Genetics Problems
1. A mother and father both find the taste of the chemical phenylthiourea very bitter. However, two
of their three children find the chemical tasteless. Assume this trait is controlled by only one gene.
Which of the two phenotypes (tastes bitter vs. tasteless) is dominant? Which is recessive? Is this
pattern consistent with complete dominance?
2. In humans, the allele B produces brown eyes and the allele b produces blue eyes. B is dominant to b.
If a brown-eyed woman and a blue-eyed man produce four brown-eyed children what are the
possible genotypes of the parents? What genotypes are most probable?
3. A dominant allele, B is responsible for wild-type body color in fruit flies; its recessive allele, b,
produces black body color. A test cross of a wild-type female gave 52 black and 58 wild-type
offspring in the F1 generation. If the wild-type females were crossed with their black F1 male
siblings, what genotypic and phenotypic ratios would be expected in the F2?
4. Assume that an individual is heterozygous for three independently located pairs of alleles. If this
organism is self-fertilized, what proportion of the offspring will carry the genotype AaBbcc? What
proportion of the offspring will have the same phenotype as the parent?
5. When a pea plant with the genotype AaBb produces gametes, what proportion will be Ab?
6. In dogs, dark coat color is dominant over albino, and short hair is dominant over long hair. Write
the genotypes of the parents in each of the crosses given in the table. Use the symbols D and d for
the color gene and the symbols H and h for the hair length genes.
number of offspring
dark, long
albino, short
parental phenotypes
dark, short
dark, short X dark, short
89
31
29
11
dark, short X dark long
18
19
0
0
dark, short X albino, short
20
0
21
0
albino, short X albino, short
0
0
28
9
dark, long X dark long
0
32
0
10
dark, short X dark, short
46
16
0
0
dark, short X dark, long
30
31
9
11
albino, long
7. Suppose a man and a woman are both heterozygous for a recessive gene for albinism (no pigment).
If they have twins, what is the probability that both of the infants will have the same phenotype
with respect to pigmentation?
8. In the mouse the gene for colored coat (P) is dominant to the gene for albino (p) and the gene for
straight whiskers (S) is dominant to the allele for bent whiskers (s). Consider a cross between two
mice heterozygous for both these alleles:
PpSs x PpSs
a. What proportion of the offspring would you expect to be albino?
b. What proportion would have bent whiskers?
c. What would be the expected phenotypic ratio of offspring with respect to both alleles?
Practice Genetics Problems
Fall 2006
1
9. Black coat color in Cocker Spaniels is governed by a dominant allele, B, and red coat color by its
recessive allele, b. Solid pattern (i.e., one color) is governed by the dominant allele, S, of an
independent gene, and spotted pattern (i.e. black and white or black and red) by a its recessive
allele, s. A solid, black female is mated to a solid, red male and produces a litter of six pups: 2 solid
black, 2 solid red, 1 black and white, 1 red and white. Determine the genotypes of the parents.
10. Imagine that, in the mountains of British Columbia, a small group of Sasquatches is discovered. A
study of four matings that occur in the group in the course of several years produces the results
shown in the table below.
a. How many genes are involved in these phenotypes?
b. Which alleles are dominant? Which are recessive?
c. Assign symbols to each allele.
d. Only five different parents participated in these matings. Give the genotypes of these five
individuals.
Mating
Parent 1
Parent 2
1
bowlegs, hairy knees
bowlegs, hairy knees
2
bowlegs, smooth legs
knock knees, smooth legs
3
bowlegs, hairy knees
knock-knees, smooth legs
4
bowlegs, hairy knees
bowlegs, hairy knees
Offspring
¾ bowlegs, hairy knees
¼ knock-knees, hairy knees
½ bowlegs, smooth legs
½ knock knees, smooth legs
¼ bowlegs, smooth legs
¼ bowlegs, hairy knees
¼ knock knees, hairy knees
¼ knock-knees, smooth legs
¾ bowlegs, hairy knees
¼ bowlegs, smooth legs
11. Tall tomato plants are produced by the action of a dominant allele, T, and dwarf plants by its
recessive allele, t. Another gene produces hairy plants when the dominant allele is present, and
hairless plants when recessive. A heterozygous tall, hairy plant is crossed. The F1 offspring were
observed to be 118 tall, hairy; 121 dwarf, hairless; 112 tall, hairless; 109 dwarf, hairy. What is the
ratio of tall to dwarf plants? What is the ratio of hairy to hairless plants? Give the genotypes of the
parent plants and all the offspring.
12. In garden peas, tall vine is dominant to dwarf; green pods is dominant to yellow. A tall, yellow plant
is crossed with a dwarf, green plant and produced 320 offspring, of which 80 were dwarf and green.
What are the genotypes of the parents?
Practice Genetics Problems
Fall 2006
2