inheritance
pattern of sickle cell anemia in a family. Known
carriers of the sickle cell gene are noted. However, not
all individuals have been tested for the sickle cell allele.
red-flowered plant. What is the likely mode of
inheritance if the offspring produced are
a. plants with pink flowers?
b. plants with flowers that are red and white spotted?
0
2. 1:mJ Describe a human genetic disorder that results
from incomplete dominance. Explain why it is
classified as incomplete dominance.
purple radishes are crossed?
b. Determine the probability that individuals 11-3 and
tillll A farmer crosses a black rooster with a white
II -4 will have another child with sickle cell anemia.
7.
a. Determine the genotypes of the parents.
of the alleles for white and black feathers?
b. Identify other phenotypes expected from this cross
b. Given the inheritance pattern you described in
and give the predicted phenotypic ratios.
S.lliJI Your friend has bred her female albino rabbit
with her male Himalayan rabbit. ''I'm hoping I'll get
some agouti rabbits:' she says. What are her chances
of getting an agouti rabbit? Explain.
.. The colour of an organism is controlled by one
gene with two alleles: an allele that produces a blue
colour and an allele that produces a yellow colour.
Using genetic notations, describe the differences in
genotypes and phenotypes of the organisms produced
by crossing a true-breeding blue organism with a
true-breeding yellow organism for the following three
inheritance patterns. Use drawings in your answers.
9.
c. blue and yellow are codominant
250
MHR Unit 2 Genetic Processes
•
.. "Human ABO blood grouping is an example
of the effects of multiple alleles, codominance, and
dominance/recessiveness:' Use a table or graphic
organizer to explain this statement.
1O.lliJI Siamese cats that spend their lives indoors tend
to have lighter-coloured fur than Siamese cats that live
outdoors. What genetic process could account for this
change?
a. blue is dominant over yellow
b. blue and yellow are incompletely dominant
tillll A chinchilla rabbit is crossed with a Himalayan
rabbit, producing an albino rabbit.
a. What can you infer about the inheritance patterns
5.
sickle cell
anem1a
phenotype
pedigree. If there are any you cannot be certain of,
explain why.
b. What is the phenotypic ratio expected when two
part (a), what are the expected genotypes and
phenotypes of the offspring produced by a cross
between a speckled hen and a black rooster?
sickle cell
trait (carrier)
a. Determine the genotype of each individual in the
radishes.
hen. Of the seven offspring, three are black, three are
speckled black and white, and one is white.
A
W'
Ill
a. Provide the genotypes for the three colours of
4.
0
II
3. tillll In radishes, colour is controlled by two alleles
that show incomplete dominance. When pure-breeding
red radishes are crossed with pure-breeding white
radishes, purple radishes are produced.
Key
normal
phenotype,
but not
tested
11.
CfJ!I What evidence is there that skin colour in
humans is a polygenic trait?
Inheritance of Linked Genes
Key Terms
As you have learned, there is no apparent interaction between non-homologous
chromosomes during meiosis. The movement of each pair of homologous chromosomes
is independent of the movement of other pairs of homologous chromosomes. This agrees
with Mendel's law of independent assortment. Recall that this law states that the alleles
for a gene segregate independently of the alleles for other genes during gamete formation.
However, Walter Sutton's research showing that alleles segregate in the same way that
homologous chromosomes do implies a very important point: alleles on the same
chromosome do not assort independently. Therefore, they do not follow the Mendelian
inheritance patterns that have been discussed in this unit. It turns out that some genes
are inherited together. Therefore, some traits are often inherited together or are "linked:'
l i n ked genes
sex-l i n ked trait
Linked Genes
In 1 905, W illiam Bateson and Reginald Punnett carried out the first study that showed
the movement of alleles that are on the same chromosome. Using sweet peas, they
that are on the same
tracked the inheritance pattern of two traits: flower colour and pollen shape. They
knew that purple flowers were dominant to white flowers, and that long pollen shape
tend to be i n herited
linked genes genes
chromosome and that
was dominant to round pollen shape. Their results are shown in Figure 6.10. All four
phenotypes that are predicted using a Punnett square were present in the F2 generation.
However, there were far more of the phenotypes from the parental generation. This
suggested that the gametes produced by the parental generation, PL and pl, tended to
assort together rather than independently when producing the F2 offspring. Genes that
do not assort independently are often called linked genes.
Phenotype
X
Cross a plant with purple
flowers and long pollen to
a plant with red flowers
and round pollen.
purple flowers,
long pollen
together
Genotype
PPLL x pp/1
Red flowers,
round pollen
Observe the phenotypes
of the F1 offspring.
PpLI
purple flowers,
long pollen
purple flowers,
long pollen
purple flowers,
long pollen
purple flowers,
long pollen
X
Allow the F2 offspring
to self-fertilize.
Meiosis
PL and pi gametes-more frequent
PI and pL gametes-less frequent
purple flowers,
long pollen
purple flowers,
long pollen
Fertilization
Observe the phenotypes
of the F2 offspring.
purple flowers,
long pollen
15.6
•
•
purple flowers,
round pollen
1.0
F2 offspring having phenotypes of
purple flowers, long pollen or red
flowers, round pollen occurred more
frequently than expected from Mendel's
red flowers,
long pollen
1.4
law of independent assortment.
Figure 6.10 A d i hybrid cross between two sweet pea plants does not
produce the expected phenotypic ratio of 9:3:3:1. These results support the
theory that al leles on the same chromosome do not assort i n dependently.
Identify Provide the genotypes of the F2 offspring.
Chapter 6 Complex Patterns of I n heritance ·
MHR
251
Crossing Over and the Inheritance of Linked Genes
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A chromosome may contain up to a few thousand genes. All of the genes on any one
chromosome are called a linkage group because they tend to be inherited together.
However, linked genes do not always stay linked-researchers have found that they
segregate on a regular basis. This is due to the process of crossing over, which you
learned about in Chapter 5. Recall that crossing over occurs in prophase I of meiosis,
when non-sister chromatids exchange pieces of chromosomes.
Suppose you are studying two genes that are on the same chromosome and,
therefore, linked. Crossing over between homologous chromosomes can occur. As
shown in Figure 6.11, this will result in the alleles of the linked genes no longer being
on the same chromosome. The alleles of the previously linked genes are now unlinked.
This means that they will migrate into different gametes. The result is that instead of
two types of gametes being produced, four different types of gametes will be produced
in differing proportions. There are fewer gametes with the recombined alleles because
crossing over is a random event and it occurs infrequently.
Figure 6.11 In most of the
no crossing over
d uring meiosis
gametes formed, there is
no crossing over-they
mainta i n the l i n kage of the
alleles. I n a small m i nority
of gametes, crossing
over occurs and a l leles of
previously l i n ked genes
become u n l i n ked.
Describe why alleles
of genes that are closer
97%
together on a chromosome
are more likely to remain
linked during meiosis.
3%
recombinant gametes
four types of gametes in unequal proportions
Using Gene Li n kage for Chromosome Mapping
Scientists have discovered that alleles for a given pair of linked genes will separate
with a predictable frequency and that this frequency is different for different pairs of
linked genes. The frequency depends on how close the alleles of the linked genes are
positioned on a chromosome. Crossing over occurs more frequently between alleles that
are far apart on a chromosome than between alleles that are close together. Therefore,
a given pair of linked genes will separate more frequently than the alleles for another
pair of linked genes if their alleles are farther apart on the chromosome. This process
of determining the relative locations of genes on chromosomes is called chromosome
mapping. These types of linkage studies are useful for mapping chromosomes in species
that reproduce rapidly and produce many offspring, such as plants and insects. But
chromosome mapping is not useful in mapping human chromosomes. Chromosome
mapping of humans only became possible when modern techniques that allow
scientists to directly see the chromosomes became available.
252
MHR U n it 2 Genetic Processes
•
Learning Check
7.
What are linked genes?
8.
How are linked genes found experimentally?
9.
What is chromosome mapping? How is gene linkage
used in chromosome mapping?
1 0. Suppose that two individuals with the genotype AaBb
are crossed, and the phenotypic ratio produced is
about 3:1 (A_B_:aabb). Are the genes for the two
traits linked? Explain.
1 1 . Some traits are described as being due to sex-linked
genes. Use your knowledge of chromosomes to
explain what this means.
1 2. Many genetic tests are based on analyzing genes that
are linked to alleles that cause disease. Explain how
testing for a linked gene could lead to an incorrect
diagnosis.
Sex-linked In heritance
An American biologist named Thomas Hunt Morgan, shown in Figure 6.12, originally did
not accept Sutton's chromosome theory of inheritance. In the early 1 900s, Morgan chose
to do research on the fruit fly, Drosophila melanogaster, to develop a new and alternative
theory. Morgan chose this organism because it is economical to maintain, reproduces
rapidly, and has traits that are fairly easy to characterize. As Morgan collected data,
however, his results soon convinced him that Sutton's theories were correct. Nevertheless,
Morgan's meticulous research provided additional information about genetic inheritance.
In 1 9 1 0, Morgan discovered an unusual white-eyed male among his fly population.
sex-linked trait
a trait controlled by
genes on the X or
the Y chromosome
He crossed the white-eyed male with a normal red-eyed female. All the F1 generation
had red eyes. This seemed to indicate that normal red eyes are dominant to the
white-eye mutation. When Morgan crossed a male and female from the F 1 generation,
however, the results surprised him. All the females of the F2 generation had red eyes,
half the F2 males had red eyes, and half the F2 males had white eyes. The discovery that
the gene for eye colour was connected to gender led Morgan to conclude that the gene
for eye colour is located on the X chromosome.
Like humans, female fruit flies have two X chromosomes, while males have one
X chromosome and oneY chromosome. The fruit fly F1 data indicated that the
white-eye phenotype is recessive, since it was masked in all of the offspring in that
generation. How did white eyes reappear in only the male fruit flies in F2, but remain
masked in the female flies? The answer lies in the sex-linked genes-the genes that are
located on the X andY chromosomes.
Traits that are controlled by genes on either the X orY chromosome are called
sex-linked traits, because they are linked to the genes that determine sex. They are
identified by their different rates of appearance between males and females.
female
Figure 6.12 (A) Drosophila
melanogastertraits that are
often studied include eye
colour and wing size and
shape. Males and females
ca n be easily identified.
(B) Thomas Morgan's
g round-breaking research
into the genetics of fruit
flies was recogn ized
in 1933, when he was
awarded the Nobel Prize in
physiology or medici ne.
male
Chapter 6 Complex Patterns of I n heritance ·
MHR
253
Sex-li n ked Genes
The X andY chromosomes, although paired together during meiosis and for
karyotyping purposes, have very little homologous DNA. The X and Y chromosomes
in humans have only a few genes in common. The human X chromosome is estimated
to contain about 2000 genes, while theY chromosome contains fewer than 1 00. The
most important genes are the sex-determination genes. For all other genes on the X
chromosome, females have two copies, while males have only one. This allows for the
difference in the expression of traits for genes that are found on the X chromosome,
which are often called X-linked genes. By comparison, only a few genes are known to
be Y-linked, because there are significantly fewer genes on theY chromosome. When
considering sex-linked traits, the allele on the sex chromosome is shown as
a superscript to an X or a Y.
The Red and White Eyes of Fruit Flies
Red and white eyes were the first sex-linked trait explored by Morgan. The possible
genotypes and phenotypes in both males and females are listed in Table 6.1. XR
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indicates red eyes, which is the dominant phenotype, and X! indicates white eyes,
which is the recessive phenotype. Notice that female flies may be a carrier for the
white-eye phenotype. However, if the allele for white eyes is present in males, it will
always be expressed. This means that X-linked traits are exhibited more often in males.
Punnett squares can be used to predict the outcome of crosses that involve sex-linked
traits. Figure 6.13 represents some of the crosses that Morgan studied.
Table 6.1 Possible Genotypes and Phenotypes for Drosophila Eye Colour
I
Genotype
xRxR
Suggested Investigation
Thoug htlab I nvestigation
6-B, Sex-Linked Crosses
Phenotype
Female with red eyes (homozygous dominant)
XRX'
Female with red eyes (heterozygous, carrier for the white-eyed allele)
X'X'
Female with white eyes (homozygous recessive)
xRy
Male with red eyes
X'Y
Male with white eyes
x
R
x
R
X'
x
R
R R
x x
R
x xr
white-eyed
male
F1 male
y
X'Y
R
x y
R
x y
X'
y
R
x y
X'Y
Figure 6.13 In Morgan's experiment on tracking the inheritance pattern of a sex-linked trait,
the wh ite-eye phenotype was passed from the father in the P generation through the daug hter
in the F1 generation.
