Simple Dominance with 2 genes
- A gene in cats causes them to be black (dominant) or brown
(recessive). A second gene causes cats to be agouti (have stripes
on their hairs) or non-agouti (solid colored hairs).
- If you did a dihybrid cross between two black haired agouti
cats, what fraction of the offspring would be expected to have each
- If a dihybrid black agouti cat mated with a brown cat that was
heterozygous for the agouti gene, what fraction of the offspring
would be expected to have each phenotype?
- In mythical dragons, the ability to breath fire (F) is dominant
over not being able to breathe fire (f). If you have a
fire-breathing dragon and want to know whether it is homozygous or
heterozygous, what could you do to find out? Explain how you would
do the experiment.
- In moths, having a long proboscis is dominant over having a
short proboscis. You’re trying to figure out whether a particular
female moth with a long proboscis is pure or a hybrid, so you cross
it with a male moth that has a short proboscis. Half the offspring
have long and half have short proboscii. What was the genotype of
- In cats, black fur is dominant over brown fur. You have a black
cat and you want to know whether it is true breeding for its black
fur color, so you mate it with a brown cat. All 10 of their kittens
are white. What was the genotype of your parent black cat? Can you
be absolutely sure?
- In rabbits, alleles for short fur (S) and long fur (L) are
incompletely dominant. Hybrids have medium length fur. If you did a
monohybrid cross with these rabbits, what ratio of short fur:
medium fur: long fur would you see in the offspring?
- In Andalusian fowl (similar to chickens), if you mate a true
breeding white fowl with a true breeding black fowl, all the
offspring will be blue. If you were to mate a blue fowl with a
white fowl, what fraction of the offspring would be blue?
- In cats, a particular gene determines whether the cat has white
patches on top of its other color(s). Homozygous dominant cats are
more than half white. Hybrid cats are less than half white, and
homozygous recessive cats have no white patches at all. If a cat
with white only on its feet mated with a cat that had no white at
all, what fraction of the kittens would have each genotype and
- In some horses, hair color is determined by one gene with two
alleles. Homozygotes may be either red or white, and heterozygotes
are roan (a mixture of red and white hairs). If you crossed two
roan horses together, what fraction of the offspring would be
- Human blood type alleles are codominant. Lucy has type A blood.
Her daughter Jenna has type O blood. Jenna’s father Ralph has never
had his blood type tested.
- What is Lucy’s genotype?
- What are the possible genotypes that Ralph could have?
- A woman with type 0 blood gives birth to a baby with type 0
blood. In a court case she claims that a man with type A blood is
the father of the child. Could he be the father? Show why he can or
cannot. Can it be proven on this evidence alone that he is the
- Hemophilia is a rare genetic disorder that results from a
deficiency in blood clotting factors. When a person without
hemophilia gets a cut, the bleeding lasts for a short time and then
the blood clots to form a scab. People with hemophilia have only
1-5% of the blood clotting factors that other people have. Thus,
when a hemophiliac gets a cut, it may continue to bleed for days or
weeks, sometimes causing dangerous amounts of blood
loss. Hemophiliacs have to do special exercises to
prevent their muscles from tearing and producing internal bleeding,
and they can take supplemental blood clotting factors, but
hemophilia has no known cure. Hemophilia is caused by a recessive
allele that is located on the X chromosome.
If XH is the allele for
normal blood clotting, and Xh is the allele for
hemophilia, draw a cross between a normal father and a mother who
is a carrier for hemophilia. Then list the percentages of offspring
that would get each genotype and phenotype.
- Colorblindness is a sex-linked recessive trait. If a woman who
was a carrier married a man who was colorblind, what percentage of
their children (males and females) would be colorblind?
- For a male to be colorblind, does he have to get the
colorblindness allele from his mother, from his father, or both?
For a female to be colorblind, does she have to get the
colorblindness allele from his mother, from his father, or