Heredity
Chapter 10, part 2
Beyond Mendel’s Laws
• Not all traits are controlled by single genes with
dominant and recessive alleles. Other patterns
of heredity involve:
• Incomplete dominance
• Sex-linked traits
• Multiple alleles
• Multiple genes (most traits involve this)
• Chromosomal abnormalities
Incomplete Dominance
• In incomplete dominance and codominance, both alleles are expressed
in the phenotype.
• incomplete dominance: two traits
appear to blend in the heterozygotes.
• co-dominance: both traits appear in
the heterozygotes.
These snapdragons
have two alleles
controlling flower color:
R1 (red) and R2
(white). Heterozygotes
(R1 R2) have pink
flowers. Unlike the
case in complete
dominance, the
phenotypes show us
which plants are
heterozygous.
R1 R1
R2 R2
R1 R2
R1 R2
R1 R1
R1 R2
R1 R2
R2 R2
mother
C1 C2
C1
father
C1 C2
eggs
C2
C1
sperm
In humans, a gene
affecting hair
texture (curly,
wavy, straight)
shows incomplete
dominance.
C1C1
C1C2
C2
C1C2
C2C2
The golden palomino
horse is a cross
between a white and
a brown horse. This
is another example
of incomplete
dominance: the
colors appear to
blend in the horse’s
hairs.
The red roan horse has both white and redbrown hairs, while the blue roan has both
white and gray hairs. The coat colors of
both parents are expressed in the hairs.
This is co-dominance.
At the gene level, incomplete
dominance and codominance are the same: in
both cases, both alleles are
expressed in the
heterozygote. The only
difference is at the
phenotype level.
Solving single-gene (monohybrid) crosses with
incomplete dominance.
One hair color in cattle is
controlled by a gene that
produces red (R1) or white
(R2) hairs. Heterozygotes
(R1 R2) are roan.
a. What color would the
offspring of a red bull and a
white cow be?
b. What are the phenotypic
ratios of a cross between a
white cow and a roan bull?
Both parents are homozygous. What
gametes can they produce?
A red bull (R1R1)
produces these
gametes:
A white cow
(R2R2)
produces
R2
these
gametes:
R1
R1R2
All offspring are roan and
heterozygous
Solving single-gene (monohybrid) crosses with
incomplete dominance.
One hair color in cattle is
controlled by a gene that
produces red (R1) or white
(R2) hairs. Heterozygotes
(R1 R2) are roan.
a. What color would the
offspring of a red bull and a
white cow be?
b. What are the phenotypic
ratios of a cross between a
white cow and a roan bull?
As always begin with the gametes
each parent can produce.
A roan bull (R1R2)
produces these
gametes:
A white cow
R1 R2
(R2R2)
produces
R2 R1R2 R2R2
these
gametes:
1/2 of offspring are roan
1/2 are white
• Pure-breeding red radishes (R1R1)
crossed with pure-breeding white
radishes (R2R2) produce purple
radishes.
• What is the genotype of the purple
radishes?
• What are the phenotypic ratios of a
cross between a purple radish and a
white radish?
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Melvin takes pollen from a plant with red flowers and pollinates
the flowers on a plant with white flowers. If color in these flowers
shows incomplete dominance, what will the offspring look like?
20%
20%
20%
2
3
20%
20%
1.
2.
3.
4.
All red
All white
All pink
Red or white, but
not pink
5. Red, white, or
pink
1
4
5
Sex Chromosomes
• In humans, genetic sex is
usually determined by the
sex chromosomes.
• Typically, women have two
X chromosomes, while men
have an X and a Y
(exception: the rare XY
female, due to nonexpression of the sry gene
on the y chromosome
during fetal development)
Gender, however, is a complex
issue, so saying XX=female, XY=
male is an oversimplification.
A Karyotype is an image of the replicated
chromosomes in prophase. All chromosomes have
matching homologues except the 23rd pair in males.
Homologous
chromosomes
Paired sister
chromatids
The X and Y
chromosome each have
their own unique genes.
To make a karyotype, a photo of a cell in prophase is
cut apart and the homologues are carefully matched.
Sex-linked Traits
• Traits that are carried on the sex
chromosomes will show different
genotypic and phenotypic rations in men
and women.
• The X chromosome has many genes,
while the Y has only a few, so there are
many more X-linked traits than Y-linked
traits.
Women pass their X
chromosomes to their
children. Men can contribute
either an X or a Y.
Which parent determines
the sex of the child?
