Ch. 14 Mendel &
The Gene Idea
MS. WHIPPLE – BRETHREN CHRISTIAN HIGH SCHOOL
1. What was the “blending” theory of
heredity that was popular during the
1800s?




One possible explanation for heredity was a “blending”
hypothesis.
This hypothesis proposes that genetic material
contributed by each parent mixes in a manner
analogous to the way blue and yellow paints blend to
make green.
With blending inheritance, a freely mating population
would eventually give rise to a uniform population of
individuals.
Everyday observations and the results of breeding
experiments tell us that heritable traits do not blend to
become uniform.
Section 14.1
2. Who first pioneered our modern understanding of
inheritance? Give me a brief description of his life before
discovering the mechanism of inheritance? What contributed
to him becoming a scientist?

Modern genetics began in an abbey garden, where a monk named Gregor
Mendel documented a particulate mechanism of inheritance.

Mendel grew up on a small farm in what is today the Czech Republic.

In 1843, Mendel entered an Augustinian monastery.

Mendel studied at the University of Vienna from 1851 to 1853, where he was
influenced by a physicist who encouraged experimentation and the application of
mathematics to science and by a botanist who stimulated Mendel’s interest in the
causes of variation in plants.

These influences came together in Mendel’s experiments.

After university, Mendel taught school and lived in the local monastery, where the
monks had a long tradition of interest in the breeding of plants, including peas.

Around 1857, Mendel began breeding garden peas to study inheritance. Section 14.1
3. What is a Character? How is this
different than a Trait?
 Organisms
within a species come in
many varieties that have distinct
heritable features, or characters, with
different variant traits.
Section 14.1
4. What were the advantages of choosing
garden peas for Mendel’s research? How did
he control their reproduction?
 There
are many varieties with distinct heritable
features, or characters (such as flower color);
character variants (such as purple or white flowers)
are called traits
 Mating can be controlled
 Each flower has sperm-producing organs (stamens)
and an egg-producing organ (carpel)
 Cross-pollination (fertilization between different plants)
involves dusting one plant with pollen from another
Section 14.1
Figure 14.3-3
EXPERIMENT
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had purple flowers
Self- or cross-pollination
F2 Generation
705 purpleflowered
plants
224 white
flowered
plants
Section 14.1
5.What is meant by the term True-Breeding?
How do you bring about a true breeding pea
plant?
Mendel
started his experiments
with varieties that were truebreeding.
When true-breeding plants selfpollinate, all their offspring have
the same traits as their parents.
Section 14.1
6. What was Mendel’s typical breeding
experiment? In your explanation, define the P
Generation, F1 Generation, & F2 Generation.

In a typical experiment, Mendel mated two
contrasting, true-breeding varieties, a process
called hybridization

The true-breeding parents are the P generation

The hybrid offspring of the P generation are
called the F1 generation

When F1 individuals self-pollinate or crosspollinate with other F1 hybrids, the F2 generation
is produced
Section 14.1
Figure 14.3-3
EXPERIMENT
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had purple flowers
Self- or cross-pollination
F2 Generation
705 purpleflowered
plants
224 white
flowered
plants
Section 14.1
7. How did Mendel’s experiment bring him to the
discovery of Dominant & Recessive traits? Give me a
specific example in your explanation.
 Mendel
reasoned that only the purple flower
factor was affecting flower color in the F1
hybrids
 However, the factor for white flowers was not
diluted or destroyed because it reappeared in
the F2 generation
 Mendel called the purple flower color a
dominant trait and the white flower color a
recessive trait.
Section 14.1
8. Alternate versions of Genes are
called:
 Alleles!!!
 Alternative
versions of genes account
for variations in inherited characters
 For example, the gene for flower color
in pea plants exists in two versions, one
for purple flowers and the other for
white flowers
 Each gene resides at a specific locus on
a specific chromosome.
Section 14.1
Figure 14.4
Allele for purple flowers
Locus for flower-color gene
Pair of
homologous
chromosomes
Allele for white flowers
Section 14.1
9. What is the Law of Segregation?
Mendel’s law of segregation states that the two
alleles for a heritable character segregate
(separate) during gamete production and end up
in different gametes.

