GENETICS
Copyright © Amy Brown Science
GENETICS
….the science that
studies how genes are
transmitted from one
generation to the
next.
Copyright © Amy Brown Science
Genes and Chromosomes
Chromosomes are
contained in the
_______
nucleus of the cell.
Chromosomes are made of:
Gene:
A segment of DNA that controls a
hereditary trait.
Chromosome: A long chain of genes.
Trait:
The characteristics that an organism has, such as
hair color, eye color, height, or skin color.
Alleles:
Different versions of a gene that produce
dis;nguishable traits in offspring.
Two ______
alleles must be present in order for a trait to appear in the
________.
each parent to the
offspring One allele is provided by __________
offspring. When __________
fertilization of sperm and egg occurs, the new
offspring will have _______
2 alleles for each gene.
The Contributions of Gregor Mendel
Gregor Mendel was an Austrian monk
who was born in 1822.
He is known as the Father of Genetics.
He discovered three laws of genetics
that would forever change biology.
He conducted a series of
experiments in a quiet
monastery garden. Mendel
spent 14 years growing and
experimen;ng with the pea
plants grown in his garden.
St. Thomas Augustinian Abbey where Mendel lived and worked.
Mendel gave us the three basic laws of
inheritance which are still used today:
Ø The Law of Dominance and
Recessiveness
Ø The Principle of Segregation
Ø The Principle of
Independent Assortment
Mendel's great
contribution was to
demonstrate that
inherited characteristics
are carried by genes.
Mendel chose the garden
pea for his experiments.
It was a good choice
because:
1. They were readily
available.
2. They were easy to grow.
3. They grew rapidly.
The sexual structures of the
flower are completely enclosed
within the petals.
There would be no accidental
cross-pollination between plants.
Before we learn about Mendel’s experiments, let’s review
the basics of sexual reproduc:on in flowering plants.
Flowers contain both male and female reproductive structures.
Pistil
The female part of the flower, the
pistil produces ________.
egg cells
_____,
Stamen
The male part of the flower, the
stamen produces ______.
pollen
_______,
sperm cells
Pollen contains ___________.
Pollen
Stamen
Pistil
Sperm cells
When the pollen is delivered to the
pistil, the sperm travels to the egg
cell, and the result is __________.
fertilization
Egg
Fertilization produces …
… a tiny embryo, which is enclosed inside a
seed.
Mendel’s Use of Pea
Plants for Genetics
Experiments
Pea flowers are
normally
_____________.
self-pollinating
Since the male and
female reproductive
structures are relatively
enclosed inside the
flower, the sperm of the
flower will fertilize the
egg of the same flower.
The resulting embryos will
have the same ____________
characteris=cs
as the parent plant.
Even though sexual
reproduction has occurred,
there is just __________.
one parent
Mendel knew that these pea plants were
“____________.”
This means that if they
true breeding
are allowed to ___________,
self-pollinate they would
produce offspring ___________________.
identical to themselves
For example: If allowed to
self-pollinate, tall plants
would always produce
tall
plants
________.
Plants with yellow seeds
would always produce
yellow seeds
offspring with ___________.
These true breeding plants were the
cornerstone of Mendel’s experiments.
Mendel’s Work
Mendel wanted to produce _____
seeds by
joining the egg and sperm from
_________________.
two different plants
To do this, he had to first
prevent the possibility of
_____________.
self-pollination
Tall Pea Plant
Dwarf Pea Plant
Mendel cut away the stamens,
the male reproductive parts of
the flower, and then dusted the
remaining female structure with
pollen from a different plant.
This is known as
cross-pollination and produces
______________
different
offspring from two ________
parents.
Now Mendel could easily crossbreed plants and experiment with
different characteristics.
Mendel’s Experiments
Essen%al Vocabulary needed for Gene%cs
Can you roll your
tongue?
Round Seed
Wrinkled Seed
1. Trait: A specific characteristic that
varies from one individual to
another.
Mendel focused on seven
_____ traits
found in pea plants.
Each of these traits had only two
contrasting variants.
For example: Pea plants were
either tall or dwarf.
Seeds were either round or
wrinkled.
Seed color was either yellow or
green.
Essen=al Vocabulary: 2. Alleles:
Do you have dimples?
3.
4.
5.
6.
Different versions of a gene
that produce distinguishable
traits in offspring
P generation:
Parental generation
F1 generation:
First generation of offspring
F2 generation:
Second generation of
offspring
Hybrids:
The offspring of parents
with different traits
Mendel crossed true-breeding tall plants with
true-breeding dwarf plants.
Tall plants x dwarf plants à Produces only tall
offspring
Tall Pea Plant
Dwarf Pea Plant
1. The F1 hybrids were all tall.
2. All of the offspring had the
appearance of only one of the
parents.
3. The trait of the other parent
seemed to have disappeared.