Predict the genotype and phenotype ratios of the offspring created by crossing a white-eyed
male and a heterozygous female.
254
MHR · U n it 2 Genetic Processes
Sex-linked Tra its in Humans
Some examples of sex-linked traits in humans are listed in Table 6.2. As you can see,
many are genetic disorders. If a disorder is X-linked dominant, affected males pass the
allele only to daughters, who have a 100 percent chance of having the disorder. Females
can pass an X-linked dominant allele to both sons and daughters, all of whom will have
the disorder. Most sex-linked inherited traits in humans are X-linked recessive traits.
Therefore, while the male only needs to inherit one allele to be affected, the female
must inherit both alleles to be affected. Thus, X-linked recessive traits affect more males
than females in a family.
Table 6.2 Sex-linked Traits in Humans
Condition
Red-green colour vision
deficiency (CVD)
I
Inheritance Pattern
X-linked recessive
I
Description
Cannot distinguish between certain
shades of red and green
Duchenne muscular
dystrophy
X-linked recessive
Progressive weakening of muscles and
loss of coordination
Hemophilia
X-linked recessive
Cannot produce a necessary blood
clotting factor
Adrenoleukodystrophy
X-linked recessive
A build-up of fatty acids that causes
progressive brain damage and death
X-linked severe combined
immunodeficiency (SCID)
X-linked recessive
Decreased immune response due to
low white blood cell counts
X-linked hypophosphatemia
X-linked dominant
Softening of bone, which leads to bone
deformity
Hairy ears
Y-linked
Hair grows on the outside of the ears
Colour Vision Deficiency: An X-linked Recessive Trait
In humans, there are inherited forms of colour vision deficiency (CVD). Individuals
affected by CVD have varying degrees of difficulty distinguishing between different
colours or shades of colours. One form, called red-green CVD, is an X-linked recessive
disorder. I ndividuals with red -green CVD have difficulty distinguishing between
shades of red and green. To track the inheritance patterns of sex-linked traits in
humans, pedigrees are often used. The inheritance pattern of red-green CVD in one
family is shown in Figure 6.14.
Key
X8X8 = normal female
X8Xb = carrier female
XbXb = CVD female
X8Y = normal male
II
xby
=
CVD male
Ill
Figure 6.14 An X-linked recessive trait l i ke CVD will affect more males than females in a fa mi ly.
Chapter 6 Complex Patterns of I n heritance
•
MHR
255
Hemophilia: A Com m o n Sex-linked Trait i n H u m a n s
Hemophilia is a condition that affects the body's ability to produce proteins involved
in blood clotting. People with hemophilia can suffer serious blood loss from simple
cuts and bruises. Hemophilia is an X-linked recessive trait that affects more than
3000 individuals in Canada.
Hemophilia is often referred to as the royal disease because it spread among the
royal families of Europe, through the descendents of Great Britain's Queen Victoria,
shown in Figure 6.15. Queen Victoria was a carrier who passed the allele on to some of
Figure 6.15 Great Britain's
Queen Victoria was a carrier
her offspring. Arranged marriages among royalty of Europe were very common until
the twentieth century. Pedigree analyses can trace the allele for hemophilia throughout
the royal families of Spain, Russia, and Prussia.
for hemophilia.
Sample Problem
Using Punnett Squares to Analyze Sex-li n ked I nheritance Patterns
Problem
Determine the probability that a woman who is a carrier for hemophilia and a man
without hemophilia will have a child with hemophilia.
What Is Required?
You need to determine the possible genotypes and phenotypes of the offspring to
determine if any of the children could have hemophilia.
What Is Given?
You know the phenotypes of the parents, and you know that the pattern of inheritance
is X-linked recessive.
Act on Your Strategy
Plan Your Strategy
Assign letters to represent each allele
for the trait, and then determine the
genotypes for the parents based on an
X-linked recessive inheritance pattern.
Since the inheritance pattern is X-linked recessive,
let X" allele for hemophilia
let XH allele for normal blood clotting
•
•
=
=
The female is a carrier, SO her genotype is XHXh.
The male is unaffected, so his genotype is XHY.
female
Use a Punnett square to predict the
genotypes of the offspring.
xH
male
y
Complete the Punnett square.
EE
female
male
Determine the predicted phenotypes
of the offspring, and the probability of
producing a child with hemophilia.
X"
xH
xH
x"
xH
xHxH
xHx"
y
xHy
X11Y
There is a 25 percent chance of having a child with
hemophilia (X11Y). All other genotypes produce a child
with normal blood clotting.
Check Your Solution
To check your solution, ensure that the genotypes of the parents accurately represent
the phenotypes, and that all possible combinations of gametes have been made.
256
MHR U n it 2 Genetic Processes
·
Sample Problem
Determi ning Sex-li n ked Inheritance Patterns in a Pedigree
Problem
The pedigree on the right shows the inheritance of red-green
CVD in a family. Identify the genotype of each family
member represented in the pedigree. How does the inheritance
pattern in the pedigree support X-linked inheritance?
II
What Is Required?
You need to determine the genotype of each individual and
describe the evidence for X-linked inheritance.
Ill
4
3
2
What Is Given?
You know that the pattern of inheritance is X-linked
recessive, and you have the phenotype of each of the
individuals (the pedigree).
Plan Your Strategy
Assign letters to represent each allele. Identify possible
genotypes for each of the phenotypes based on an X-linked
recessive inheritance pattern.
Act on Your Strategy
•
•
let X'
let XC
=
=
allele for CVD
allele for normal vision
XCXC unaffected female
XCX' female carrier
X'X' female with CVD
=
=
XCY
X'Y
=
=
unaffected male
male with CVD
=
Assign all possible genotypes, according to the information
in the pedigree.
At this point, you cannot be certain of the genotypes for
individuals I - 1 , Il- l , and Il-3. Since they are unaffected
females, the possible genotypes are xcxc and xcxc.
•
II
Ill
Complete the pedigree with genotypes that you can infer
based on the data in the pedigree.
You know that individual Il-3 must be xcxc to produce
a son who has CVD.
Individual Il- l must be xcx'. The X chromosome she
received from her father is X' and, since she is unaffected,
she must have received xcxc from her mother.
You cannot be certain of the genotype of l-2 because
both genotypes are possible cxcxc and xcxc), given the
genotypes of the offspring.
2
3
4
2
3
4
•
•
II
•
Describe how the inheritance pattern supports X-linked
recessive inheritance.
Ill
The allele for CVD is passed from the grandfather (I - 1 )
through his unaffected daughter (II-3) to her affected son
(III-4). This pattern is indicative of X-linked recessive
inheritance. As well, more males are affected than females,
which also indicates X-linked recessive inheritance.
Check Your Solution
To check the pedigree, ensure that all the offspring genotypes are possible given the
genotypes of the parents.
Chapter 6 Complex Patterns of I n heritance ·
MHR
257
Practice Problems
1 1 . A woman who is a carrier for CVD and a man who
b. Determine the genotypes of the flies described in
the F2 generation.
has CVD decide to have children.
a.
c. What is the probability of producing tan offspring
Determine the genotypes of these two people.
from a yellow female and a tan male?
b. What is the expected ratio of genotypes and
phenotypes among their children?
1 2. The mother and father of a boy who has CVD both
have normal colour vision. Use a Punnett quare to
explain how this can occur.
1 7.
Given the pedigree below, determine whether
the pattern of inheritance of this trait is X-linked
recessive, X-linked dominant, or Y-linked dominant.
Explain your answer.
1 3. A woman with hemophilia and a man without
hemophilia decide to have children. What is the
probability that their sons will have hemophilia?
1 4. Nystagmus is a condition in which involuntary eye
movement leads to poor vision. This condition is
caused by an X-linked recessive allele. Suppose that
a man and woman, both with normal vision, have
two children. The boy is affected with nystagmus,
and the girl is unaffected.
a. Determine the genotype of the parents.
b. Is it possible to determine the genotypes of the
children? Why or why not?
1 5. A woman has X-linked hypophosphatemia, which
affects bone development. She marries a man with
normal bone structure. If the woman's father also
has normal bone structure, what is the probability
that the woman and her husband will have a child
with the disorder?
1 6. A true-breeding tan-bodied female fruit fly is
crossed with a yellow-bodied male. All of the
offspring in F 1 have tan bodies. In the F2 generation,
all the females have tan bodies, 50 percent of the
males have tan bodies, and 50 percent of the males
have yellow bodies.
a. Describe the pattern of inheritance for body
colour in fruit flies. Explain your answer.
II
Ill
2
3
4
1 8. In one breed of dog, a mutant gene that causes
hearing impairment is found on the Y chromosome.
What are the possible phenotypes of offspring from
each of the following crosses?
a. a male dog whose father is hearing impaired and
a female dog whose father is not hearing impaired
b. a female dog whose father is hearing impaired and
a male dog whose father is not hearing impaired
1 9. Suppose you have one homozygous dominant
red-eyed female fly and one white-eyed male
fly. What steps would you follow to produce a
white-eyed female fly?
20. The allele for short fingers is dominant to the allele
for long fingers. What is the genotype of a male who
has CVD and long fingers? If all of his children have
normal vision and short fingers, what is the likely
genotype of the children's mother?
Barr Bodies: I nactive X Chromosomes
Since females carry two X chromosomes and males only one, why is there no difference
in the expression of X-linked genes between males and females? The answer is that
every cell has only one functioning X chromosome. In every female cell, one of the
X chromosomes is inactive. The inactive X chromosome is condensed tightly into
a structure known as a Barr body. At an early stage of embryonic development, one
Figure 6.16 I n cats, the
al leles fo r black or orange
coat are carried o n the
X chromosome.
258
X chromosome in each cell is deactivated. Which X chromosome is deactivated can
vary among cells. One visible effect of one X chromosome being inactive is the calico,
or tortoiseshell, coat colour in cats, shown in Figure 6.16. In heterozygous females,
roughly 50 percent of the cells have an active X chromosome with the allele for black
coat colour, and 50 percent of the cells h ave an active X chromosome with the allele for
orange coat colour. This results in a tortoiseshell coat with patches of both black and
orange. The patches of white are the result of the interaction with a different gene.
MHR Unit 2 Genetic Processes
•
Section Summary
•
Alleles of different genes that are on the same
chromosome do not assort independently. These genes
are said to be linked and their associated traits tend to be
inherited together.
•
•
Sex-linked traits are expressed in different ratios by male
and female offspring because they are determined by the
segregation of X and Y chromosomes.
Although sex-linked genes are linked to the X and Y
chromosomes, Punnett squares can be used to predict
genotypes and phenotypes.
Review Questions
1 . .m Design an experimental procedure that you
could follow to determine whether two plant genes are
linked.
9. .. Explain how a girl with Turner syndrome could
have red-green CVD, even though both of her parents
have normal vision.
2. ID Describe how the process of crossing over
1 0 .m The following pedigree was given to a group of
of non-sister chromatids can affect linked genes.
students to analyze. They believe it indicates X-linked
recessive inheritance. Do you agree or disagree?
Explain your answer.
•
3. 1D What experimental evidence would lead
scientists to suspect that two genes are linked?
4. .m A chromosome contains three genes, P, Q,
andR. The percentage of gametes produced that have
the genes separated due to crossing over is shown in
the table below.
II
Linked Genes
Genes
P and Q
I
Gametes with
Unlinked Genes (%)
5
P and R
18
Q and R
13
From these data, identify the gene pair with alleles
that are closest together on the chromosome. Explain
your answer.
5. .. Draw a diagram that shows how crossing over
can cause linked genes to become unlinked.
6. 1D List two features of Drosophila melanogaster
7.
that make this species a good choice for the study of
sex-linked inheritance.
.m A woman with regular vision and a man with
regular vision have three children, one of whom
has CVD.
a. What can you conclude about the genotypes of
the parents?
b. What sex is the child who has CVD? How do
you know?