This segregation of alleles corresponds to the distribution of
homologous chromosomes to different gametes in meiosis.

If an organism has two identical alleles for a particular character,
then that allele is present as a single copy in all gametes.

If different alleles are present, then 50% of the gametes will receive
one allele and 50% will receive the other.
Section 14.1
10. Review: During the process of Meiosis, how do the
homologous chromosomes (one from mom and one from
pop) reshuffle their alleles? When do they do this? What is the
end result of this while making gametes?
 Homologous
Chromosomes
reshuffle their alleles during
prophase I of Meiosis. This
reshuffling of alleles called
CROSSING OVER and then
independent assortment of
chromosomes results in nonidentical gametes by the
end of meiosis.
Section 14.1
11. What is a Punnett Square? What is
its purpose? Give me an example.

A Punnett square may
be used to predict the
results of a genetic
cross between
individuals of known
genotype by showing
all possible
combinations of
parental alleles.
Section 14.1
12. Define the following terms:
 Homozygous:
An organism with two
identical alleles for a character
 Heterozygous: An organism that has
two different alleles for a gene
 Phenotype: Physical Expression of a
Trait
 Genotype: Alleles present in an
individual for a specific character (i.e.
WW; Bb)
Section 14.1
13. How can two organisms have the same
Phenotype but not the same Genotype?
A
Homozygous Dominant Individual for
Brown eyes (BB) and Heterozygous
Individual (Bb) will both have the same
Phenotype – Brown Eyes.
Section 14.1
14. What is the purpose of a test
cross?

How can we tell the genotype of an individual with the
dominant phenotype?
Such an individual could be either homozygous
dominant or heterozygous
 The answer is to carry out a testcross: breeding the
mystery individual with a homozygous recessive
individual
 If any offspring display the recessive phenotype, the
mystery parent must be heterozygous

Section 14.1
15. What is a Monohybrid Cross? A
Dihybrid Cross?
 Monohybrid
Crosses look at a single trait and
what possible offspring can be produced from
two parental genotypes.
 Dihybrid
crosses involve two traits. Dihybrid crosses
are predictions of how two traits will show up in
the offspring produced by a mating between two
parent organisms.
Section 14.1
16. From Figure 14.8, how would the results differ in
the F2 generation if alleles assorted dependently or
independently? What was the result?
 If
the alleles assort dependently you would
predict the same outcome as a
monohybrid cross with a phenotypic ratio
of 3:1.
 If
the alleles assort independently you
would predict a phenotypic ratio of 9:3:3:1
 Mendel’s
experiments show that alleles
assort independently with a ratio of 9:3:3:1. Section 14.1
17. What is the Law of Independent Assortment? Does
this law pertain to genes found on separate
chromosomes or the same chromosomes (homologous
chromosomes)?
Using a dihybrid cross, Mendel developed the law of
independent assortment
 The law of independent assortment states that each pair
of alleles segregates independently of each other pair
of alleles during gamete formation
 Strictly speaking, this law applies only to genes on
different, nonhomologous chromosomes or those far
apart on the same chromosome
 Genes located near each other on the same
chromosome tend to be inherited together
Section 14.1cc

1. What is the Multiplication Rule? How
does it apply to Mendelian inheritance?
We can use the multiplication rule to determine the probability
that two or more independent events will occur together in
some specific combination.
 Compute the probability of each independent event and then
multiply the individual probabilities to obtain the overall
probability of these events occurring together.
 The probability that two coins tossed at the same time will both
land heads up is 1/2 × 1/2 = 1/4.
 Similarly, the probability that a heterozygous pea plant (Pp) will
self-fertilize to produce a white-flowered offspring (pp) is the
probability that a sperm with a white allele will fertilize an ovum
with a white allele. This probability is 1/2 × 1/2 = 1/4.

Section 14.2
2. What is the Addition Rule? How does it
apply to Mendelian inheritance?

We can use the addition rule to determine the probability that an F2
plant from a monohybrid cross will be heterozygous rather than
homozygous.

The probability of an event that can occur in two or more mutually
exclusive ways is the sum of the individual probabilities of those ways.

The probability of obtaining an F2 heterozygote by combining the
dominant allele from the egg and the recessive allele from the sperm
is 1⁄4.