Mendel thought that the dwarf
trait had been lost.
Mendel’s Conclusions:
Biological inheritance is determined by
“______
factors ” that are passed from one
generaIon to the next.
Today, we know these factors to be ______.
genes
Each of the traits that Mendel observed in
the pea plants was controlled by ________
one gene
two contrasting forms
that occurred in ___________________.
For example: The gene for the height of pea plants occurs in a
tall form and in a ______
dwarf form.
____
alleles
The different forms of a gene are called ______.
dominant
Mendel realized that some alleles are __________
over other alleles.
Principal of
Dominance and
Recessiveness:
Some alleles are dominant, and others are
recessive. A dominant allele can cover up or
mask a recessive allele.
Dominant allele:
If the dominant allele is present
in an offspring, the dominant
trait will show up in the
offspring.
Recessive allele:
This trait will show up in the
offspring only if the dominant
allele is not present.
Copyright © Amy Brown Science
In Mendel’s experiments, the allele for
tall pea plants was dominant over the
allele for dwarf pea plants.
The Principal of Segregation
Mendel had another question:
Had the dwarf trait
(recessive allele)
disappeared, or
was it still present
in the F1 offspring?
Mendel allowed the _________
hybrid tall offspring
from the first generation to ___________.
self-pollinate
F1 Tall x F1 Tall à Produces offspring that
are ¾ tall and ¼ dwarf
1. He found that ¾ of the
offspring were tall and ¼ of
the offspring were dwarf.
F1 Tall Pea Plant allowed
to self-pollinate
F1 Tall Pea Plant allowed
to self-pollinate
F2 Offspring
2.Evidently the F1 "tall" offspring must have been carrying the
dwarf trait, but it had been hidden.
3.The dwarf trait had been passed down to the offspring, and
it reappeared in the ____________.
F2 generation
Why did the recessive allele seem to disappear in the F1
generation and then reappear in the F2 generation?
Mendel realized that organisms have
two
alleles for every ____.
trait
_________
These two alleles are inherited, one
from each ______.
parent
If the offspring receives a dominant
allele from one parent, that dominant
trait will ______
appear in the offspring.
Recessive traits show up in the
offspring only if the offspring receives
… recessive alleles from both parents.
If a parent has two alleles for a trait, how does the
parent pass only one allele to the offspring?
Today, we know
that the answer to
this lies in the type
of cell division
known as _______,
meiosis
the formation of
________.
gametes
Gametes are sex cells or egg and sperm cells.
The capital letter, T, represents
a dominant allele.
The lower-case letter, t,
represents a recessive allele.
During meiosis, the DNA is
_________
replicated and then separated
into _________.
4 gametes
Each gamete has ____
half the number of chromosomes as
the parent cell. In this way, a parent passes one allele for
each gene to their offspring.
Mendel’s Principle of Segregation
Mendel’s Principle of
Segregation says that
every individual carries
two alleles for each
trait.
These two alleles
separate or segregate
during the formation of
the egg and sperm cells.
Homozygous or Heterozygous?
An offspring will inherit two alleles for a trait, one allele from
each parent. The combination of alleles received by the
offspring may be either homozygous or heterozygous.
Homozygous means that …
… the two alleles are the
same: TT or tt
T
T
Homozygous
t
t
Homozygous
T
t
Heterozygous
Heterozygous means that …
… the two alleles are
different: Tt
Genotypes and Phenotypes
A phenotype is the …
… physical characteristics
of an organism or what
the organism looks like.
Genotypes
A genotype is the …
… genetic makeup of
an organism.
Phenotype: One flower is
purple, and one flower is
red.
For example, in Mendel’s
pea plants, the tall allele
was dominant over the
dwarf allele:
Genotype
TT
Tt
tt
Phenotype
Tall plant
Tall plant
Dwarf plant
Using Probability and Punnett Squares to Work Genetics Problems
If we know the geneIc makeup
of parents, what type of
offspring might they produce?
What is the probability of
producing different types of
offspring?
Probability:
The likelihood that a
particular event will
occur.
Using Probability and Punnett Squares to Work Genetics Problems
A
a
a
a
Punnett Square
1. A Punnett square is a
diagram showing the
________________
allele combina=ons that
might result from a
genetic cross between
two parents.
2. The ______
alleles of the first
parent will be placed
across the ___
top of the
square.
3. The ______
alleles of the second
parent will be placed
along the _______
left side of the
square.
Using Probability and PunneR Squares to Work Gene;cs Problems
A
a
a
a
Monohybrid Cross:
4. The possible gene combina;ons
offspring will be placed
of the ________
________________.
inside the squares
5. LeRers will represent the
______.
alleles
6. A capital leRer represents a
dominant allele.