8. 1D Describe the possible genotypes of the parents
of a woman who has hemophilia. Explain your answer.
Ill
2
3
4
5
6
1 1 . 1D How do pedigrees for autosomal recessive traits
and X-linked recessive traits differ?
1 2 .. A boy has Duchenne muscular dystrophy. His
.
mother's brother also has this disorder. The boy's father
and his two younger sisters do not appear to be
affected by the disease. Draw a pedigree to illustrate
the inheritance of Duchenne muscular dystrophy in
this family. What is the probability that his sisters are
carriers of the disease?
1 3. 18 The symptoms associated with X-linked
dominant diseases are often more severe in males.
Explain.
1 4. .. Draw a sample pedigree to illustrate inheritance
of hemophilia in a family. Make sure that your
pedigree reflects that particular inheritance pattern.
1 5. .. Some women are heterozygous for an X -linked
genetic disorder that results in a non-uniform
distribution of sweat glands on their skin. These
women have patches of skin that lack sweat glands and
patches of skin that have sweat glands. How can the
Barr body cause this phenomenon?
Chapter 6 Complex Patterns of I n heritance ·
MHR
259
The Future of Genetics Research
Genetics research is continually changing and developing in response to new
Key Terms
discoveries. Many genetics researchers now focus on obtaining more and more detailed
information. In addition to wanting to know the sequences of genes that are associated
with certain inherited traits, investigators want to know how those genes play a role
bioi nformatics
genomics
genetic profile
in determining those p articular traits. Looking for answers to these types of questions
has led to the development of more sophisticated technologies and equipment, and
has resulted in new scientific fields of study. In addition, many studies now require the
collaboration of scientists from very different disciplines, such as biology, chemistry,
physics, sociology, bioethics, and political science.
The Human Genome Project
In the opener for this chapter, you were introduced to the the Human Genome Project.
Determining the DNA sequence of the human genome is considered to be one of the
Figure 6.17 The Human
Genome Project ach ieved
many milestones and has
provided a springboard
for decades of future
research. Nevertheless,
this project would not have
been possible without
several essential preceding
discoveries-including
Mendel's studies of pea
most pivotal contributions to science ever made. Nevertheless, achieving this scientific
landmark depended on many discoveries that came before it. Figure 6.17 highlights only
a small number of developments since Mendel's work that formed the foundations of
this project.
An important component of the 1 3 -year Human Genome Project was determining the
DNA sequences of other organisms. This allows scientists to make comparisons between
species and learn even more about important features of genomes. Overall, identifying
the genome sequences of humans and many other organisms allows for a much more
comprehensive understanding of biological systems. This knowledge will have a wide
range of applications in fields such as human health, agriculture, and the environment.
!
plants.
NA as the hereditary
l :r;
work
' '"''""'""'J
.
1991
1992
the Human
Genome Project
is launched
first US genome
centres established
I
T
I
T
ethical, legal, and
social implications
(ELSI) program
founded
260
MHR U n it 2 Genetic Processes
•
rapid-data-release
g uidelines
established
''""""""j
of DNA is
determined
map of genes
is produced
1990
.
1995
T
US Equal Employment
Opportunity Commission
issues policy on
genetic discrimination
in the workplace
...
<he geo,.ic cod
is identified
j
1996
1997
(Saccharomyces
cerevisiae)
Escherichia coli
T
yeast
genome sequenced
I
genome
sequenced
What's in Our Genome?
In addition to determining the actual sequence of the nucleotides in the human genome,
scientists had to make sense of the sequence. Trying to make sense of the sequence
can be compared to reading a book written in a language nobody knows or understands.
Imagine the genome as words in a book written without capitalization, punctuation,
or breaks between words, sentences, or paragraphs. Also, suppose there are strings of
additional letters scattered randomly between and within sentences. Figure 6.18 shows
how a page from such a book might look. To understand what is written, you h ave to
decode the jumbled text. Similarly, scientists had to decode the sequence of our DNA
to learn about the human genome. When the Human Genome Project began, there
was a great deal that was not known about our genome. For example, it was not known
how many genes humans actually had and how much of our DNA is part of those genes.
After sequencing the entire human genome, scientists observed many things that
surprised them. Some of these discoveries include the following:
o
o
o
o
oecodingthehumansurrrtodgenomeseque
ncecanhfgeteirunfhdbewmparedtorefdt
wiqppnbfteadingabookthatwaswregdfst
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ginethegenomeMterd1ortgpmUwordstkfh
gnviinabooknvhgytpwmlwrittenwithoutc
apftalizationkghtowkfgcbvjorpunctuation
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weenwordssentflltetorvhgotwpqmnkpar
agraphsandwithfoagwitostringsoflettersd
hfiruwqscatteredbetweenandwithin
sentencesinghomlaordertobdqpvundei'Wt
ndwhatiswrittenthejumbledbghfqomkslte
xthastobeghqpmsddetoded.
Figure 6.18 Decoding the
DNA sequence of the h u m a n
g e n o m e is l i ke fig u ring o u t
where t h e pu nctuation a n d
capitalization m u st g o to
unde rstand what is written
on this page.
Only about 2 percent of the nucleotides in the human genome make up our genes
and code for all the proteins in the body.
The estimated 25 000 total number of genes is much less than scientists predicted.
Previous estimates were between 80 000 and 1 40 000.
Go to scienceontario
to find out more
Over 50 percent of our DNA consists of stretches of repeating sequences.
There is very little genetic variation within our species. About 99.9 percent of the DNA
sequence is almost exactly the same in all people.
H aving the sequence of the human genome only represents a starting point. It is
like being given the pages of an instruction manual for the human body. The next steps
involve figuring out how to interpret all of the information and use that to understand
how everything works together. Scientists agree that this process will take many more
years of research.
1
methods for determining the
'"'"" of DNA ' " de"loped
=====
1
cystic fibrosis
gene " ldemlfied
•
1 •
•
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h"m" ''"'"
gene-for Hu ntington
disease-is mapped
2000
T
free access to genome
information established
I
(Drosophila melanogaster)
fruit fly
i
2003
2002
2001
draft version
of human
genome sequence
p ublished
i
sequences of mouse, rat, and
rice genomes completed
T
human genome
sequence
completed
genome sequenced
I
Chapter 6 Complex Patterns of I n heritance ·
MHR
261
The Development of Bioi nformatics
bioinformatics a
field of study that
deals with using
computer technology
to create and ana lyze
large databases of
i nformation
In the Launch Activity at the beginning of this chapter, you simulated the work
required to piece together the sequence of a small fragment of DNA. Imagine
doing this work by hand for the over three billion base pairs of the human genome.
Sequencing the human genome and the genomes of other organisms generated
exceptionally large amounts of data that needed to be organized and shared among
labs around the world. A new field of study, called bioinformatics, arose from this need.
is a branch of biology that deals with applying computer technology to
create and maintain databases of information that can be analyzed to better understand
biological processes.
Bioinformatics is a relatively new branch of biology. American chemist M argaret
Bioinformatics
Dayhoff, shown in Figure 6.19, is the founder of bioinformatics. Her work, which
began in the late 1 940s, involved creating a computerized protein and DNA sequence
database-the first bioinformatics project. Today's bioinformatics exists because of
simultaneous advances in three areas: techniques to sequence biological molecules such
as DNA and proteins, computer database software to sort and store massive amounts of
genetic information, and communication technology to share information around the
world efficiently. Today, there are many on-line genetics databases available that allow
easy access to vast amounts of genetic information by all members of the public-not
just scientific researchers.
Figure 6.19 A chemist
named Margaret Dayhoff
is co nsid ered to be the
founder of bioi nformatics.
Activity
Bioinformatics is just one of a number of newly developed fields, all of which
involve using computers to study biological problems. For example, computational
biology involves developing mathematical models and computer simulations of
biological processes.
Accessing Genetic Information
I n this activity, you w i l l join the worldwide com m u n ity
of scientists who explore i nformation stored in the many
on-line databases that are ava i lable.
3. Spend some time looking at the d ifferent databases
that are available from this site. What d ifferent types of
i nformation about a genetic d i sorder can be obtained
from them?
Materials
computer with I nternet access
•
Procedure
1. Choose one of the genetic d i sorders provided by you r
teacher.
2. Use the I nternet to access the website that you will be
using. Yo u r teacher will provide a demonstration to help
you get started.
4. Choose three or fo u r types of i nfo rmation that are
available about the genetic disorder that you selected.
Questions
1. S u m marize the i nformation you col lected on the genetic
disorder you i nvestigated.
2. Based on you r experience with the on-line databases,
what was the most effective way of obta i ning the
i nformation that you were looking for?
Learning Check
1 3. What were some achievements of the Human
Genome Project?
1 4. How much of the human genome is actually used to
code for proteins?
1 5. List three types of technologies that contribute to
developing the tools used in bioinformatics.
262
MHR U n it 2 Genetic Processes
•
1 6. Explain how bioinformatics contributed to the
Human Genome Project.
1 7. Why was development of the Internet crucial for the
Human Genome Project?
1 8. Describe an experiment that requires bioinformatics.
Genom ics: The Study of Genomes
Just as genetics is the study of genes, genomics is the study of genomes and how genes
work together to control phenotype, as illustrated in Figure 6.20. Although some traits
are determined by only one gene, most traits involve multiple genes. To understand
how an individual gene produces a specific phenotype, researchers such as Mendel and
Morgan chose one gene and studied it and its phenotype across many individuals. A
significant advantage that came from the Human Genome Project was the ability to
consider multiple genes and the genome as a whole. This allows scientists to study the
genomics the study
of genomes and the
complex i nteractions
of genes that result i n
phenotypes
interactions among many genes and how they all contribute to a phenotype. Computer
technology and fields such as bioinformatics play a vital role in this by allowing
scientists to analyze large amounts of information from a variety of sources.
Although there is considered to be little variation in the sequence of the human
genome, it is important to keep in mind that the 0. 1 percent difference represents
potential for variation in about three million nucleotides. Some of this variation is
associated with many diseases. Scientists believe that almost all human diseases have
a genetic component, either directly or indirectly. Comparing genome sequences has
been particularly useful in studying the genetic basis for many human diseases, such
as cancer. For example, bioinformatics and computational biology have been used to
compare the DNA sequences of certain regions of the genome in individuals affected by
a particular type of cancer with the DNA sequences of the same regions in those who
are not. Differences in DNA sequence indicate a potential genetic basis for the disease.
While this represents a good starting point for the study of the genetics of a disease,
scientists are discovering that many diseases are the result of a complex array of factors,
and studying them requires more elaborate methods.
C G T T C T C T A T T A A C A .. .
G C A A G A G A T A A T T G T .. .
three billion DNA base pairs
i n the cell n ucleus
phenotypes
are expressed
thousands of different proteins
are produced i n tri llions of cells
Figure 6.20 Genomics is the study of how a n orga nism's genome contributes to its phenotype.
Chapter 6 Complex Patterns of I n heritance · MHR
263
Linking Genetic Variations to Disease
In previous sections, you learned about diseases that are associated with a mutation
or mutations in a single gene, such as sickle cell anemia. Many other diseases, such as
cancer, stroke, heart disease, diabetes, and asthma, are influenced by a combination of
environmental and genetic factors.
Many scientists consider determining what variations in DNA sequence contribute
to different diseases to be one of the best opportunities to understand the complex
causes of many human diseases. The most common type of variation between people is
differences in individual nucleotides, as shown in Figure 6.21. For example, one person
may have a C at a certain location, while another person may h ave a T. This type of
genetic variation is called a single nucleotide polymorphism, or SNP (pronounced
"snip"). A SNP can act as a marker for a gene or be associated with a gene if it is
genetically linked to it. Recall that sequences of DNA are genetically linked when
they are physically close to each other on a chromosome and tend to be inherited
together. For example, if a SNP is common among people with high blood pressure,
that could provide a marker for the location of a gene that is involved in the disease.
However, there are almost 10 million different SNPs that commonly occur in the
human genome. Testing all of these is not feasible. Nevertheless, SNPs that are near
each other on a chromosome tend to be inherited together. These regions of genetically
linked variations are called haplotypes. Certain tag SNPs can uniquely identify these
haplotypes. Since there are far fewer of these types of SNPs, they can be used as a basis
for comparing genetic variations and identifying genes that influence the health of an
individual.