The probability of combining the recessive allele from the egg and
the dominant allele from the sperm also 1⁄4.

Using the rule of addition, we can calculate the probability of an F2
Section 14.2
heterozygote as 1⁄4 + 1⁄4 = 1⁄2.
1. How is Complete Dominance different than
Incomplete Dominance? Give me an example
of each.

Complete dominance is exhibited when one
allele completely masks the expression of
another, this is characteristic of Mendel’s
crosses.

However, some alleles show Incomplete
dominance, in which heterozygotes show a
distinct intermediate phenotype not seen in
homozygotes.
Section 14.3
2. How is Incomplete Dominance
different than Co-Dominance? Give me
an example of each.
 In
incomplete dominance, the phenotype
of F1 hybrids is somewhere between the
phenotypes of the two parental varieties.
Example, Flower color (white, red, pink)
 In codominance, two dominant alleles
affect the phenotype in separate,
distinguishable ways. Example, blood type
(Type A, B, AB, & O)
Section 14.3
3. Briefly describe the relationship of
Dominance & Phenotype in the
expression of Tay-Sach’s Disease.


For any character, dominance/recessiveness relationships depend on the level at
which we examine the phenotype. For example, humans with Tay-Sachs disease
lack a functioning enzyme to metabolize certain lipids. These lipids accumulate in
the brain, harming brain cells and ultimately leading to death.

Children with two Tay-Sachs alleles (homozygotes) have the disease.

Both heterozygotes, with one allele coding for a functional enzyme, and homozygotes,
with two such alleles, are healthy and normal.

Thus at the organismal level, the allele for the functional enzyme is dominant to the
Tay-Sachs allele.
The activity level of the lipid-metabolizing enzyme is reduced in heterozygotes.


Thus, at the biochemical level, the alleles show incomplete dominance.
Heterozygous individuals produce equal numbers of normal and dysfunctional
enzyme molecules.

At the molecular level, the Tay-Sachs and functional alleles are codominant.
Section 14.3
4. What is Multiple Alleles? Give me
an example.
Most genes exist in populations in more than two
allelic forms
 For example, the four phenotypes of the ABO blood
group in humans are determined by three alleles for
the enzyme (I) that attaches A or B carbohydrates to
red blood cells: IA, IB, and i.
 The enzyme encoded by the IA allele adds the A
carbohydrate, whereas the enzyme encoded by the
IB allele adds the B carbohydrate; the enzyme
encoded by the i allele adds neither

Section 14.3
Figure 14.11
(a) The three alleles for the ABO blood groups and their
carbohydrates
IA
Allele
Carbohydrate
IB
i
none
B
A
(b) Blood group genotypes and phenotypes
Genotype
IAIA or IAi
IBIB or IBi
IAIB
ii
A
B
AB
O
Red blood cell
appearance
Phenotype
(blood group)
5. What is Pleiotropy? Give me an
example.
 Most
genes have multiple
phenotypic effects, a property
called pleiotropy
 For example, pleiotropic alleles are
responsible for the multiple
symptoms of certain hereditary
diseases, such as cystic fibrosis and
sickle-cell disease
Section 14.3
6. What is Epistasis? Give me an
example.

In epistasis, a gene at one locus alters the phenotypic
expression of a gene at a second locus
For example, in Labrador retrievers and many other
mammals, coat color depends on two genes
 One gene determines the pigment color (with alleles B
for black and b for brown)
 The other gene (with alleles C for color and c for no
color) determines whether the pigment will be
deposited in the hair

Section 14.3
Figure 14.12
BbEe
Eggs
1/
4 BE
1/
4 bE
1/
4 Be
1/
4
be
Sperm
1/ BE
4
1/
BbEe
4 bE
1/
4 Be
1/
4 be
BBEE
BbEE
BBEe
BbEe
BbEE
bbEE
BbEe
bbEe
BBEe
BbEe
BBee
Bbee
BbEe
bbEe
Bbee
bbee
9
: 3
: 4
7. Describe Polygenic Inheritance of Human
skin color. Use the term Quantitative
Characters in your description.
 Quantitative
characters are those characteristics
that vary in the population along a continuum.
 Quantitative
variation usually indicates polygenic
inheritance, an additive effect of two or more
genes on a single phenotype
 Skin
color in humans is an example of polygenic
inheritance
Section 14.3
7. Describe Polygenic Inheritance of Human
skin color. Use the term Quantitative
Characters in your description.
Let’s assume that skin color in humans is controlled by three independent genes.
 Imagine that each gene has two alleles, one light and one dark, which
demonstrate incomplete dominance.