_________
7. A lower-case leRer represents
a ________
recessive allele.
one trait or characteristic is being
• A cross in which only ______________________
considered.
boxes
• Monohybrid crosses use Punnett squares consisting of 4
_______.
Practice
Problem
#1
Mendel began his experiments using true-breeding
parents. He soon discovered that the tall trait was
dominant over the dwarf trait.
Cross a true-breeding tall pea plant to a true-breeding
dwarf pea plant.
What is the genotype
of the first parent?
TT
What is the genotype
of the second parent?
tt
Place the alleles of the first
parent on the top of the square.
Place the alleles for the second
parent on the left of the square.
T
T
Tt
Tt
t Tt
Tt
t
Fill in the squares to
show all the possible
combinations
of alleles that the
offspring might inherit.
Use this table to show all possible genotypes
and phenotypes of the offspring, and the
probabilities of each.
4⁄4 Tt
4⁄4 tall
Practice
Problem
#2
T
In the above problem, none of the offspring will show
the dwarf trait. As we learned earlier, Mendel wondered
what had happened to the dwarf trait. He allowed the F1
generation to self-pollinate.
Show this cross using the Punnett square below.
What is the genotype of each parent?
t
T TT Tt
t Tt
%
1/4 TT
2/4 Tt
1/4 tt
Tt x Tt
¾ Tall
¼ dwarf
Practice
Problem
#3
Having dimples is dominant over the absence of dimples.
Cross a heterozygous dimpled man with a woman who
does not have dimples. Show all work in the Punnett
square and summarize your findings in the table.
Dd
What is the genotype of the woman? dd
What is the genotype of the man?
D d
d Dd dd
d Dd dd
2/4 Dd
2/4 dd
2/4 dimples
2/4 no dimples
Normal skin is dominant over albino
skin. A woman who has normal skin,
but whose father was albino, marries
a heterozygous, normal skinned man.
What type of offspring might they
expect?
Practice
Problem
#4
What is the genotype of the woman? Aa
A
What is the genotype of the man?
a
A AA Aa
a Aa aa
1/4 AA
2/4 Aa
1/4 aa
Aa
¾ Normal
¼ albino
Practice
Problem
#4
How many different genotypes are possible among the offspring? 3
How many different phenotypes are possible among the offspring? 2
What is the probability of getting homozygous offspring? 2/4
What is the probability of getting heterozygous offspring? 2/4
What is the probability of getting normal offspring? 3/4
What is the probability of getting albino offspring? 1/4
Practice
Problem
#5
In dogs, the allele for short hair (B) is
dominant over the allele for long hair (b).
Two short haired dogs have a litter of puppies.
Some of the puppies have short hair and
some of the puppies have long hair.
What are the
genotypes of the
parents?
Bb and Bb
B b
B BB Bb
b Bb bb
1/4 BB
2/4 Bb
1/4 bb
¾ short
hair
¼ long
hair
Practice
Problem
#5
If the litter of puppies contained 12 pups,
how many would you expect to have
short hair?
¾ of the 12 should have short hair. ¾ of 12 = 9 pups
How many would you expect to have long
hair?
¼ of the 12 should have long hair. ¼ of 12 = 3 pups
The Principal of Independent Assortment
Mendel needed to answer one more quesJon.
When alleles are being segregated during gamete
formaJon, does the segregaJon of one pair of alleles
have any effect on the segregaJon of a different pair
of alleles?
In other words, does the gene that determines if a pea plant
is tall or dwarf have any effect on the gene for seed color?
Mendel designed a
second set of
experiments to
follow two different
genes as they passed
from parent to
offspring.
This is known as a …
… two-factor cross
or a dihybrid cross.
One parent had peas that were round and
yellow. The second parent had peas that
were wrinkled and green.
The round and yellow traits are dominant.
First, Mendel crossed true-breeding parents.
Round, yellow peas
RRYY
X
Wrinkled, green peas
rryy
All F1 offspring
produced round,
yellow peas.
If round and yellow are
dominant, what is the
genotype of all of the
F1 offspring?
RrYy
When the first generation was allowed to self-pollinate (RrYy x RrYy),
it resulted in the production of 556 seeds:
Ø 315 round, yellow (dominant, dominant)
Ø 105 round, green (dominant, recessive)
Ø 104 wrinkled, yellow (recessive, dominant)
Ø 32 wrinkled, green (recessive, recessive)
This meant that the alleles for seed
shape had segregated independently
of the alleles for seed color.
The alleles for one trait had
___________
no influence on the alleles of
another trait.
This is known as
independent assortment
_____________________.
The Principle of Independent Assortment states that …
… when gametes are formed, the alleles of a gene for one trait
segregate independently of the alleles of a gene for another trait.
NOTE: This is assuming that the two genes are located on separate
nonhomologous chromosomes and are not linked on the same
chromosome.
Using a Punnett square for a two-factor or dihybrid cross
ü A dihybrid cross considers two traits at the same time.