In 2002, an international group of researchers from Canada, the United States,
Japan, China, Nigeria, and the United Kingdom collectively began the International
HapMap Project. The major aim of this project is to develop a haplotype map
( HapMap) of the human genome, which represents a map of the variations in the
human genome. This can then be used by other scientists to identify the genetic basis
for many human diseases.
Ch romosome 1
1
Ch romosome 1
Ch romosome
Chromosome
1
SNP
SNP
SNP
. . .
T T CGGG T C . . . .
AG T C GA C C G . . . .
AAC AT GC CA . . . .
T T CGGG T C . . . .
AGT CAA C CG . . . .
AAC ACG C CA
AACA CG C CA
.
.
. . .
AAC ACGC C A . . . .
T T CGAG T C . . . .
T T CGGG T C . . . .
AGT CAA C CG . . . .
AG T CGAC CG . . . .
Haplotype map of the human genome
Figure 6.21 A hap lotype map is constructed by identifying single n u cleotide polymorph isms
(SNPs) among a n u m ber of i ndividuals.
Beyond the Genome Sequence
Analysis of the data generated from the Human Genome Project will continue for many
decades. A significant part of that research involves more than working at the level of the
DNA sequence. For example, the field of proteomics began when scientists recognized
how important it is to understand the products of our genes-proteins. Based on the
term genome, the term p ro teome was developed to refer to all of the proteins in an
organism. Research studies in proteomics focus on studying the three-dimensional
shape of proteins and eventually determining the functions of all the cellular proteins.
264 MHR U n it 2 Genetic Processes
•
Studying Gene Expression
Today, many scientists are studying what regulates the expression of genes. That is, they
look at what influences whether a particular protein is produced from a certain gene
and, if so, how much of the protein is made. An individual's phenotype is the result
of which genes are active-are being expressed-and which genes are inactive, or not
being expressed. While all cells of an individual have the identical genetic material,
the same genes are not expressed in the same way in every type of cell. For example,
differences in gene activity can exist between healthy cells and cells of diseased tissue,
such as cells of cancerous tumours.
Scientists now realize that some factors that affect gene expression can be inherited,
but they are not due to changes in DNA sequence. Epigenetics is the study of how
changes in the inheritance of certain traits or phenotypes are based on changes
to gene function and not to changes in DNA sequence. Epigenetics differs from
evolution because there is no change to the DNA sequence of a gene and epigenetic
changes are not necessarily permanent. Epigenetic changes represent a response to an
environmental condition that may be reversed once that condition changes, or soon
after the change. The term epigenome refers to cellular material that is not part of the
genome but that influences whether a gene is "turned on" or "turned off:' Identifying
epigenetic factors is believed to be a next major frontier in biological sciences.
Studying Gene Expression Using Microarrays
A very important method that is used to study differences in gene activity is DNA
microarray technology. In this technique, DNA is placed as spots on a glass plate,
called a microarray plate. One slide can contain thousands of spots of DNA that
correspond to certain parts of a genome, and that contain different genes. Figure 6.22
shows an example of a microarray plate. This technique allows scientists to study the
activity of up to thousands of genes at a time, under particular conditions. Studying the
activity of so many genes at once tells scientists which genes are active or inactive under
certain conditions, and gives them information on how this activity is co-ordinated
among different genes.
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. il
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Figure 6.22 DNA microarrays allow scientists to see the activity of genes
u nder certain conditions. The colou r of each ci rcle o n a microarray plate l i ke
this one corresponds to the activity of a gene in the DNA spot on the plate.
Chapter 6 Complex Patterns of I n heritance ·
MHR
265
Genetic I nformation: Public Benefits and Concerns
genetic profile the
complete genotype of
an individual, including
various m utations
Some of the most important benefits of genetic research are in the area of human
medicine. Figure 6.23 illustrates this link between genetics and medical treatment.
Studying the human genome as a whole may make it possible to develop drugs that are
tailored to the expression of the genes associated with particular disorders, and to the
unique genome of a patient.
In the future, researchers hope to use established links between genetic variation
and risk of disease to provide better medical advice to patients. If the cost of DNA
sequencing continues to decrease, individuals may have access to their genetic
profile-their complete genotype, including all of the various mutations linked to
disease. Currently, doctors are only able to make generalized risk assessments based on
medical history. Armed with a genetic profile, however, genetic counsellors and doctors
will be able to provide more specific risk assessments, design individualized prevention
plans, and design genetically precise treatment programs.
What Can Happen to Information from a Genetic Profile?
Establishing genetic profiles for individuals, and making these profiles available to
health-care providers, also creates ethical concerns. For example,
•
Could insurance companies deny coverage to people who have a genetic
predisposition for a particular disease?
•
Figure 6.23 This a ltered
representation of the
caduceus-a com mon
symbol for medical
practice-illustrates the
•
Could potential employers have access to an individual's genetic profile and use it in
assessing whether to hire the person?
Should researchers be allowed to use the genetic profiles of individuals to help them
better understand the link between genome and phenotype?
The central issue in all of these ethical questions is who should have access to the
information in a genetic profile.
l i n k between g e netics and
treatment of disease.
Ownership of Genetic Information
All the data gathered through the Human Genome Project is publicly available. Having
access to the data made it possible for scientists to share what they learned about
human genetics. In other areas of genetics research, however, the relationship between
public and private information is more complex.
Go to scienceontario
to find out more
In 2005, the National Geographic Society and the IBM company jointly launched
the Genographic Project. This project uses DNA samples provided by hundreds of
thousands of volunteers around the world to learn more about the migrations of
ancient peoples. Using high-tech genetics tools and computer facilities, DNA sequences
of the individuals are analyzed to better understand human genetic roots and how we
all "connect" at the level of our DNA.
Studies such as the Genographic Project can contribute valuable information to
researchers in many fields. But who owns the genetic information? For example, should
companies have the right to sell DNA information to other companies without the
permission of the people who provided the samples? Should companies that use DNA
in medical research be required to share the results of their work with the individuals
or communities whose genetic information was used?
Some people argue that genetic information is a natural resource that belongs
to everyone. Others believe that genetic information about a person belongs only to
that person. In addition, many think that if companies cannot earn a profit from their
research, there is little incentive for them to invest in genetic studies. In the world of
genetics research, where is the boundary between public and private property?
•
266
MHR U n it 2 Genetic Processes
g
Section Summary
o
o
The complete DNA sequence of the human genome was
determined as part of the Human Genome Project.
The field of bioinformatics arose from the need to share
and maintain the large quantities of data collected from
genomic research. It also provides tools for analyzing
genomic data.
o
o
There is still much to be learned from the data generated
from the Human Genome Project, particularly in
identifying genes that are associated with human health.
Current and future research in genomics may allow
scientists to tailor preventative and curative treatments
for individual patients based on their specific genetic
profiles. However, ethical questions about who owns an
individual's genetic information continue to be debated.
Review Questions
1 . 1D The Human Genome Project involved
sequencing the genomes of other organisms as well as
of humans. Provide two reasons for why this was done.
2 . .. In this section, the human genome was
compared to a book. Illustrate how the parts of a
book-pages, paragraphs, sentences, words, and
letters-can be used to represent chromosomes,
chromatids, genes, and nucleotides.
3. 1D Describe three things about the human genome
that scientists learned from the Human Genome
Project.
4. 1D Although the Human Genome Project is
complete, research based on its findings continues.
Describe two areas of current research that developed
from it.
5. lilt The Human Genome Project cost billions of
dollars to complete. Do you think it was worth it?
Provide reasons that support your opinion.
6. .:g The two pictures below show scientists
conducting genetic research in labs. The photo on the
left was taken in the 1 980s, and the photo on the right
was taken in the 2000s. Describe how these photos
reflect the changes in genetic research that took place
over this time period.
7. .:g Describe why you think the field of
bioinformatics was given that name. Provide a suitable
alternative name for this field of science.
S. ID What is genomics? Describe the type of research
that is involved and how it may help society.
9.
1D What is the HapMap project? What is its main
goal?
1 0 .:g Explain how epigenetics suggests that the traits
•
we inherit may not be due only to the DNA we receive.
1 1 . 1D Describe how DNA micro array technology is
used to study gene expression.
1 2. .. Determining a genetic profile can have its
benefits and its risks. Use a table to list as many
benefits and risks as you can.
1 3 . .. Should people be encouraged to have their
genetic profiles determined, since this might prevent
them from developing certain illnesses? Justify your
answer using examples.
1 4. lilt Imagine that you have been hired by an
international organization that establishes practices
for scientists to follow when doing genetics research.
Your job is to develop a policy on the collection and
ownership of genetic information.
a.
What are some of the issues you should consider?
b. Based on the issues you listed, decide where you
stand on those issues and develop a policy that
reflects that stance.
c. Briefly summarize how your policy will balance
public and private interests.
1 5 . .. Write a paragraph expressing your opinion on
whether employers should have to provide a work
environment that suits a person's genetic profile.
Chapter 6 Complex Patterns of I n heritance ·
MHR
267
Plan Your Own
I N V E S T I G AT I O N
Skill C heck
./ Initiating and Planning
.t Performing and Recording
./ Analyzing and Interpreting
.t Communicating
Safety Precautions
•
Wash your hands when you have
completed this investigation
Suggested Materials
•
•
•
•
•
•
•
•
seeds (Brassica rapa, radish,
or bean)
Environmental Influences
on the Production of Chlorophyll
Chlorophyll is the molecule that allows plants to capture light energy from the
Sun and use the energy to produce food in the form of sugars. Chlorophyll
is also the pigment that gives leaves their green colour. Plants that produce
chlorophyll appear green. If the production of chlorophyll is "turned off;' the
plant will become pale yellow, or even white. The production of chlorophyll is
under genetic control.
Working in groups and using the materials provided, you will design and
conduct an investigation to test the influence of light on the production of
chlorophyll. Your investigation must enable you to draw conclusions about each
of the following.
•
labels
paper towels
•
water
What is the minimum duration of exposure to light required to turn on the
production of chlorophyll?
Is the triggering event reversible? That is, does chlorophyll production start
and stop as environmental conditions change?
shoe boxes
petri dishes
graduated cylinder
light source
Organizing Data in a Table in
Appendix A for help with designing
When chlorophyll is no longer present, a green plant will become pale yellow or even
o toble for data.
the change i n environmental conditions that accompanies autumn, ch lorophyll i s no
Go to
Go ta Constructing Graphs
in Appendix A
jar information about making graphs.
268
white. This i s similar to what happens to many trees d u ring the a utumn. I n the spring
and summer, tree leaves a ppear green because chlorophyll is being produced. With
longer prod u ced and other pigments in the tree leaves become visible. This results in
the ye llow, orange, and red "fall colours" of some trees.
MHR · U n it 2 Genetic Processes
rt
m
Pre-Lab Questions
1 . Describe the genotype of the organisms you should use
that will allow you to test the effect of the environment
on phenotype.
2. What is the difference between qualitative and
quantitative data?
3. Differentiate among independent, dependent, and
Analyze and Interpret
1 . Did your observations support or refute your
hypothesis? Explain.
2. Did your investigation allow you to draw conclusions
about the inheritance of the genes that are involved in
the production of chlorophyll? Why or why not?
3. Identify the variables you considered when designing
your investigation. Explain why you needed to consider
each variable to obtain scientifically valid results.
controlled variables.
Question
How does light influence the production of chlorophyll in
germinating plants?
Conclude and Communicate
4. State your conclusions about the relationship between
exposure to light and the activity of the genes that are
involved in the production of chlorophyll.
Hypothesis
Formulate a hypothesis to explain how light influences the
activity of the genes responsible for chlorophyll production.
Use this hypothesis as the basis of your experimental design.
Plan and Conduct
1 . With your group, brainstorm several methods you
could use to test your hypothesis given the materials
provided. As a group, select one method for your
experimental design.
2. Identify the independent, dependent, and controlled
Extend Further
5.
6.
INQUIRY Could a different hypothesis be consistent
with the results of your investigation? How could you
design an investigation to test this different hypothesis?
What social benefit could come from
understanding the effect of light on chlorophyll
production?