An AABBCC individual is very dark; an aabbcc individual is very light.
A cross between two AaBbCc individuals (with intermediate skin shade) produces
offspring with a wide range of shades.
Individuals with intermediate skin shades are most common, but some very light
and very dark individuals may be produced as well.
The range of phenotypes forms a normal distribution if the number of offspring is
great enough.
The majority of offspring would have intermediate phenotypes (skin color in the
middle range).
Environmental factors, such as sun exposure, also affect skin color and contribute
to a smooth normal distribution.
Section 14.3
Figure 14.13
AaBbCc
AaBbCc
Sperm
1/
1/
8
8
1/
1/
Eggs
8
1/
1/
8
8
1/
8
1/
1/
8
8
8
8
1/
8
1/
8
1/
1/
8
1/
8
1/
8
1/
8
Phenotypes:
Number of
dark-skin alleles:
1/
64
0
6/
64
1
15/
64
2
20/
64
3
15/
64
4
6/
64
5
1/
64
6
8. What does the phrase “Nature vs. Nurture”
mean when talking about genes & their
expression?

Phenotype depends on both environment and genes,
this is meant by the term “Nature vs. Nurture”
A
single tree may have leaves that vary in size, shape, and
greenness, depending on exposure to wind and sun.
 For
humans, nutrition influences height, exercise alters build,
sun-tanning darkens skin, and experience improves
performance on intelligence tests.
 Even
identical twins, who are genetically identical, accumulate
phenotypic differences as a result of their unique experiences.
Section 14.3
9. What is the Norm of Reaction? What
does it mean if a character is
Multifactorial? Give me examples.
 The
norm of reaction is the phenotypic range of a
genotype influenced by the environment
 For
example, hydrangea flowers of the same genotype
range from blue-violet to pink, depending on soil acidity

Norms of reaction are generally broadest for
polygenic characters

Such characters are called multifactorial because
genetic and environmental factors collectively
influence phenotype
Section 14.3
Figure 14.14
1. What is a Pedigree? How are
these useful to society?





A pedigree is a family tree that describes the interrelationships
of parents and children across generations
Inheritance patterns of particular traits can be traced and
described using pedigrees
This is especially useful for studying human genetics as many
other methods for studying genetics are unethical to apply to
humans.
A pedigree can help us understand the past and predict the
future.
We can use normal Mendelian rules, including the
multiplication and addition rules of probability, to predict the
probabilities of specific phenotypes.
Section 14.4
Figure 14.15
Key
Male
1st
generation
Affected
male
Female
Affected
female
Mating
1st
generation
Ww
ww
Ww
ww
2nd
generation
Ww
ww
3rd
generation
WW
or
Ww
Widow’s
peak
ff
ff
(a) Is a widow’s peak a dominant or
recessive trait?
Ff
Ff
Ff
ff
ff
FF
or
Ff
3rd
generation
ww
No widow’s
peak
ff
Ff
2nd
generation
FF or Ff
Ww ww ww Ww
Ff
Offspring
Attached
earlobe
Free
earlobe
b) Is an attached earlobe a dominant
or recessive trait?
2. For recessively inherited disorders, only homozygous
recessive individuals exhibit the disorder. Why don’t
heterozygotes have the disorder since they have one
recessive allele?
 Recessively
inherited disorders show up only
in individuals homozygous for the allele
 Carriers are heterozygous individuals who
carry the recessive allele but are
phenotypically normal; most individuals with
recessive disorders are born to carrier parents
 Albinism is a recessive condition
characterized by a lack of pigmentation in
skin and hair
Section 14.4
3. What factors contribute to certain disorders are
found in higher prevalence among a certain group of
individuals (such as Tay-Sachs disease among
Ashkenazi Jews)
 Genetic
disorders are not evenly
distributed among all groups of humans.
 This
is due to the different genetic histories
of the world’s people during times when
populations were more geographically
and genetically isolated.
Section 14.4
4. A taboo on inbreeding (the mating of close relatives) is
found in almost every human society on earth. Why do you
think this taboo is necessary for a healthy population? What is
an example of inbreeding being detrimental to a species?