ü When two traits are being considered, the Punnett square
will need 16 squares.
ü Each parent will pass one allele of each
gene pair to the offspring.
Given the following parental genotypes,
what alleles could each parent pass to
their offspring?
If the parent was AaBb: AB, Ab, aB, ab
If the parent was Aabb: Ab, Ab, ab, ab
If the parent was aaBb: aB, ab, aB, ab
If the parent was AABB: AB, AB, AB, AB
Use the following Punnett square to illustrate
Mendel’s experiments
True-breeding Round and Yellow (RRYY) x True-breeding wrinkled and green (rryy)
RY
True breeding
Round, yellow
seed (RRYY)
RY
RY
RY
ry RrYy RrYy RrYy RrYy
ry RrYy RrYy RrYy RrYy
ry RrYy RrYy RrYy RrYy
True breeding
Wrinkled, green
seed (rryy)
ry RrYy RrYy RrYy RrYy
16/16 RrYy 16/16 Round,
yellow
seeds
If the offspring from the above cross
are allowed to self-pollinate:
Round and Yellow x Round and Yellow
RrYy x RrYy
RY
Ry
rY
Practice
Problem
ry
RY RRYY RRYy RrYY RrYy
Ry RRYy RRyy RrYy Rryy
rY RrYY RrYy rrYY rrYy
ry RrYy Rryy rrYy rryy
1/16 RRYY
2/16 RRYy
1/16 RRyy
2/16 RrYY
4/16 RrYy
2/16 Rryy
1/16 rrYY
2/16 rrYy
1/16 rryy
Round,yellow
9/16
Round, green
3/16
Wrinkled,
Yellow 3/16
Wrinkled, green
1/16
Right handedness (R) is dominant over left handedness
(r). The ability to roll your tongue (T) is dominant over
the inability to roll your tongue (t).
What offspring might be expected from a cross
involving the following parents:
RRR x RRTt
Rt
Rt
Rt
Practice
Problem
Rt
RT RRTt RRTt RRTt RRTt
Rt RRtt RRtt RRtt RRtt
RT RRTt RRTt RRTt RRTt
8/16 RRTt
Rt RRtt RRR RRR RRR
8/16 RRR
8/16 Right-handed,
tongue roller
8/16 Right-handed,
nonroller
Practice
Problem
A woman, who is right handed and a tongue roller, has a
father who is left handed and cannot roll his tongue. She
marries a heterozygous right handed, tongue rolling man.
What possible offspring might they expect?
What is the genotype of the woman? RrTt
What is the genotype of the man? RrTt
RT
1/16 RRTT
2/16 RRTt
1/16 RR4
2/16 RrTT
4/16 RrTt
2/16 Rr4
1/16 rrTT
2/16 rrTt
1/16 rr4
9/16 Right handed
tongue rollers
3/16 right handed
nonrollers
3/16 le< handed
tongue rollers
1/16 le< handed
nonrollers
Rt
rT
rt
RT
RRTT RRTt RrTT RrTt
Rt
RRTt RRtt RrTt
Rrtt
rT
RrTT RrTt rrTT
rrTt
rt
RrTt
rrtt
Rrtt
rrTt
A Summary of Mendel’s Principles
Mendel’s principles form the basis of modern gene;cs.
Mendel’s principles include the following:
1. The inheritance of traits is determined by
individual units known as genes.
2. Genes are passed from parent to offspring.
3. Each gene has two or more forms called
alleles.
4. Some alleles are dominant, while other
alleles are recessive.
A Summary of Mendel’s Principles
two alleles for a
5. Each parent has __________
particular trait that they inherited from
their parents.
one allele to their offspring
They will pass _________
when the alleles are
segregated into gametes
______________________.
6. The alleles for one trait segregate
independently of the alleles for another
trait, when the genes are located on
separate, nonhomologous chromosomes.
Today we know that
there are exceptions to
the principles set forth
by Gregor Mendel.
These exceptions
include the following:
1. Some alleles are neither
dominant nor recessive.
2. The majority of genes
have more than 2 alleles.
3. Some traits are controlled
by more than one gene.
Genes and the Environment
Gene expression is the result of the interaction of genetic
potential with the environment.
A seedling may have the geneHc
capacity to be green, to flower, and to
fruit, but it will never do these things if
it is kept in the dark. A tree may never
grow tall if the soil is poor and no water
is available.
In other words, the presence of
the gene is not all that is required
for the expression of a trait.
The gene must be present along
with the proper environmental
condi;ons.
Examples: Primrose plants that are red
flowered at room temperature are
white flowered when raised at hoRer
temperatures. Himalayan rabbits are
white at high temperatures and black at
low temperatures. Snowshoe rabbits
are white at low temperatures and
brown at high temperatures.
Snowshoe Rabbits
The phenotype of any
organism is the result of
interaction between genes
and the environment.