RESEARCH
variables, and the type of data you will collect.
3. As you prepare your procedure, be sure to consider the
time required for each step.
4. Prepare the data table you will use to record your
observations. Decide what form (such as the type of
graph) you will use to present your results.
5. Review your procedure with your teacher. Do not
begin doing the investigation until your teacher has
approved your group's procedure.
6. Record your observations in your table. Make notes
about any findings that do not fit in your data table.
Record any questions that come up as you conduct
your investigation.
Chapter 6 Complex Patterns of I n heritance · MHR
269
Thoughtlab
I N V E S T I G AT I O N
Skill Check
Initiating and Planning
./ Performing and Recording
./ Analyzing and Interpreting
-
-
-
./ Communicating
Materials
•
data on crosses
Sex-linked Crosses
Thomas Morgan used Drosophila melanogaster, the common fruit fly, extensively
in his studies of sex-linked traits. In this investigation, you will model Morgan's
experiments using Drosophila melanogaster and use your results to confirm
sex-linked inheritance for the trait you chose to study.
Pre-Lab Questions
1 . How is a sex-linked recessive trait distinguished from an autosomal
recessive trait?
2. Describe the genotype of the P generation that could be used to model
Morgan's studies of sex-linked genes in Drosophila.
3. What phenotype is expected in the F1 generation produced from the cross
described in question 2?
Question
How are sex-linked traits inherited in Drosophila melanogaster? How do actual
results compare with theoretical ratios?
Organize the Data
1 . Choose one trait from the table below (eye colour, eye shape, or body
Go to Organizing Data in a Table in
Appendix A for help with designing
colour) to investigate.
Common Sex-linked Traits in Drosophila melanogaster
Trait
o table for data.
Eye colour
I
Phenotype 1
White
I
Phenotype 2
Red
Eye shape
Round
Bar
Body colour
Black
Yellow
Two forms of eye colour in fruit flies are white and red (A}. Eye shape can be round (A} or appear as na rrow bars
270
MHR U n it 2 Genetic Processes
·
(B).
Part 1: The F1 Generation
Conclude and Communicate
2. Determine the genotype of the flies use for the P
4. Describe the inheritance pattern for the trait you
generation.
studied in this investigation.
3. Construct a table to record your results.
5. How does the actual phenotypic ratio you obtained
4. Use a computer simulation program or obtain results
compare to the theoretical phenotypic ratio? Account
for any differences.
for the F 1 generation from your teacher. Record the
results in your table.
5. Before beginning Part II, complete the Analysis section
of the investigation for Part I.
Extend Further
6.
Part II: The F2 Generation
6. Determine the genotype of the flies for the F 1 cross.
7. Construct a table to record your results.
8. Use a computer simulation program or obtain results
7.
for the F2 generation from your teacher. Record the
results in your table.
Analyze and Interpret
I NQUIRY In this investigation, you tracked the
inheritance pattern of one sex-linked trait. Design an
investigation that would track the inheritance of one
of these traits and the autosomal trait of normal versus
vestigial wings. Describe the results you expect.
RESEARCH Drosophila melanogaster has been used
extensively in genetics research. What other traits have
been studied in Drosophila? On which chromosomes
are the genes for these traits located?
Part I
1 . From the data you recorded on the appearance of the
flies in the F1 generation, which trait is dominant?
Explain your answer.
2. Given your response to question 1 , form a hypothesis
about the phenotypic ratio that you will observe in the
F2 generation.
Part II
3. Calculate an actual phenotypic ratio of the F2
generation from your results.
Two forms of body colour in fruit flies are black (A) and yellow
(8).
Chapter 6 Complex Patterns of I n heritance · MHR
271
Section 6.1
Beyond Mendel's Observations of Inheritance
Some patterns of inheritance are more complex
than those first proposed by Mendel. These include
codominant and incomplete dominant inheritance
KEY CONCEPTS
I ncom plete dominance leads to the expression of an
•
intermediate phenotype. I n the case of codomina nce,
both al leles are fu lly expressed.
patterns. In addition, for some traits multiple alleles
for a gene can exist within the population.
•
Although an individual has only two a l leles for any gene,
•
Environmental conditions can infl uence the expression of
m u ltiple alleles for a gene may exist with i n the population.
KEY TERMS
codom ina nce
i ncom plete dominance
continuous variation
polygenic trait
heterozygous advantage
certa i n traits.
•
Polygenic traits are controlled by more than one gene
a nd can usually be identified by conti n uous variation in
phenotype.
Section 6.2
Inheritance of Linked Genes
Some traits are inherited together, due to linked genes.
Gene linkage includes sex-linked genes, which are on
the sex chromosomes.
KEY CONCEPTS
Alleles of d ifferent genes that are on the same chromosome
do not assort independently. These genes are said to be
•
linked and their associated traits tend to be i n herited
KEY TERMS
linked genes
together.
sex- l i n ked trait
•
Sex-linked traits are expressed in different ratios by male
and female offspring because they are determi ned by the
segregation of X and Y chromosomes.
•
Although sex-l i n ked genes are l i n ked to the X and Y
chromosomes, Pun nett squares can be used to predict
genotypes and phenotypes.
Section 6.3
The Future of Genetics Research
Current and future research in genetics involves
studying how phenotypes result from complex
interactions between genes and gene products.
KEY TERMS
bioinformatics
genetic profile
KEY CONCEPTS
The complete DNA sequence of the h u man genome was
•
determi ned as part of the Human Genome Project.
•
The field of bioi nformatics arose from the need to share
and mainta in the large q uantities of data col lected from
genomics
genomic research. It also provides tools for analyzing
genomic data.
•
There i s sti l l much to be learned from the data generated
from the Human Genome Project, particularly in identifying
genes that are associated with h u man health.
•
Current and future research i n genomics may allow
scientists to tailor preventative and cu rative treatments for
individual patients based on their specific genetic profiles.
However, ethical questions a bout who owns an i ndividual's
genetic information continue to be debated.
272
MHR · U n it 2 Genetic Processes
5. The following pedigree follows the inheritance pattern
Knowledge and U nderstanding
Select the letter of the best answer below.
1 . The seed colour of a particular species of plant
of sickle cell anemia in a family. What is the sex,
genotype, and phenotype of individual II-5?
is inherited through incomplete dominance. If a
true-breeding plant with blue seeds is crossed with
a true-breeding plant with yellow seeds, what is the
expected seed colour of the offspring?
II
a. yellow
b. green
c.
blue
Ill
d. yellow and blue spots
2
e. You cannot predict seed colour from the
2. Roan cows are the result of a codominant inheritance
pattern. In roan cows, the allele for white hair and the
allele for red hair are both expressed. Which of the
following is the most appropriate representation for
codominant alleles?
=
allele for white hair, and let R
=
allele for
=
allele for white hair, and let r = allele for
red hair.
Let w = allele for white hair, and let R = allele for
red hair.
d. Let cw = allele for white hair, and let cR = allele for
red hair.
e. Let C" = allele for white hair, and let cR = allele for
c.
red hair.
3. A man with blood type 0 and a woman with blood
type AB have a child. Which of the following are
possible blood type(s) for the child?
a. 0 only
b. AB only
c.
A or B
d. A, B, or O
4.
b. affected female, HbAHb5
c.
unaffected male, Hb5Hb5
Hb5Hb5
unaffected male, HbAHbA
d. affected male,
e.
6. An X-linked dominant allele is inherited from a
heterozygous female by
a. all of her sons
red hair.
b. Let W
5
4
a. unaffected female, HbAHb5
information given.
a. Let W
3
e. A, B, AB, or 0
b. half of her sons
c.
all of her daughters
d. none of her daughters
e. all of her children
7. Which of the following most accurately describes the
field of genomics?
a. the study of haplotypes
b. the study of how DNA is copied
c.
the study of how genes interact to produce a
phenotype
d. the study of how genomes are formed
e. the study of the inheritance pattern of genes
8. How has DNA microarray technology revolutionized
the study of gene activity?
a. Gene expression in cells can now be studied.
b. The proteins produced by genes have been
Skin colour in humans ranges from very dark to very
light. Which of the following most likely describes how
skin colour is inherited?
d. The human genome has been completely sequenced.
a. principle of dominance
e. All of the proteins produced in a cell can now be
b. incomplete dominance
c.
discovered.
c.
Many genes can be studied at the same time.
studied.
codominance
d. polygenic inheritance
e. environmental influence
Chapter 6 Complex Patterns of I n heritance ·
MHR
273
Answer the q uestions below.
A plant that produces white flowers is crossed with a
9.
1 9.
•
plant that produces red flowers. Describe the pattern
of inheritance if the flowers produced are
c.
all red
1 0. What is the predicted phenotypic ratio in the F2
What is heterozygous advantage? Provide an example.
1 2.
Describe how multiple alleles influence inheritance of
a trait. Provide an example.
1 3. Height is an example of a polygenic trait. What aspect
of height suggests this?
20. A man with type B blood and a woman with type
AB blood have children. What blood types are possible
among their children? What would tell you that the
man is heterozygous for type B blood?
21 .
A woman with type AB blood has a child with the
same blood type. What are the possible genotypes of
the father?
22. What could be a genetic reason for the black area of fur
forming after a cold pack has been placed on the back
of this Himalayan rabbit?
What are linked genes? Explain why their inheritance
is not according to the law of independent assortment.
1 4.
CPO' produces platinum-coloured fur
0'0' produces silver foxes.
Could a fox breeder establish a true-breeding variety
of platinum foxes? Explain.
•
generation if two alleles are inherited by incomplete
dominance?
11.
cPcP is lethal, usually during an embryonic stage
•
a. pink
b. red and white spotted
In foxes, a pair of alleles, cP and 0', interact as follows:
1 5. Parents who do not have symptoms of Duchenne
muscular dystrophy have a son with Duchenne
muscular dystrophy. Which parent has passed the
disease to their son? Explain your answer.
1 6. What is a person's genetic profile? What are some
ethical issues concerning access to this information?
Th inking and I nvestigation
A man with straight hair has two children with a
1 7.
woman who has curly hair. One child has straight hair,
and one has wavy hair. What pattern of inheritance for
hair type does this suggest?
1 8. Use the pedigree below to answer the following
questions. The letters in the symbols indicate the blood
type of each individual.
a . Determine the blood types of individuals I -4
and I-6.
b. Individual III-2 and a man with blood type AB have
four children. Will any of these children have blood
type 0? Explain.
23.
Explain why genes that are far apart on a single
chromosome may be inherited as though they are on
different chromosomes.
24. A horse breeder finds that one of his stallions has a
genetic defect that affects the production of sperm.
The gene associated with this trait is located on the Y
chromosome. What is the possibility that the stallion's
female offspring could pass on this trait to their sons?
Explain.
25. Fruit flies can have normal wings or stunted wings.
In an investigation, you mate several normal-winged
females with a male that has stunted wings. In the F 1
generations, only the males have stunted wings. What
can you conclude from this investigation?
26. Suppose that the first dihybrid crosses Mendel
performed had involved traits controlled by closely
linked genes.
a. How would Mendel's results have differed from the
results he obtained for a dihybrid cross involving
non-linked genes?
b. What hypothesis might Mendel have developed to
II
explain his results?
c.
Ill
274
2
3
M H R · U nit 2 Genetic Processes
4
5
What investigation could Mendel have conducted to
test this hypothesis? What would he have observed?
Com m u n i cation
27.
Rudy and Maria are expecting a baby. They have
normal vision, but both of their fathers are colour
vision deficient (CVD). Their mothers have normal
vision.
a.
Application
35. A farmer wants to breed a variety of taller corn.
a.
b. Will the farmer's work be most effective if height in
Draw a pedigree for their family.
corn plants is determined by polygenic inheritance,
multiple alleles, or codominant alleles? Explain.
b. What is the probability that the baby will be a girl
with CVD?
c.
28.
29.
30.
c.
What is the probability that the baby will be a boy
with normal vision?
The closer genes are together on a chromosome, the
more likely they will assort together. Illustrate this
concept with a model or diagram.
BI G
Variability and diversity of living organisms
e result from the distribution of genetic
materials during the process of meiosis. Mendel
proposed the idea that all genes assort independently,
producing offspring with a variety of traits whose
distribution can be predicted mathematically. William
Bateson and Reginald Punnett found that not all genes
do assort independently. Develop a diagram that shows
independent assortment and how linked genes
contradict this theory.