Mother or father?
Can men be carriers of a
recessive X-linked trait?
No!
If a boy has an X-linked trait,
which parent did he inherit
the trait from? Mother
female parent
When determining the
outcome of a cross that
involves an X-linked trait, we
have to take into account
how the two sex
chromosomes are
distributed in the offspring.
eggs
male parent
XA
Xa
XA
Y
XA XA
sperm
Nettie Stevens
was one of the
first researchers
to discover the
patterns of Xlinked
inheritance.
XA
Xa
Xa XA
XA
female offspring
Y
XA Y
Xa
Y
male offspring
This diagram
illustrates a cross for
an X-linked trait in
fruit flies. Red eyes
are dominant, white
are recessive.
R
female parent
R
r
Red eyed carrier XR Xr
R
r
eggs
XR
R
Xr
R
r
R
sperm
male parent
Red eyed
XR Y
XR
Y
XR XR
female offspring
R
Xr XR
r
XR Y
male offspring
all the F2
females
have red eyes
half the F2
males have
red eyes, half
have white eyes
Xr Y
Where does a boy inherit his X
chromosome from?
33%
33%
33%
1. His mother
2. His father
3. Equal chances
that it could come
from either parent.
1
2
3
Red-green color blindness is an X-linked trait.
Charts such as these are used to diagnose red-green
color blindness. They look very different for those with
normal vision and those with RG-color blindness.
Normal
RG color-blind
A
29
70
B
45
nothing
C
abstract
5
D
26
nothing
I have one brother
who is red-green
color-blind.
Where did he
inherit the colorblind gene from?
Mom or Dad?
What are the odds
that I am a
carrier?
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Solving X-linked crosses.
In sex-linked problems, we must
track the gene and the chromosome
Red-green color-blindness is
that carries it.
X-linked. If a man and
An X with a
An X with an
woman with normal vision
normal allele is
affected allele is
have a color-blind son:
shown like this
shown like this
a. What are the genotypes
b
X
B
X
of the parents?
b. What are the odds of
The father is not
The mother is not
having another color-blind color-blind so must
color-blind, but
son?
not have an
can be a carrier.
affected allele. His
Her genotype
c. What are the odds of
genotype is:
must be:
having a color-blind
daughter?
XB Y
XB Xb
Solving X-linked crosses.
To set up the Punnet square, keep
the chromosome and its allele
together as one unit.
Red-green color-blindness is
X-linked. If a man and
Father can make
woman with normal vision
these gametes:
have a color-blind son:
XB
Y
Mother can
a. What are the genotypes
B XB XB XB Y
X
make
these
of the parents?
gametes:
b. What are the odds of
Xb XB Xb Xb Y
having another color-blind
Notice there are both girls and boys
son?
here, so we have to express the odds
c. What are the odds of
separately for each
having a color-blind
Boys: 50% odds of being color-blind
daughter?
Girls: 0% odds of being color blind but
50% are carriers.
• Hemophilia is an X-linked trait. If a man
who is normal for the blood-clotting
protein marries a woman who is a
carrier, what are the odds that a
daughter of theirs will have hemophilia?
What are the odds that a son will have
this disorder?
• What would the phenotypes of the
parents have to be to produce a
daughter with hemophilia?
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• Adeline is red-green color blind. Her
husband, Gaston, has normal vision.
They’ve come to you, a genetic
counselor, to find out what the chances
are that any of their children will inherit
Adeline’s red-green color blindness.
“The chances for girls will be different
from the chances that boys will inherit
your red-green color blindness,” you
say. They look puzzled and ask why that
is? Create a Punnet square to help them
understand.
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Licorice – Black Male
In cats, one coat
color gene on
the X
chromosome has
two alleles:
orange and
black.
Odin – Orange Male
Sprocket –
Calico
(orange and
black) Female
• Cats also have X and Y chromosomes. In
cats, one coat color gene is on the X
chromosome. One allele for this gene
produces black fur. The other allele
produces orange fur. Use this to explain why
male cats can be orange OR black, but
normally only females can be calico or
tortoise shell (orange AND black).
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In humans, whose gamete determines
the (genetic) sex of the child?
25%
25%
25%
2
3
25%
1. Mom’s
2. Dad’s
3. Equal odds that it
could be either
4. No one knows
1
4
Duschenne’s muscular dystrophy (DMD) is a devastating
neurological disorder that often causes death before the teen
years. It is caused by a recessive allele carried on the X
chromosome. Is it likely that a girl could be born with this disorder?