Incomplete Dominance or
Nondominance
not controlled by
Some traits are ____________
dominant and recessive alleles.
_____________________
In some cases, the offspring will
in between
have a phenotype __________
the phenotypes of the parents.
In radishes, a pure-breeding white
radish plant crossed with a purebreeding red radish plant produces
plants with purple radishes.
The phenotypes of the parents
blend or mix together
appear to ___________
in the phenotype of the
offspring
________.
Incomplete Dominance
This is known as incomplete
dominance.
Alleles are neither dominant nor
recessive.
The alleles blend together in the
offspring.
For example:
In some flowers, such
as snapdragons and
four o'clocks, a
homozygous red
_______________
flower crossed with a
________________
homozygous
white
flower yields a
heterozygous
pink
________________
flower.
Since there is no
recessive allele, use only
capital
______ letters.
For example: A red
flower would be ___,
RR and
white flower would be
WW and the
____,
heterozygous pink hybrid
RW
would be ___.
In eggplants, a pure-breeding white
eggplant crossed with a purebreeding purple eggplant produces
eggplants with a violet color.
What type of offspring might be produced by
two pink flowering plants?
What are the genotypes
of the parents?
Practice
Problem
RW and RW
1/4 RR
1/4 Red
2/4 RW 2/4 Pink
1/4 WW 1/4 White
R
W
R RR
RW
W RW
WW
e
c
i
t
c
Pra m
e
l
b
o
r
P
In a certain plant, flower color shows incomplete dominance,
but the stem length shows dominance. The allele for long stem
is dominant over the allele for short stem. Cross a
heterozygous long-stemmed red plant with a short-stemmed
pink plant.
What is the genotype of the first parent? LlRR
What is the genotype of the second parent? llRW
LR
lR
lR
lR
LlRR LlRR llRR llRR
lW LlRW
lR
LR
LlRW llRW llRW
LlRR LlRR llRR llRR
lW LlRW
LlRW llRW llRW
4/16 LlRR
4/16 LlRW
4/16 llRR
4/16 llRW
4/16 Long, red
4/16 Long, pink
4/16 short, red
4/16 short, pink
Codominance
In codominance …
… neither allele is
dominant or recessive,
nor do the alleles blend
together in the
phenotype of the
offspring.
Both codominant alleles contribute
to the phenotype of the offspring.
Both _________
dominant alleles are apparent in the phenotype of the
heterozygous offspring.
____________
Codominance in Human Blood Types
In humans, four blood types are possible:
A, B, AB, and O
three alleles that determine
There are ___________
blood type.
A and B alleles are ___________
codominant
The _______
O blood is recessive
and the allele for _________________.
These three alleles are written as
follows: IA, IB, and i.
Alleles IA and IB are codominant, and the
allele “i” is recessive.
Blood Genotypes and Phenotypes
Genotypes Phenotypes
IA IA
IA i
IB IB
IB i
IA IB
ii
Type A blood
Type A blood
Type B blood
Type B blood
Type AB blood
Type O blood
What types of offspring might be expected if one
parent has type AB blood and the other parent is
heterozygous for type A blood?
What is the genotype of the
first parent? IA IB
Practice
Problem
What is the genotype of the
second parent? IA i
IA
IB
IA
IA IA
IA IB
i
IA i
IB i
¼
¼
¼
¼
IA IA
IA IB
IA i
IB i
2/4 Type A blood
1/4 Type AB blood
1/4 Type B blood
A man and a woman have four children. Each child has
a different blood type. What are the genotypes of the
parents and the four children?
What are the genotypes of the parents? IA i and IB i.
What are the genotypes of
the four children?
The type O child is ii.
The type AB child is IA IB.
The type A child is IA i .
The type B child is IB i .
Blood Type and Rh Factor
Another component of our blood type is the Rh factor.
Some people have Rh positive blood and others have Rh
negative blood. The Rh factor is determined by one gene with
two alleles. The allele for Rh positive is dominant over the allele
for Rh negative. Let’s use “R” to represent the positive allele
and “r” to represent the negative allele.
A woman whose blood type is AB negative
marries a man with blood type O positive.
The man’s mother had blood that was A
negative.
Practice
Problem
What is the genotype of the woman? IA IB rr
What is the genotype of the man? ii Rr
What is the genotype of the man’s mother? IA i rr
IAr
IAr IBr IBr
iR
IAi Rr IAi Rr IBi Rr IBi Rr
ir
IAi rr
IAi rr
IBi rr
IBi rr
4/16 IAi rr
iR
IAi
IAi
IBi
IBi
4/16 IBi Rr
ir
Rr
IAi rr
Rr
IAi rr
Rr
IBi rr
Rr
IBi rr
4/16 IAi Rr
4/16 IBi rr
4/16 Type A Rh positive
4/16 Type A Rh negative
4/16 Type B Rh positive
4/16 Type B Rh negative
Mul;ple Alleles
more than two alleles
Many genes have ___________________.