BI G
Genetic and genomic research can have
e social and environmental implications.
Identify a potential scientific outcome of genomics
research. Develop an illustration showing the possible
social implications of achieving that outcome.
32. Use a graphic organizer to summarize the uses of
bioinformatics in genetics research.
36.
take to create a breed of corn that is taller and more
disease-resistant than the current corn crop?
Figure 6.17 provides a summary of some important
discoveries in genetics research, including the Human
Genome Project.
a.
34. Summarize your learning in this chapter using
a graphic organizer. To help you, the Chapter 6
Summary lists the Key Terms and Key Concepts. Refer
to Using Graphic Organizers in Appendix A to help
you decide which graphic organizer to use.
Research one development or discovery that is
in the figure, including an aspect of the Human
Genome Project. Choose a subject that you have not
learned about in this unit.
b. As part of your research, find out about at least one
individual who is associated with the discovery or
invention.
c.
Summarize your findings and develop a
presentation that you could present to the
class or another general audience. Make sure
your presentation includes a discussion on
the importance of the discovery in terms of its
contribution to scientific research.
37. Genome Canada was established in 2000 to develop a
national program for financial support of genomic and
proteomic research in Canada.
a.
33. There are many benefits to genetics research, but there
are also significant ethical concerns. Use a concept map
to illustrate some of the benefits and concerns that are
associated with the different genetics research topics
discussed in this chapter.
The farmer finds that many of the tallest corn plants
are also very susceptible to a particular disease.
How could the farmer design an investigation to
find out if the genes for height are linked to the
genes that cause susceptibility to the disease?
d. If these genes are linked, what steps could the farmer
3 1 . In this chapter, DNA sequences in a genome are
compared to letters strung together in a book. Develop
another analogy for DNA, chromosomes, genes, and
nucleotides. Illustrate your analogy with a diagram or
model.
How can the farmer use variation in the height of
the current corn plants to produce taller corn plants?
Choose a research project that is funded by Genome
Canada and that is listed on the Genome Canada
website.
b. Write a brief description that summarizes what
the project is studying. Include the names of the
individuals associated with the project and at what
institution(s) they work.
c.
Research Genome Canada's GE3LS program. What
does this acronym stand for and what are the main
objectives of this program? Develop an argument for
or against the importance of having such a program.
Chapter 6 Complex Patterns of I n heritance ·
MHR
275
Select the letter of the best answer below.
1 . 1m!J Incomplete dominance occurs when
a. one allele masks the expression of the other allele
b. one trait is masked by the presence of another trait
c.
both alleles are expressed when the alleles occur
together
Use thefollowing information to answer questions 6 and 7.
The gene that controls coat colour in rabbits has four alleles:
agouti (C), chinchilla (cch), Himalayan (c'' ) , and albino (c) .
The order of dominance is C > c"h > c'• > c.
6. 1m!J What is the phenotype of a rabbit with the
genotype c"hc?
d. an intermediate phenotype is expressed when the
a. agouti
alleles occur together
b. chinchilla
e. an unpredictable phenotype is expressed when the
c.
alleles occur together
d. Himalayan
2. 1m!J Which of the following is an example of
codominance?
e. albino
7.
a. A plant with green seeds is crossed with a plant with
a.
b. Individuals who are heterozygous for sickle cell
b. 25 percent
A red snapdragon crossed with a white snapdragon
produces pink snapdragons.
d. 75 percent
e. A litter of kittens often display a wide variety of
c.
3 . .. A man who is homozygous for blood type A
and a woman who is homozygous for blood type B
have a child. Which of the following could be the
child's genotype?
a. JA i
b. JAJA
8
C. J i
8.
JAJB
1m!J Which two terms are most relevant to the
inheritance of human blood types?
a. incomplete dominance and codominance
b. codominance and multiple alleles
incomplete dominance and multiple alleles
d. codominance and polygenic inheritance
e. dominance and codominance
5. 1m!J Traits that exhibit continuous variation are
usually
a. controlled by one gene
b. the result of codominance
c.
dominant
d. polygenic
e. affected by the environment
276
1m!J How can linked genes become "unlinked"?
a. During meiosis, they sort independently.
b. During crossing over, they are separated.
c.
During anaphase, they segregate to opposite poles
in the cell.
d. During mutation, the genes are separated.
e. During DNA replication, the genes are rearranged.
9.
.. Hemophilia is an X-linked recessive disorder.
If a female with hemophilia and a male without
hemophilia had children, what is the predicted
percentage of children who would have hemophilia?
d. JBJB
c.
50 percent
e. 100 percent
traits.
4.
0 percent
disease produce both normal and sickle-shaped
red blood cells.
d. There are many genes that control eye colour.
e.
.. If a rabbit with the phenotype cchch is crossed
with an albino rabbit, what is the probability of
producing a Himalayan rabbit?
white seeds; the offspring produce white seeds.
c.
chinchilla and albino mix
MHR U n it 2 Genetic Processes
·
a.
0 percent
b. 25 percent
c.
50 percent
d. 75 percent
e. 1 00 percent
1 0. 1m!J Which of the following statements about the
Human Genome Project is false?
a. It involved sequencing the human genome.
b. It identified coding and non-coding sections of
DNA.
c.
It involved sequencing the genome of common
representative organisms.
d. It identified genes in the human genome.
e. It determined the functions of the genes in the
human genome.
Use sentences and diagrams as appropriate to answer the
1 7.
questions below.
1 1.
.:lllt In radishes, colour is controlled by two alleles,
one for red colour and one for white colour.
Inheritance of these alleles shows incomplete
dominance. The photographs below show the
phenotype for each possible colour: red, purple, and
white. What phenotypic ratio would you expect from
crossing two heterozygous radish plants?
.:lllt The pedigree below illustrates the sex-linked
inheritance pattern of a trait in a family.
II
Ill
2
3
4
5
6
a. Explain how this pedigree shows sex-linked
inheritance. What type of sex-linked inheritance is
it? Explain.
b. From the pattern of inheritance you determined in
part (a), determine the genotype of ii-2.
c.
1 2.
.:lllt A student crosses a true-breeding plant that
produces green seeds with a true-breeding plant that
produces yellow seeds. Predict the possible offspring
when
1 8.
b. the allele for green seeds is codominant with the
allele for yellow seeds
c.
1 3.
.
.. Blood type ABO is determined by three alleles.
X-linked trait.
21.
.. A female fruit fly that is homozygous dominant
for red eyes is crossed with a white-eyed male fruit fly.
Use a Punnett square to predict the genotype(s) and
phenotype(s) of their offspring.
Self-Check
If you missed
question...
Review
section(s) ...
1 I 2
3
4
5
6
7
8
9
10 1 1
.. "Decoding the human genome can be compared
to reading a book in a language that nobody knows or
understands:' Explain this statement using diagrams or
a graphic organizer.
.. Investigating environmental effects on gene
.. Draw a diagram that illustrates the concept of
c:m:J Why did the Human Genome Project include
the sequencing of other organisms?
22 .
linked alleles of genes. In your diagram, show how they
can become unlinked.
1 6.
c:m:J Explain why males cannot be carriers of an
colours of female calico cats.
expression is an important aspect of genetics research
on plant crops. Explain why, using an example of a trait
to illustrate your answer.
1 5.
1 9.
20. c:m:J Explain how Barr bodies account for the patchy
the alleles for green and yellow seeds are
incompletely dominant
Draw a diagram that shows how blood type is
determined by a combination of the three alleles.
14
.. Duchenne muscular dystrophy affects many
more males than females. Explain why and draw a
pedigree to illustrate its inheritance pattern.
a. the allele for green seeds is dominant to the allele
for yellow seeds
Based on your answer to part (a), determine the
probability that individuals II - 1 and II-2 would have
an affected child.
23 .
.. What is genomics research? How can it be used
to improve human health?
24 .
25 .
.. What is bioinformatics? Describe a scientific
study that uses bioinformatics.
.. The human genome has long stretches of DNA
that do not code for proteins. Describe how the
variation between individuals in these regions can
be useful to study.
I
12 13 14 15 l 16 17 18 19 20 21 22 23 24 25
1
i
6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.2 6.2 6.3 6.1 6.1 6.1 6.1 6.2 6.2 6.2 6.2 6.2 6.2 6.3 6.3 6.3 6.3 6.3
Chapter 6 Complex Patterns of I n h eritance · M H R
277
Analyzing the Risks and Benefits of GMOs
For many years farmers have used reproductive technologies such as selective breeding to
produce the strongest and most profitable plants and animals possible. Now genetic engineering
allows scientists to manipulate genomes. All of the organisms shown below have been genetically
modified to become transgenic organisms. Recall that a genetically modified organism, or GMO,
is one whose genome has been altered. A transgenic organism is produced when this alteration
involves the insertion of a gene from another organism. The merits of producing such organisms
are continually debated. The long-term effects that GMOs have on humans who are exposed to
them, on the modified organisms themselves, and on the environment are not yet known.
Assume the role of a science writer who contributes to an on-line magazine that is a
forum for discussions on the impact of GMOs on society. Choose a GMO and research its
application(s). Analyze the benefits and risks associated with the use of this organism and develop
recommendations regarding its application(s).
What are the issues related to the use of genetically modified organisms, and do the benefits
outweigh the possible risks?
Initiate and Plan
1.
Select one GMO and its application(s). Some options
for you to consider are
•
•
organisms genetically modified to be more resistant
to disease
•
•
organisms genetically modified to be more resistant
to very cold temperatures
organisms genetically modified to allow for
easier harvesting
•
organisms genetically modified to improve medical
treatment for humans (for example, plants or animals
used in pharmaceutical production)
organisms genetically modified to provide alternative
and/or higher yield food products for human
consumption
organisms genetically modified to improve
environmental conditions
Most canola plants g rown in western
This EnviropigTM has had a bacterial gene
These GloFish® have had a gene from sea
Canada have had a bacterial gene
inserted into its genome that enables it
anem ones i n serted i nto their genomes
i nserted into their genomes that makes
to break down a phosphorus-containing
that ma kes them g l ow in the dark.
them resistant to herbicides.
compound in feed.
Perform and Record
2.
Research your chosen GMO. Focus your initial research
on the scientific technology associated with the
development of the organism, and on the regulatory
processes that must be followed.
3. Consider the following questions to guide your
research:
•
•
•
4.
5.
What type of genetic modification have scientists
made to the organism? Does this modification
involve inserting a gene from another organism, to
produce a transgenic organism?
What government regulatory bodies are involved in
reviewing the research, development, and use of the
organism (for example, local municipal government,
provincial government, Health Canada, Environment
Canada, U.S. Food and Drug Administration)?
At what stage of research and development is
the genetic engineering of the organism and its
application(s)? Is research still at a preliminary
stage? Has research and development received some
kind of government approval? Has the organism
or its products received government approval for
commercial use?
Research the economic, political, societal, ethical, and
environmental issues related to the application(s) of the
GMO that you have selected.
Consider the following questions to guide your research:
•
Who stands to benefit the most from the application(s)?
•
What is the most significant benefit?
•
•
•
What is the most significant risk to the environment
(if any), and to society as a whole?
What, if any, long-term benefits and risks have citizens
or scientists identified regarding the use of this GMO?
What are the sources of the information you have
gathered? How trustworthy and credible are your
sources? How do you know?
Analyze and Interpret
1.
Prepare a risk-benefit analysis that outlines the risks
and benefits associated with the development of the
GMO and its application(s). Refer to Analyzing STSE
Issues in Appendix A for help with how to do
this analysis.
2.
Make recommendations about whether development
and use of the GMO should continue as is, should
stop, or requires stricter regulations. Support your
recommendations using specific examples from your
risk-benefit analysis.
Com m u nicate You r Findings
3. Choose a form of communication to convey your
recommendations that is appropriate for an on-line
magazine (such as a web page, blog, podcast, or
Internet video) .
Assessment Criteria
Once you complete your project, ask yourself these
q uestions. Did you . . .
0 CfJ!I select a n appropriate GMO?
0 CfJ!I describe the scientific and technical principles
related to the technology and the regulatory
processes that m ust be followed?
