Excelsior BIOL 360 - Suppose a gene has two alleles

1. Suppose a gene has two alleles, one of which is dominant over the other. An
individual whose genotype is homozygous for the dominant allele has which of the
following?
a. Two copies of the allele that determines phenotype whenever the allele is present
b. Two copies of the allele whose effect is hidden unless the other allele is absent
c. One copy of the allele that determines phenotype when present and one copy of the other
allele
d. One copy of each of two alleles that both contribute equally to determining phenotype
Use the following information for the next 2 questions.
Pigeons have two alleles of a single gene that determines whether they have feathers on their
lower legs. The alleles are called no grouse (associated with the featherless phenotype)
and grouse (associated with feathery legs).
2. Suppose a pigeon that is homozygous for the no grouse allele mates with a
heterozygous pigeon. What is the expected frequency of the homozygous no grouse
GENOTYPE in the offspring?
a.
b.
c.
d.

0%
50%
75%
100%

3. The no grouse allele is dominant over the grouse allele. If a pigeon homozygous for
the no grouse allele mates with a heterozygous pigeon, what is the expected
frequency of the feathery-legged (grouse) PHENOTYPE in the offspring?
a.
b.
c.
d.

0%
50%
75%
100%

4. In a certain species of flowering plant, the allele for white petal color is dominant
and the allele for red petal color is recessive. If you mated a homozygous red flower
with a homozygous white flower, what would you expect the offspring to look like?
a. All offspring will have red petals.
b. All offspring will have pink petals.
c. All offspring will have white petals.
d. Half of the offspring will have red petals and the other half will have white petals.

5. A scientist came across two populations of beetle species,
each pure-breeding. One population was all red in color; the other
was all black. The scientist mated one red beetle with one black
beetle, and the offspring were all black. What is the dominance
relationship between the two alleles?
a.
b.
c.
d.

Black is dominant over red.
Red is dominant over black.
Black and red are codominant.
More crosses are needed to determine the dominance relationship.

6.

Imagine a species whose eye color is determined by a receptor
molecule called EyeC. When EyeC is not activated, cAMP levels in
the cell are low and yellow pigment is produced. When EyeC is
activated by the binding of a signal molecule, cAMP levels rise
and blue pigment is produced.
The P allele codes for a version of EyeC that responds to signal
molecules and therefore turns on and off, resulting in green eye
color (from both blue and yellow pigment). The Qallele codes for
a version of EyeC that is always activated, even when signal
molecules are not present.
Assuming EyeC functions similarly to MC1R, which of the
following statements is the best prediction of the dominance
relationship between alleles P and Q as well as the phenotype
for PQ individuals?

a.
b.
c.
d.

Q is dominant over P, and the eye color of PQ individuals is blue.
Q is dominant over P, and the eye color of PQ individuals is yellow.
P is dominant over Q, and the eye color of PQ individuals is blue.
P and Q are codominant, and the eye color of PQ individuals is green.

7. A large sunflower population is established in a field. The flowers
mate randomly, and all individuals are equally likely to survive
and reproduce. In this population, 60% of the alleles of a gene for
petal color are dominant and 40% of the alleles are recessive.
Given this information, after many generations, which of the
following options would be most likely?
a.
b.
c.
d.

The allele frequencies will be 100% dominant and 0% recessive.
The allele frequencies will be 80% dominant and 20% recessive.
The allele frequencies will be 60% dominant and 40% recessive.
There is no way to predict what the allele frequencies will be.

8. A lizard population has two alleles for horn length. Long horns
help protect the lizard from predation by birds. Homozygotes
with allele 1 have long horns, while homozygotes with
allele 2 have short horns. Given this information, which of the
following is most likely true about the mode of inheritance for
allele 1, and why?
a.
b.
c.
d.

Allele 1 is dominant, because it gives lizards an advantage.
Allele 1 is recessive, because it gives lizards an advantage.
Alleles 1 and 2 are codominant, because each codes for a different horn length.
It is impossible to say, because dominance cannot be inferred from fitness.

9. The allele for black noses in wolves is dominant over the allele for
brown noses. There is no known selective advantage for one nose
color over another in wolves. If this remains true, which of the
following statements is most likely TRUE about the change in wolf
nose colors over many generations?
a.
b.
c.
d.

Black noses will become more common than they are now.
Black noses will stay about the same frequency as now.
Black noses will become less common than they are now.
Brown noses will disappear after enough generations pass.

10. A large population of land turtles on an isolated island has two
alleles for a gene that determines shell thickness. The allele for
thinner shells is dominant over the allele for thicker shells. The
thinner-shell allele occurs at a frequency of 20%. Assuming there
is no net advantage to thick or thin shells, what should the
frequency of homozygous thicker-shelled turtles be in the
population? (Hint: Use a large Punnett square to calculate the
answer.)
a.
b.
c.
d.

4%
16%
50%
64%