Genes with three or more alleles are said to have
multiple alleles
“______________.”
In genetics problems with
multiple alleles, there are
three
__________________
or more alleles for
the trait.
This does not mean that an individual will have more than
two alleles for a given trait. “Multiple alleles” means that
there are three or more alleles for a trait in a population.
The best example for multiple alleles involves coat color in rabbits. Coat
color in rabbits is determined by a single gene that has at least four
different alleles. These four alleles demonstrate a “dominance
hierarchy” in which some alleles are dominant over others. The four
alleles for coat color in rabbits in order of dominance are as follows:
Allele
Phenotype
C
Full color ( oJen called
wild type or agouK)
cch
Light gray or Chinchilla
ch
Albino with black
extremiKes or Himalayan
c
Albino
These alleles are listed in order of their dominance. What
would be the possible genotypes of each of these rabbits?
CC, Ccch, Cch, Cc
Full color: ________________
Full Color
cchcch, cchch, cchc
Chinchilla: _______________
hch, chc
Himalayan: c________
cc
Albino: ___
Albino
Himalayan
Chinchilla
Practice
Problem
C
What types of offspring could be produced by a
full color rabbit that had a genotype of C cch that
was bred with a Himalayan rabbit that was ch c?
cch
ch C ch cch ch
c
Cc
cch c
¼ C ch
¼ cch ch
¼ Cc
¼ cch c
2/4 Full color
2/4 chinchilla
Polygenic Inheritance
In polygenic inheritance, the
determination of a given
characteristic is the result of
the …
… interaction of many genes.
Some traits, such as size,
height, shape, weight, color,
metabolic rate, and behavior
are not determined by one
pair of alleles.
These traits are the cumulaJve result of the combined effects of
many genes.
This is known as polygenic inheritance.
A trait affected
by a number of
genes
(polygenes)
does not show a
clear difference
between groups
of individuals.
Instead, it
shows a
graduation of
small
differences.
Many normal
human traits
are thought to
be polygenic.
Examples
include hair
color, eye
color, weight,
height, and
skin color.
Chromosomes
Gene Linkage
Independent Assortment states that when
Mendel’s Principle of ______________________
gametes are formed, the alleles of a gene for one trait segregate
independently of the alleles of a gene for another trait.
for genes that are
We now know that this principle is true only
____________
nonhomologous chromosomes
located on separate ___________________________.
A
B
If Gene A and Gene B are located on
separate nonhomologous
chromosomes, they will assort into
gametes independently of one another.
A
B
Independent assortment
cannot occur if Gene A and
Gene B are linked together
on the same chromosome.
Chromosomes
Gene Linkage
A chromosome is a group of ___________.
linked genes
chromosomes
During meiosis, it is the _____________
that assort independently in gametes, not
genes
the individual ______.
As luck would have it, most of the traits Mendel
studied in pea plants are located on separate
chromosomes.
Chromosomes
Gene Linkage
Does this mean that two genes located on the same chromosome
are forever linked together? The answer is no.
Remember that “____________”
crossing-over sometimes occurs during meiosis.
Crossing-over is the process in which homologous chromosomes
may exchange portions of their chromatids. This produces new
combinations of genes and contributes to genetic diversity.
Sex Determination
Human cells contain _______
23 pairs of chromosomes.
There are 22 pairs of __________
autosomes and one pair
of ________________.
sex chromosomes
In males and
females, all of
the pairs of
chromosomes
are the same
except one pair.
The pairs that are the same in both males and females are
called autosomes.
Autosomes are all of the chromosomes within a cell except
for the sex chromosomes.
One pair differs between males and
females.
sex chromosomes
This pair is called the ________________.
The sex chromosomes differ in structure
contain genes that determine the
and _____________
____
sex of the individual.
Females have 2
copies of a large
X chromosome.
Males have one X
and one small Y
chromosome.
There are many genes found
on the X chromosome.
The Y chromosome appears
to contain only a few genes.
Since the X and Y chromosomes determine
the sex of an individual, all genes found on
these chromosomes are said to be
sex-linked
_________.
More than 100 sex-linked genetic
disorders have now been associated with
the X chromosome.
Sex-linked traits in humans include …
… colorblindness, hemophilia, and muscular dystrophy.
recessive alleles.
These are caused by _________
Since males have only one copy
of the X chromosome, they will
have the disorder if they inherit
just one copy of the allele.
Females must inherit __________
two copies of the allele, one on each of
X chromosomes, in order for the trait to appear in the
their ___
offspring.
Therefore, sex linked genetic disorders are much more
common in ______
males than ________.
females
Colorblindness in Humans
The table below summarizes the possible genotypes and phenotypes.