0 .. identify the economic, political, societal,
ethical, and environmental issues related to
the technology?
0 .. make recommendations based on specific
exa mples from the risk-benefit ana lysis of whether
use of the GMO should continue as is, be stopped, or
be under stricter regulations?
0 .. organ ize your research using an appropriate
format and a ppropriate academic documentation?
0 .. select a format for you r recommendations
that is appropriate for the audience and pu rpose?
0 .. use scientific vocabulary appropriately?
BIG
•
•
Overall Expectations
Genetic and genomic research can have social and
environmental implications.
Variability and diversity of living organisms result from
the distribution of genetic materials during the process
of meiosis.
Chapter 4
In this unit you learned how to . . .
evaluate the importance of some recent contributions to
our knowledge of genetic processes, and analyze social
and ethical implications of genetic and genomic research
•
investigate
•
demonstrate
an understanding of concepts, processes,
and technologies related to the transmission of hereditary
characteristics
Cel l Division and Reproduction
KEY IDEAS
Chromosomes i n h u m a n somatic cells are organ ized into
23 pairs. One pair is the sex ch romosomes, and the other
22 pairs are the autosomes.
•
Errors d u ring meiosis ca n result
in changes to the structure and
•
n u m ber of chromosomes.
•
Modern technologies al low
scienti sts to manipu late the
Meiosis produces haploid g am etes from d i ploid parent
•
genetic processes, including those that occur
during meiosis, and analyze data to solve basic genetic
problems involving monohybrid and dihybrid crosses
•
cells. It leads to genetic variation i n ga metes through the
genetic make-up of organisms.
i ndependent assortment of chromosomes and crossing
This has led to many benefits.
over of genetic material.
Chapter 5
Patterns of I nheritance
KEY IDEAS
Mendel's monohybrid and d i hybrid cross experiments
demonstrated the existence of dominant and recessive
forms of traits.
•
The combination of al leles in an individual is its genotype.
•
•
Ped i g rees provide information
about the in heritance of genotypes
•
and phenotypes of i ndivid uals
across generations within a fa m i ly.
•
Ka ryotyping, fluorescence
The expression of the genotype in an individual is the
in situ hybridization (FISH),
phenotype. A dominant phenotype is expressed when
and gene testing are used to
a dominant allele is present. A recessive phenotype
monitor chromosome structu re,
req u i res two copies of the recessive al lele.
chromosome n u m ber, and
Pun nett squares are used to study the genotypes and
disease-ca using genes.
phenotypes of offspring.
Chapter 6
Complex Patterns of Inheritance
KEY I DEAS
Some patterns of i n heritance are more complex than those
fi rst proposed by Mendel. These i nclude codominant and
•
sequence of the h u m a n genome.
Many new research fields and
incomplete dominant i n heritance patterns. I n addition, for
•
some traits m ultiple a l leles of a gene exist in the population.
methods have developed from
Linked genes occur on the same chromosome and tend to
this project.
be i n herited together. However, crossing over can u n l i n k
these genes.
•
The H u man Genome Project
determined the complete DNA
•
Sex-linked traits are expressed i n different ratios by male
and female offspring because they are governed by the
segregation of X and Y chromosomes.
280
MHR Unit 2 Genetic Processes
•
•
C u rrent and future research i n
genomics may a llow scientists
to tailor medical treatments
for individual patients based on their genetic profiles.
However, ethical q uestions about who owns genetic
information continue to be debated.
Knowledge and Understanding
5.
Select the letter of the best answer below.
1.
Which of the following statements best describes an
individual whose genetic make-up is shown below?
••
Which phase of meiosis is shown in the illustration
below?
2
4
3
5
..
I
9
8
II ir
a. prophase I
b. prophase II
c. metaphase I
d.
metaphase II
19
20
;
.at
••
16
15
17
12
I
18
t
21
22
X
y
Which of the following statements best describes the
difference between a daughter cell produced by mitosis
and one produced by meiosis?
b. The individual is a female with the correct number
a. A cell produced by mitosis is genetically identical to
a cell produced by meiosis.
d.
b. A cell produced by mitosis has half the DNA
content of a cell produced by meiosis.
of chromosomes.
c. The individual is a male with trisomy.
6. Blue flowers (B) is dominant to white flowers (b).
A true-breeding plant with blue flowers is crossed with
a true-breeding plant with white flowers. Which of the
following statements represents a result of this cross?
content of a cell produced by mitosis.
d.
A cell produced by mitosis is genetically altered due
to crossing over, but a cell produced by meiosis is
not.
a. The offspring all have the genotype Bb.
b. The offspring are all homozygous recessive for blue
flowers.
e. A cell produced by mitosis can produce an egg or
c. The offspring are all homozygous recessive for white
sperm cell, but a cell produced by meiosis cannot.
flowers.
3. Which of the following processes contributes to genetic
variation?
d.
c. crossing over
d.
interphase
e. cytokinesis
A cross is performed between two pea plants, one with
the genotype Tt, and the other with the genotype tt. If
250 offspring are produced, approximately how many
have the genotype Tt?
a. O
b. 63
The offspring all have the phenotype bb.
e. The offspring are all homozygous dominant for blue
a. cloning
b. mitosis
The individual is a female with trisomy.
e. The individual is a female with monosomy.
c. A cell produced by meiosis has half the DNA
4.
14
11
a. The individual is a male with the correct number
of chromosomes.
e. interphase
2.
13
10
)
flowers.
7.
What is the predicted phenotypic ratio of the offspring
from a dihybrid cross between two individuals that
are heterozygous for both traits? Assume that the two
genes involved are not linked.
a.
3:1
b. 9:3: 3: 1
c. 1 :2 :2 : 1
d. 1:1:1:1
e. 1 :3
c. 1 2 5
d. 1 80
e. 250
U n it 2 Review · M HR
281
What is a key indicator of autosomal dominant
inheritance?
8.
a.
1 7.
Mendel performed his ground-breaking genetic
experiments using pea plants. List three characteristics
of pea plants that helped Mendel obtain such conclusive
results, and thus allowed him to develop his theory
of inheritance.
1 8.
Describe what the terms dominant and recessive mean.
How are they used to describe the forms of a trait at
the genotype level and at the phenotype level?
1 9.
What are monohybrid and dihybrid crosses? How can
Punnett squares be used to represent these crosses?
20.
What is meant by the phrase autosomal recessive
inheritance? In your explanation, use an example of
a genetic disorder that is inherited in this manner.
21.
Describe the chromosome theory of inheritance and
the contribution that Walter Sutton's research made to
the development of this theory.
22.
Describe three types of genetic tests that are done and
the information that each provides.
23.
Why is sickle cell anemia an example of codominant
inheritance?
24.
Explain how a single gene may have multiple alleles.
Include an example of a trait affected by multiple alleles
in your explanation, and describe how multiple alleles
affect phenotypes.
25.
Colour vision deficiency (CVD) is a sex-linked trait.
Explain why males cannot be carriers for CVD.
26.
Describe the role that bioinformatics played in the
Human Genome Project.
The trait is passed from father to son.
b. The trait is passed from father through an
unaffected daughter to her sons.
c. The trait skips generations.
d.
Two unaffected parents have an affected child.
e. Two affected parents have an unaffected child.
Incomplete dominance is expected when
9.
a.
one allele prevents the expression of the other allele
b. the expression of one allele is masked by the
presence of another allele
c. an intermediate phenotype is expressed when the
alleles occur together
d.
both phenotypes are expressed when the alleles
occur together
e. the phenotypes are expressed randomly when the
alleles occur together
1 0.
A man with blood type AB married a woman with
blood type B who carries an allele for blood type 0.
What are the possible blood types of their children?
a. 0
b. A and B
c. A and AB
d.
B and AB
e. A, B, and AB
Answer the questions below.
11.
What happens during each phase of interphase?
1 2.
What is a karyotype and what is it used for?
27.
1 3.
What are the important features that make
chromosomes homologous pairs? Why are
homologous chromosomes not identical?
Describe the similarities and differences between
mitosis and meiosis.
28.
What is the difference between a gene and an allele?
1 4.
What are haploid and diploid cells? Where is each cell
type found?
1 5.
What are the two essential outcomes of meiosis?
Identify the phases of meiosis where these outcomes
are achieved.
1 6.
The diploid cells of a fruit fly (Drosophila melanogaster)
contain four chromosomes.
a.
How many pairs of chromosomes does a diploid cell
of a fruit fly contain?
b. How many chromosomes does a haploid cell of a
fruit fly contain?
c. How many genetically distinct gametes can be
produced from a parent?
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M H R · U n it 2 Genetic Processes
Th in king and I nvestigation
29.
Errors can occur during meiosis that result
in alterations to the number and structure of
chromosomes.
a.
Describe the different types of errors.
b. What methods are used to detect and differentiate
between these errors?
30. How do artificial insemination and embryo transfer
differ in terms of controlling genetic variation?
31.
Compare and contrast oogenesis and spermatogenesis.
List their similarities and their differences.
32.
If black coat colour is dominant to white coat colour in
an animal, what is the
a. genotype of a homozygous black-coated animal?
37.
The pedigree below traces a genetic disorder in a
family.
b. genotype of a homozygous white-coated animal?
33.
c.
genotype of a heterozygous animal?
d.
genotypes of the gametes produced by each of the
animals in parts (a) to (c)?
II
3
2
The following data were obtained from an initial cross
between a true-breeding round-seeded pea plant and a
true-breeding wrinkled-seeded pea plant.
a. Based on the data, what are the dominant and
recessive forms of seed shape? Explain your answer.
a.
Do you think the disorder has an autosomal
dominant or autosomal recessive inheritance
pattern? Why?
b. Provide the genotypes and phenotypes for all
individuals in this pedigree. Explain your answer.
If there is a genotype you cannot be sure of, explain
why.
b. Do the data in the tables support the Mendelian ratio?
Explain your answer, and any differences observed.
Results for the F1 Generation
38.
In snapdragons, the alleles for flower colour display
incomplete dominance.
a.
A red-flowered plant is crossed with a
white-flowered plant. What are the predicted
genotypes and phenotypes of the offspring? Explain
your answer.
b. An offspring produced from the mating in part
34.
In humans, the allele for peaked hairline is dominant
to the allele for smooth hairline. Is it possible for two
adults with peaked hairlines to have a child with a
smooth hairline? Explain.
35.
Copy and complete the table below in your notebook,
given the information about pea plants in Table 5.1 and
the following:
G = green pod colour
T tall plant
Y
yellow seed colour
(a) is crossed with a white-coloured snapdragon.
What are the predicted phenotypes and genotypes
of the offspring? Include the phenotypic ratio of the
offspring.
39.
Baby 1 - blood type 0
Baby 2 - blood type B
Mr. Jones - blood type A
=
Mrs. Jones - blood type A
=
GomMe
from Male
Parent
TY
Gt
Yg
36.
I
GomMe
from Female
Parent
tY
gt
I
Genotype
of Offspring
I
Mr. Guttierez - blood type A
Phenotype
of Offspring
Mrs. Guttierez - blood type AB
40.
Determine the probability of a hemophiliac child
being born when neither the father nor the mother has
hemophilia, but the mother's father has hemophilia. Is
there any chance that their daughters will be affected?
Why or why not?
41 .
How do epigenetics and genetics differ? Provide two
examples of investigations that illustrate the differences
between these fields of study.
yg
In pea plants, the allele for purple flowers is dominant
to the allele for white flowers and the allele for tall
plants is dominant to the allele for short plants. Two
pea plants that are heterozygous for both traits are
crossed, producing 272 offspring.
a. Provide the genotype of each parent.
b. What are the genotypes of the gametes from each
parent?
c.
From the following blood types, determine which baby
belongs to which parents. Explain your answer.
What is the expected number of offspring that are
short plants with white flowers?
Comm u n i cation
42.
Draw an illustration that shows the relationship
between DNA, chromatin fibre, a chromosome, a gene,
an allele, and homologous chromosomes.
Unit 2 Review · M H R
283
43.
44.
45.
Summarize the process of meiosis in graphic
form, illustrating the movement and number of
chromosomes in each cell.
51.
BIG
Variability and diversity of living organisms
9 result from the distribution of genetic
materials during the process of meiosis. Crossing over
and independent assortment play an important role in
genetic recombination. Draw labelled diagrams to
show how they provide genetic variation.