1. Yes, if her mother is a
carrier.
2. Yes, if both parents are
carriers.
3. No, she’d have to have
a father with DMD,
which is nearly
25%
25%
25%
25%
impossible.
4. No, girls never show Xlinked traits.
1
2
3
4
Multiple Alleles
• Human blood type (A, B, AB, and O) is
determined by a gene that has three
alleles.
• A and B are co-dominant
• O is recessive to both
• Though there are three alleles, each
person still has only two copies of the
ABO gene.
A and B alleles produce A and B proteins on the
surface of red blood cells. The O allele produces
neither of these proteins.
• Edmund has type B blood. His father
had type AB, while his mother had type
O. His wife, Francesca, has type A
blood. Both of her parents had type AB.
What possible blood types can Edmund
and Francesca’s children have?
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• Barney has type AB blood. Betty has
type A blood. What blood types can their
children have? What types can they
NOT have?
• Bernadine has type B blood. Her
husband, Rupert, has type A blood.
Their daughter, Opal, has type O blood.
Assuming no infidelities, how did this
happen?
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• Orson and his wife, Laurita, just had a
child. But Orson is not pleased. He has
his suspicions about his neighbor,
Horatio, and suspects Horatio and
Laurita have been a little too friendly. He
orders blood tests done and finds that
he has Type A blood. Laurita is also
Type A. But the baby is Type O. Horatio
gallantly offers to have his blood tested,
and finds he has Type AB. Who can and
who cannot be the father of this child?
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Genevieve had a blood test done to find out her blood type and
learned that she inherited an A allele from her father and a B
allele from her mother. What is her blood type?
25%
1.
2.
3.
4.
25%
25%
2
3
25%
A
B
AB
O
1
4
Agnes decides to give blood for the first time. She’s looking
forward to getting her blood donor card so that she can find out
what her blood type is. “Both of my parents are type AB,” she
says, “so I know my blood type can’t be:
25%
1.
2.
3.
4.
25%
25%
2
3
25%
A
B
AB
O
1
4
Multiple Genes
• Most human traits are the result of multiple
genes.
• In some cases (such as skin and hair color),
there are multiple copies of the same gene
(such as the melanin gene).
• In many others, there are many different
genes controlling a trait, and the environment
may affect how a trait is expressed.
(Example: human height)
Human skin color is
controlled by at least
three melaninproducing genes,
which are
incompletely
dominant.
sperm
This massive Punnet
square shows a cross
between two people
who are heterozygous
for all three genes.
eggs
Chromosomal Abnormalities
• Chromosomal abnormalities include:
• nondisjunctions (failure of chromatids
to separate during meiosis)
• deletions of parts of chromosomes
• Most chromosomal abnormalities cause
cell death, but a few are survivable.
Cri-du-chat syndrome is caused
by a deletion of a large segment
of chromosome 5.
Cri-du-chat children often have
small head circumference and
are severely cognitively
challenged. Some may have
heart defects, muscular or
skeletal problems, or vision
problems.
Trisomy 21 results in Down
Syndrome.
Using what you know about
meiosis, explain how a
fertilized human egg cell can
end up with three copies of
chromosome 21.
If a person with Down
Syndrome planned to have a
child, could the child inherit
Down Syndrome?
Nondisjunction of the sex chromosomes is more
often survivable than nondisjunctions of somatic
chromosomes. As long as the fetus has at least one
X chromosome, it can survive.
Klinefelter syndrome
produces an XXY male.
At puberty, Klinefelter
males fail to fully develop
secondary sex
characteristics. Men with
this syndrome may or may
not be sterile. They have a
tendency to gain weight
easily and their muscle
mass is underdeveloped,
but mental function is
usually normal.
Turner syndrome occurs when a
girl inherits only one X
chromosome.
Turner children are often short,
and may show swelling in the
hands and feet. Some have
heart defects, but most are
cognitively normal. Hormone
therapy at puberty can help
Turner girls grow to normal
height and develop secondary
sex characteristics.
• Use what you know about meiosis and
the sex chromosomes to explain:
• how a boy could be born with two X
chromosomes.
• how a girl could be born with only one
X chromosome.
• why there are no Turner boys.
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Recap
• Traits inherited by classic Mendelian
genetics are by far in the minority.
• Incomplete dominance, sex linkage, and
multiple alleles involve single-gene traits
that show unique patterns of inheritance.
• Most traits involve multiple genes and
gene-environment interactions.