Genotype
Phenotype
XCXC
Normal vision female
XCXc
Normal vision female, but a carrier
of the colorblind allele
XcXc
Colorblind female
XCY
Normal vision male
XcY
Colorblind male
Hemophilia in Humans
The table below summarizes the possible genotypes and phenotypes.
Genotype
XHXH
Phenotype
Normal blood clolng female
XHXh
Normal clotting female, but a
carrier of hemophilia
XhXh
Hemophiliac female
XHY
Normal blood clotting male
XhY
Hemophiliac male
Eye Color in Fruit Flies
Sex-linked traits were discovered in the early 1900’s in the lab of Thomas
Hunt Morgan. It was discovered that the gene for eye color in fruit Clies
was carried on the X chromosome. In fruit Clies, the allele for red eyes is
dominant over the allele for white eyes.
Genotype
Phenotype
XRXR
Red-eyed female
XRXr
Red-eyed female
XrXr
White-eyed female
XRY
Red-eyed male
XrY
White-eyed male
A normal woman, whose father had
hemophilia, married a normal man. What is
the chance of hemophilia in their children?
Practice
Problem
What is the genotype of the woman’s father? XhY
What is the genotype of the woman? XHXh
What is the genotype of the man? XHY
XH
XH XHXH
Y
XHY
Xh
XHXh
XhY
1/4
1/4
1/4
1/4
XHXH 2/4 Normal
cloSng
XHXh
female
H
X Y 1/4 Normal
XhY
cloSng male
1/4 Hemophiliac
male
The gene for colorblindness is carried on the X
chromosome and is recessive. A man, whose
father was colorblind, has a colorblind daughter.
1. Is this man colorblind? How do you know?
Yes. The colorblind daughter had to get one allele for
colorblindness from each parent. This would require
her father to be colorblind.
Practice
Problem
2. Where did the man get his allele for
colorblindness?
A man gets his allele for colorblindness
from his mother. He gets his Y
chromosome from his father.
3. Must the fathers of all colorblind girls be
colorblind? Explain.
Yes. For a girl to be colorblind, she must
inherit the colorblind allele from each
parent.
A. A pedigree chart
shows …
… relationships
within a family.
B. Squares represent
males and circles
represent females.
C. A shaded circle or square indicates that a person
has the trait being considered.
The following table shows three generations of
guinea pigs. In guinea pigs, rough coat (R) is
dominant over smooth coat (r). Shaded individuals
have smooth coat. What is the genotype of each
individual on the table below?
Practice
Problem
rr
Rr
RR or Rr*
Rr
Rr
Rr
Rr
Rr
Rr
rr
Rr / RR
There is no way to
know!
rr
Rr / RR
rr
There is no way to
know!
* #2 is most likely RR since no offspring have smooth coat.)
The following pedigree table is for
colorblindness. Colorblindness is a sex-linked
trait. Shaded individuals have colorblindness.
Determine the genotype of each of the
following family members.
XB Xb
XB Xb
Xb Y
XB Xb
Xb Y
Xb Y
XBY
Xb Xb
XB Xb
XBY
Practice
Problem
XB Y
XB Xb
XB XB
XB Xb
XB Xb
XB XB
Mutations
Normally genes are copied accurately during
DNA replication
______________.
Occasionally there is an error when a gene is
not
copied correctly or chromosomes do not
_________________,
segregate properly
_________________.
These mistakes are called _________.
mutations
Mutations are changes …
… in the nucleotide-base sequence
of a gene or DNA molecule.
Germ-Cell Mutations and Somatic-Cell Mutations
Germ-cell mutations occur in an organism’s ________.
gametes
do not affect the organism itself.
These mutations ____________
These mutations are very important, however, because they can
passed along to future offspring
be ___________________________.
Somatic-cell mutations occur in
the _________
body cells of an organism.
These mutations _________
can affect
the organism.
For example: Some skin
cancers and leukemias are the
result of somatic-cell
mutations.
These mutations are not
inherited by offspring.
The Importance of Mutations
1. Mutations range from lethal to
beneficial.
However, most mutations are neutral
and have little or no effect.
2. Some mutations cause such
________________
dramatic changes that normal cell
functions are _________
disrupted and may
genetic disorder
result in a ______________.
3. Some mutations may actually be
_________.
beneficial
The mutation may cause a change in
the _________
phenotype of the organism that
better suited for its
makes it ____________
environment.
4. Those organisms that are better suited are more
likely to survive, reproduce, and pass these
favorable traits on to their offspring.
Natural Selection
This is the mechanism of _______________.
Types of Muta;ons
1. Mutations can involve an entire ____________
chromosome or a
single ______________.
DNA nucleotide
2. Gene mutation: A gene mutation produces a
change within a single gene.
It involves the substitution, addition, or
removal of a single nucleotide along the DNA
strand.
Usually, a gene mutation results in a certain
amino acid being left out of a protein.