BIG
Genetic and genomic research can have
9 social and environmental implications.
Through genetic modification, some crop plants can
be engineered to be more resistant to disease. Many
organizations and citizen groups oppose the use of
these crops. Choose a crop plant that has been
genetically modified to be more resistant to disease.
Research the risks and benefits associated with this
technology. Illustrate these benefits and risks in a
pamphlet, poster, or graphic organizer.
46.
Use a diagram to illustrate how transgenic organisms
are created.
47.
A Punnett square can be used to predict the possible
outcomes of a genetic cross. Explain graphically
how a Punnett square uses the laws of probability
by diagramming a cross between two pea plants
heterozygous for height (given that the allele for tall
plants is dominant to the allele for short plants) .
Predict the genotypic and phenotypic ratios for the
offspring based on the results of your Punnett square.
Since Mendel performed his experiments with pea
plants, scientists have discovered that there are more
complex patterns of inheritance. Use examples and
diagrams to illustrate the differences among the
following mechanisms:
•
•
•
•
dominance
incomplete dominance
codominance
sex-linked inheritance
52.
Draw a pedigree that could represent the inheritance
of hemophilia in a family. When drawing the pedigree,
ensure that you choose genotypes that will clearly
illustrate the pattern of inheritance for hemophilia.
Provide a brief rationale for why the pedigree shows
the correct inheritance pattern.
53.
In this unit, you have learned how different fields of
study are applied to provide a better understanding
of genetic processes and human disease. For example,
bioinformatics was essential for the success of the
Human Genome Project. Develop an illustration
using one or two examples of different fields of study
or technologies that have worked together to provide
a more complete understanding of a genetic process.
Appl ication
54.
Type 1 diabetes is managed effectively with synthetic
insulin produced by bacteria. Why do scientists
continue to research this disease in hopes of finding
other treatments or a cure?
48.
Using Punnett squares, illustrate how someone could
determine whether an organism with a dominant
phenotype is heterozygous for that trait.
55.
What is the risk of relying on artificial insemination or
embryo transfer to produce the offspring in a herd of
animals?
49.
Assume you write a monthly blog for an on-line
magazine that provides information to the general
public about various genetic disorders. You have been
asked to write about Huntington disease.
56.
Stem cell research has led to many ground-breaking
discoveries, as well as thought-provoking controversies.
a. Describe some of the controversy surrounding stem
cell research and how new research has managed to
reduce the controversy.
a. Provide a description of the genetic abnormality
b. Research a development in regenerative medicine
that causes Huntington disease and the inheritance
pattern of the disorder. Also include a brief
statement about the symptoms, diagnosis, and
treatment options that are available.
b. Write a brief paragraph describing your opinion
SO.
284
on whether genetic testing for Huntington disease
should be mandatory for family members when
there is a family history of the disorder. Include
valid points to support your argument.
Draw a diagram that illustrates gene linkage.
MHR
•
Unit 2
Genetic Processes
that has come from stem cell research in Canada.
Describe what the research involved, as well as its
potential benefit to society.
57.
Scientists believe that most human diseases involve
a complex array of interactions between genetic and
environmental factors. Why is it not possible to follow
a trait such as high blood pressure by performing
a monohybrid cross, as done by Mendel with pea
plants? Be sure to include both scientific and ethical
considerations in your answer.
58.
Many breeds of dogs are known for a high incidence of
genetic disorders. German shepherd and Saint Bernard
dogs, like the one shown below, are predisposed to
developing a crippling condition called hip dysplasia.
a.
61 .
Why are purebred dogs more at risk for such
conditions than mixed breeds?
Many organisms undergo a heat shock response when
they are placed at higher temperatures than they
normally live at. One part of this response involves
increased expression of certain genes, which helps the
organisms to cope with the higher temperature.
a.
b. What advice would you give to dog breeders who
want to maintain their dogs' purebred pedigrees,
but also want their dogs to be as healthy as possible?
b. Saccharomyces cerevisiea, shown below, is a type of
yeast that undergoes a heat shock response. This
organism has been extensively used in genetics
studies. Research the use of Saccharomyces cerevisiea
in genetics studies. Provide a summary of why it is
such a useful organism for this type of research.
62.
59.
Cystic fibrosis is a genetic disorder that leads to the
build-up of thickened mucus in the lungs and other
organs. Individuals affected by cystic fibrosis are more
susceptible to respiratory illnesses and must undergo
physical therapy regularly to manage the symptoms
of the disease.
a.
Describe the genetic basis of cystic fibrosis and its
pattern of inheritance.
b. How can a genetic counsellor help affected
Develop a plot for a movie or play that involves the use
of gene therapy. Ensure that the application is accurate
scientifically.
a.
Research how bioinformatics is playing an
important role in cancer research.
bioinformatics as part of its studies in cancer
research.
c. Summarize the information you gather and present
your findings to the class, using a format of your
choice.
63.
How can having your genetic profile determined
pose both potential risks and benefits? How has this
development of genetics research brought to light the
need for new social and political policies?
64.
Our knowledge in the areas of genetics and genomics
has grown incredibly since 2000. Choosing one specific
example, discuss how this research has increased our
understanding of human health and disease.
individuals and their families?
60.
Bioinformatics has applications in many fields of study.
b. Identify a research group in Canada that is using
c. One hope for a cure for cystic fibrosis is gene
therapy. Describe how gene therapy could be used
to cure cystic fibrosis, and the obstacles that must be
overcome for gene therapy to provide that cure.
Describe a technique that could be used to monitor
this response in an organism.
Unit 2
Review · MHR
285
Select the letter of the best answer below.
cm.J Below is a list of characteristics of chromosomes.
1.
6.
Which allele(s) will be present in the gametes of a
heterozygous individual?
a. T
d. Tt
Which combination of characteristics is the same for
each chromosome of a homologous pair?
I - same size
IV - same gene location
II - same genes
a.
I and II
b. I and III
b. t
7.
d.
cm.J What is the difference between a karyotype for
a normal male and a karyotype for a male with
trisomy 2 1 ?
a.
A normal male has 20 chromosomes; a male with
trisomy 2 1 has 2 1 chromosomes.
b. 25 percent
8.
cm.J In guinea pigs, black coat colour is dominant to
brown coat colour, and short hair is dominant to long
hair. How could you determine the genotype of a black
short-haired guinea pig?
a. Perform a cross between it and a brown guinea pig
with either hair length and examine the offspring.
Y chromosome; a male with trisomy 2 1 has two
Y chromosomes.
c. A normal male has one X chromosome and one
Y chromosome; a male with trisomy 2 1 has two
X chromosomes.
b. Perform a cross between it and a brown
short-haired guinea pig and examine the offspring.
A normal male has 46 chromosomes; a male with
trisomy 2 1 has 47 chromosomes.
c. Perform a cross between it and a brown long-haired
guinea pig and examine the offspring.
e. A normal male has 46 chromosomes; a male with
d.
trisomy 2 1 has 45 chromosomes.
3. cm.J Which of the following reproductive
technologies produces offspring that are the most
similar genetically?
a.
Perform a cross between it and a black short-haired
guinea pig and examine the offspring.
e. The genotype cannot be determined.
9.
preimplantation genetic diagnosis
b. in vitro fertilization
c. artificial insemination
d.
e. 100 percent
c. 50 percent
b. A normal male has one X chromosome and one
d.
.:m1 The allele for round seeds is dominant to the
allele for wrinkled seeds in pea plants. Two pea plants
that are heterozygous for seed shape are crossed. What
is the probability of producing a plant with wrinkled
seeds?
a. 0 percent
d. 75 percent
I, II, and IV
e. I, II, and V
c. I, II, and III
2.
e. TT or Tt or tt
c. T or t
V - same mutations
III - same alleles
cm.J A gene exists in two different forms, T and t.
embryo transfer
.:ml Attached earlobes and albinism, a lack of skin
pigment production, have autosomal recessive
inheritance patterns. Using the pedigree below,
determine the probability of individuals I - 1 and I-2
having a child with albinism and unattached earlobes.
e. embryo splitting
4.
a.
to produce genetically identical organisms
II
b. to produce multiple copies of genes for mass
2
production
a. 6.25
c. to produce multiple copies of genes for further
d.
b. 12.5 percent
to produce identical cells to treat disease
c. 25 percent
cm.J What term describes the form of a trait that
seemed to disappear in Mendel's crosses of
true-breeding pea plants?
a. dominant
d. heterozygous
b. recessive
e. sex-linked
c. homozygous
•
286
percent
research
e. to repopulate endangered species
5.
t
t
cm.J What is the goal of therapeutic cloning?
MHR U n it 2 Genetic Processes
1 0.
3
d. 50
Key
= albino
= attached
earlobes
percent
e. 100 percent
cm.J Which of the following will inherit an X-linked
allele from a heterozygous female?
a. only her sons
b. only her daughters
c. half of her sons
d.
all of her daughters
e. one-quarter of her daughters
1 9.
Use sentences and diagrams as appropriate to answer the
q uestions below.
11.
1 2.
lm!l Describe the two key outcomes of meiosis.
tllJ:. Errors that occur during meiosis are present in
20 .
.. Using labelled diagrams, illustrate how meiosis
contributes to genetic variation.
1 4.
.. Prenatal testing can be used to determine
21
genetic abnormalities. Describe a genetic disorder in
terms of the source of the disorder, the chromosome(s)
affected, the associated symptoms, and any prevention,
diagnosis, or treatment options.
1 5.
1 6.
.. Gene therapy holds much promise for curing
a number of diseases, including diabetes and cystic
fibrosis. There are also a number of ethical issues
related to gene therapy. Using a graphic organizer of
your choice, summarize the pros and cons associated
with gene therapy. Refer to Using Graphic Organizers
in Appendix A for help choosing a graphic organizer.
all cells of the body, whereas errors that occur during
mitosis may occur in only a small number of cells.
Explain why this occurs.
1 3.
.. Why is genetic testing usually not recommended
until after a genetic counsellor looks at a family
pedigree?
. .. Using an organism with black hair and another
organism with white hair as an example, illustrate the
difference between incomplete dominance and
codominance with a cross between these organisms.
22 .
.. A variety of reproductive technologies,
including cloning, artificial insemination, and in vitro
fertilization, are used to control the genetic diversity of
farm animals or plant crops. Choose one method and
describe how it is used in this manner.
.. Use your knowledge of blood types to match
the baby with the correct set of parents. Explain your
answer using Punnett squares.
Baby 1 - blood type A
Baby 2 - blood type 0
tllJ:. In tomatoes, round shape is dominant to pear
Mr. Rousseau - blood type AB
Mrs. Rousseau - blood type B
shape. A student crossed a plant that had round
tomatoes with a plant that had pear-shaped tomatoes
and obtained the data below. What are the genotypes of
the plants that were crossed? Explain your answer.
Mr. Sakic - blood type A
Mrs. Sakic - blood type B
23.
tllJ:. Duchenne muscular dystrophy is an example of
a sex-linked recessive trait found on the X chromosome.
Results of a Tomato Plant Cross
a. Write the genotypes of a female carrier, a normal
male, and an affected male.
b. Determine the probability of a female carrier and
1 7.
a normal male having an affected child.
tllJ:. A long-haired cat and a short-haired cat have a
c. Is it possible for the parents in (b) to have an
litter of kittens. The litter has both short- haired and
long-haired kittens. If the allele for short hair is
dominant to the allele for long hair, determine the
genotypes of the parents. Explain your answer.
1 8.
affected daughter? Explain why or why not.
24.
gain a better understanding of the genetic reasons for
different human diseases?
tllJ:. In mice, the allele for black fur is dominant to
25.
the allele for brown fur, and the allele for deafness is
recessive to the allele for normal hearing. If a mouse
that is heterozygous for both traits is crossed with a
homozygous black mouse carrying the deafness allele,
determine the probability of producing a deaf black
mouse.
Self-Check
If you missed
question...
Review
section(s) ...
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2
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4
5
6
7
8
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1m!1 How will the HapMap project help scientists
lm!l Describe the Human Genome Project and its
achievements. How has the completion of this project
been important for the advancement of genetics
research?
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Unit 2 Self-Assessment · M H R
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