If a recessive allele that causes a disease is rare,
then the chance of two carriers meeting and
mating is low

Consanguineous matings (i.e., matings between
close relatives) increase the chance of mating
between two carriers of the same rare allele

Most societies and cultures have laws or taboos
against marriages between close relatives
Section 14.4
5. Please describe the evolution of Sickle Cell Anemia
and the theorized reason for its prevalence among
people of African descent.
Sickle-cell disease affects one out of 400
African-Americans
 The disease is caused by the substitution of a
single amino acid in the hemoglobin protein in
red blood cells
 In homozygous individuals, all hemoglobin is
abnormal (sickle-cell)
 Symptoms include physical weakness, pain,
organ damage, and even paralysis

Section 14.4
5. Please describe the evolution of Sickle Cell Anemia
and the theorized reason for its prevalence among
people of African descent.



Heterozygotes (said to have sickle-cell trait) are usually healthy
but may suffer some symptoms
About one out of ten African Americans has sickle cell trait, an
unusually high frequency of an allele with detrimental effects in
homozygotes
Individuals with one sickle-cell allele have increased resistance to
the malaria parasite, which spends part of its life cycle in red
blood cells.



In tropical Africa, where malaria is common, the sickle-cell allele is both a
boon and a bane.
Homozygous normal individuals die of malaria and homozygous recessive
individuals die of sickle-cell disease, while carriers are relatively free of both.
The relatively high frequency of sickle-cell trait in African-Americans is a
vestige of their African roots.
Section 14.4
6. Why do you think a lethal recessive allele
would be more reproductively successful than
a lethal dominant allele?
A
lethal recessive allele will most likely be
the most successful because it can “hide”
in heterozygote carriers. In this way, it can
be passed on without killing the individual
with the gene.
Section 14.4
7. What characteristic of Huntington ’s disease makes it
successful at being passed on even though it’s a dominant
trait? What technology is helping to eliminate/lessen the
effect of this factor?

The timing of onset of a disease significantly
affects its inheritance

Huntington’s disease is a degenerative disease
of the nervous system

The disease has no obvious phenotypic effects
until the individual is about 35 to 40 years of age

Once the deterioration of the nervous system
begins the condition is irreversible and fatal
Section 14.4
8. What is a Multifactorial disease? Why is
it so difficult to study the genetic basis of
these diseases?
 Many
diseases, such as heart disease,
diabetes, alcoholism, mental illnesses, and
cancer have both genetic and
environmental components
 Little
is understood about the genetic
contribution to most multifactorial diseases
Section 14.4
9. What is the Genetic Information
Nondiscrimination Act of 2008? Why was this
Act necessary?
Recently developed tests for many disorders can distinguish
normal phenotypes in heterozygotes from homozygous
dominants.
 These results allow individuals with a family history of a genetic
disorder to make informed decisions about having children.
 Issues of confidentiality, discrimination, and counseling may
arise.


The Genetic Information Nondiscrimination Act, signed into US
law in 2008, prohibits discrimination in employment or insurance
coverage based on genetic test results.
Section 14.4
10. Describe the following types of
fetal testing:


Amniocentesis

can be used from the 14th to 16th week of pregnancy to assess whether
the fetus has a specific disease.

Fetal cells extracted from amniotic fluid are cultured and karyotyped to
identify some disorders.

Other disorders can be identified from chemicals in the amniotic fluids.
Chorionic Villus Sampling

allows faster karyotyping and can be performed as early as the 8th to
10th week of pregnancy.

A sample of fetal tissue is extracted from the chorionic villi of the
placenta.
Section 14.4
10. Describe the following types of
fetal testing:
 Ultrasound
uses
sound waves to produce an image
of the fetus
 Fetoscopy
a
needle-thin tube containing fiber
optics and a viewing scope is inserted
into the uterus.
Section 14.4