The proper protein is not formed. These
“point mutations” will be studied in more
detail when you learn about DNA, RNA, and
Protein Synthesis.
3. Chromosome mutations produce changes in the
whole chromosome.
1. A chromosome mutation involves
either a change in the ________
structure of a
chromosome, or the gain or loss of
___________________.
an entire chromosome
2. Chromosome mutations have a much
more drastic
___________
effect on the organism
than gene mutations because they
involve the _________________
entire chromosome
rather than just ________.
one gene
3. Types of chromosome mutations
include:
Deletion, inversion, translocation,
and nondisjunction.
Deletion:
The loss of a piece of
chromosome due to
breakage.
Inversion:
A segment of
chromosome breaks off,
flips around, and
reattaches to the same
chromosome.
Translocation:
A piece of one
chromosome breaks
off and reattaches to
a nonhomologous
chromosome.
Nondisjunction
a) In nondisjunction the members of a pair of homologous
chromosomes do not move apart properly during meiosis I, or
sister chromatids fail to separate during meiosis II.
b) The result is that one gamete receives two of the same type
of chromosome and another gamete receives no copy. The
other chromosomes are usually distributed normally.
Nondisjunction
c) If either of these gametes is then fertilized by a normal
gamete, the zygote will have an abnormal chromosome
number for that one particular chromosome.
One zygote will have an extra chromosome, and the other
zygote will be missing a chromosome.
d) If the organism survives, it usually has a set of
symptoms caused by an abnormal dose of the
genes associated with the extra or missing
chromosome.
An example
is Down
Syndrome.
“Crossing
Over”
It should be noted that “crossing over” during meiosis is not a
chromosome mutation.
In crossing over, the order of the genes on a chromosome is not
altered.
The chromatids break at exactly corresponding points.
There is no loss or gain of genetic information, and they always rejoin
to a homologous chromosome.
This is a source of genetic variation, but it is not considered a
mutation.
GeneAc Disorders
A genetic disorder is
an inherited disease
or disorder that is
caused by a mutation
in a gene or by a
chromosomal defect.
Genetic disorders can be caused by inheriting autosomal
________
recessive alleles, autosomal _________
dominant alleles, or alleles
linked on the sex
_______________.
chromosomes
Some genetic disorders are caused when chromosomes fail
to separate correctly during meiosis.
Huntington’s Disease
1. Huntington’s disease is caused by inheriting
a single dominant allele.
It develops when the affected person
reaches the age of 30 to 40 years.
2. Symptoms include:
Forgetfulness, muscle spasms, severe
mental illness, and finally, death.
Cystic Fibrosis
1. This disease is caused by a recessive allele
on autosomal chromosome 7.
2. Two recessive alleles must be inherited to
cause the disease.
People with one copy of the normal
dominant allele are unaffected.
3. Those suffering from this disease have …
… digestive problems and produce a thick,
heavy mucus that clogs the lungs and
airways.
Duchenne Muscular Dystrophy
1. This is a sex-linked disorder caused by a
defective version of the gene that codes for a
muscle protein.
2. This disease results in …
… the progressive weakening and loss of
skeletal muscle tissue.
Down Syndrome
1. Down Syndrome is caused when autosomal chromosome
21 fails to separate during meiosis.
It results in an extra chromosome 21 in each cell of the
body.
2. Symptoms: Short, stocky body, thick neck and tongue,
mental retardation, and heart defects.
3. Most cases are the result of nondisjunction.
When the gamete is formed, it has 2 copies of
chromosome #21 instead of one.
When fertilized, the resulting embryo then has three
copies.
4. The older the mother, the greater the probability of
nondisjunction occurring.
5. This affects 1 out of every 800 children born in the USA.
Nondisjunction of the Sex Chromosomes
1. Klinefelter’s Syndrome is the result of a male
receiving an extra X chromosome.
They are XXY. It occurs once in every 2000
births. The male has sex organs, but they are
not functional.
2. Turner’s syndrome occurs in females. The
female inherits a single X chromosome. This
occurs once in 5000 births. They appear to be
female, but the sex organs never develop
during puberty. They are sterile.
Human disorders are also
caused by chromosomal
translocation.
This is the attachment of a
fragment from one
chromosome to another
nonhomologous
chromosome.
Chromosomal translocations have been implicated in certain
cancers, and a small fraction of the people with Down
Syndrome are the result of the translocation of chromosome
#21 to another nonhomologous chromosome.
Created by Amy Brown
Copyright © Amy Brown Science
All rights reserved by author.
This document is for your classroom use only.
This document may not be electronically distributed or
posted to a public web site,
except for permitted use in password-protected Google
Drive, Google Classroom, or Microsoft OneDrive.
http://www.teacherspayteachers.com/Store/Amy-Brown-Science
Clipart Credits:
• Adobe Photo
• Deposit Photo