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More Genetics Problems
Name _____________
1. In thoroughbred race horses, a solid colored leg (S) is completely dominant over white
stocking (s). In addition, black coat color (LL) is incompletely dominant over brown coat
color (RR), with the heterozygote possessing “bay” coat color (LR).
Cross a heterozygous solid bay mare (female) with a bay stallion that has white
stockings.
A. What is the probability that their offspring will be heterozygous for both traits?
_________
B. What is the probability that they will have a brown foal with white stockings?
___________
2. Mouse coat color is black, white, or a blend of the two colors producing a gray color
known in genetics as agouti. The trait is inherited by incomplete dominance with agouti
being the hybrid phenotype.
A. When you cross two agouti mice, out of 84 total F1 offspring, how many would you
expect to be white? ____________
B. Diagram this cross to the F2 when the two F1 genotypes with the lowest frequency
are mated.
3. In corn, a completely recessive gene produces albino leaves, which causes the plant to
die because no chlorophyll needed for photosynthesis is produced. Plants with the dominant
allele produce green leaves. In addition, the kernel type “sweet” is incompletely dominant
with “starchy” with the hybrid phenotype being “sugary.”
If two heterozygous green sugary plants are crossed:
A. What fraction of the F1 is heterozygous for both traits? _____________
B. Out of 36 plants produced in the F1, how many would you expect to produce ears
of corn? ________________
4. In a certain breed of chicken, straight feathers are incompletely dominant over rough
feathers with the intermediate phenotype being frizzled. Also, a short beak is dominant
over a long beak. Answer the following questions based upon a cross between two
chickens, one frizzled and long-beaked and the other frizzled and heterozygous for
short beak.
a.
b.
c.
d.
What
What
What
What
%
%
%
%
of
of
of
of
the
the
the
the
offspring
offspring
offspring
offspring
will
will
will
will
be
be
be
be
frizzled with a short beak? _______
rough-feathered with a long beak? _______
straight-feathered with a short beak? _______
straight-feathered with a long beak? _______
5. Unattached earlobes is due to a dominant allele and attached earlobes is the recessive
phenotype. A cross between two individuals with unattached earlobes produces 6 individuals
with unattached earlobe and 2 individuals with attached earlobes.
a. What are the parental genotypes?
b. What type of cross is this called?
c. What percent of the F1 population would you expect to have attached
earlobes?
d. Testcross the F1 genotype with the highest frequency. What percent of the F2
offspring should have unattached earlobes?
6. In summer squash, WW produces white fruit while WY produces green and YY produces
yellow. Another allele (D) causes the fruit to appear disk-shaped, and its recessive makes
the fruit appear sphere-shaped. A white squash plant, heterozygous for shape is crossed
with a green plant also heterozygous for shape.
a. What proportion of fruit will be white and disk shaped?
b. What proportion of fruit will be green and sphere shaped?
c. How many fruit will be yellow?
d. What is the mode of inheritance for squash color?
7. Malaria is a parasitic disease that’s prevalent in tropical areas. When a mosquito that’s
carrying the parasites bites someone, the parasites enter the person’s bloodstream and
invade and live in the person’s RBCs. However, if a person has sickle-cell anemia (ss), the
presence of a parasite in an RBC is so stressful, it causes the RBC to sickle (crinkle up), and
when that happens, the parasite dies before it can multiply and spread to other RBCs. Thus
a person who is ss is also “immune” to malaria.
If a person is Ss and a malaria parasite tries to invade an RBC with abnormal hemoglobin,
again, the RBC will sickle, killing the parasite before it has a chance to reproduce. If a
parasite invades an RBC with normal hemoglobin, it will be able to live and multiply, but if
its offspring invade other RBCs with abnormal hemoglobin, they, too, will be killed. Thus, a
person who is Ss is “resistant” (though not totally immune) to malaria.
If a person is SS and has all normal hemoglobin, the malaria parasites do just fine, invading
RBCs, growing and multiplying, and invading more RBCs. Thus, an SS person often dies
from causes tied to the malaria.
A man and woman living in a tropical area where malaria is prevalent and health care is not
accessible have seven children: 2 that are immune to malaria, 3 that are resistant to
malaria, and 2 that have normal hemoglobin. What are the genotypes of the parents?
8. An aquatic arthropod called a cyclops has antennae that are either smooth or barbed. The
allele for barbed antennae (B) is dominant. In the same organism, there is a resistance to
pesticides. Reacting to pesticides is the dominant trait (P) while the resistance to pesticides
is a recessive trait. A cyclops that has smooth antennae and is resistant to pesticides is
crossed with one that is heterozygous for both traits.
a. How many offspring will have barbed antennae with a resistance to pesticide?
b. How many offspring will have a reaction to pesticides?
c. Of the offspring that have barbed antennae, what percentage is resistant to
pesticides?
9. In foxes, silver-black coat color is governed by a recessive allele “r” and red coat color is
governed by the expression of the dominant allele “R.” Determine the genotypic and
phenotypic ratios expected from the following matings:
A. homozygous red x heterozygous red
B. heterozygous red x heterozygous red
C. homozygous red x silver-black
10. In humans, there is a gene that controls formation (or lack thereof) of muscles in the
tongue that allow people with those muscles to roll their tongues, while people who lack
those muscles cannot roll their tongues. The ability to roll one’s tongue is dominant over
non-rolling. The ability to taste certain substances is also genetically controlled. For
example, there is a substance called phenylthiocarbamide (PTC for short), which some
people can taste (the dominant trait), while others cannot (the recessive trait). Let R
represent tongue-rolling, r represent a non-roller, T represent ability to taste PTC, and t
represent non-tasting. A male heterozygous for both traits has children with a woman
who is also heterozygous for both traits. What are the expected phenotypic and
genotypic ratios of their children?