AP Biology Genetics Problems
Monohybrid Crosses
1. Two drosophila with normal wings are crossed. Among 123 progeny, 88 have normal wings
and 35 have “dumpy” wings.
a. What inheritance pattern is shown by the normal and “dumpy” alleles?
b. What were the genotypes of the two parents?
2. If a dumpy-winged female (from above) is crossed with her father, how many normal-winged
flies will be expected among 80 offspring?
3. In human beings, brown eyes are dominant over blue eyes. Suppose a blue eyed male marries
a brown-eyed woman whose father was blue-eyed. What proportion of their children would
you predict will have blue eyes?
4. Sickle cell anemia is a human genetic disorder caused by a recessive allele. A couple plans to
marry and wants to know the probability that they will have an affected child. With your
knowledge of Mendelian inheritance, what can you tell them if:
a. Both are normal, but each has one affected parent and the other parent has no history of
sickle cell.
b. The man is affected by the disorder, but the woman has no family history of sickle cell.
Dihybrid Crosses
5. How many unique gametes can organisms with each of the following genotypes produce?
a. rrss
b. RRss
c. RrSS
d. RrSs
6. In tomato plants, the gene for purple stems (A) is dominant to its allele for green stems (a) and
the gene for red fruit (R) is dominant to its allele for yellow fruit (r). if two tomato plants
heterozygous for both traits are crossed, what proportion of the offspring are expected to have:
a. Purple stems and yellow fruit
b. Purple stems and red fruit
c. Green stems and red fruit
d. Green stems and yellow fruit
7. In sesame plants, the one-pod condition (P) is dominant to the three-pod condition (p), and
normal leaf (L) is dominant to wrinkled leaf (l). These traits are inherited independently.
Determine the genotypes for the two parents for all possible matings producing the following
offspring:
a. 323 three-pod normal, 106 three-pod wrinkled
b. 150 one-pod normal, 147 one-pod wrinkled, 51 three-pod normal, 47 three-pod wrinkled
8. Two traits are simultaneously examined in a cross of two pure-breeding pea-plant varieties.
Pod shape can either be swollen or pinched. Seed color can be either green or yellow. A plant
that is swollen and green is crossed with a plant that is pinched and yellow. One of the
resulting F1 plants is self-crossed. A total of 640 F2 progeny are phenotypically categorized as
follows:
360 swollen, yellow; 120 swollen, green; 120 pinched, yellow; 40 pinched, green
a. What is the phenotypic ratio observed for pod shape? Seed color?
b. What is the phenotypic ratio observed for both traits considered together?
c. What is the dominance relationship for pod shape? Seed color?
Blending/Incomplete Dominance
9. Winnie the Pooh had a colony of tiggers whose stripes went across the body. His pen-pal, Yogi,
sent him a tigger whose stripes ran lengthwise. When Pooh crossed it with one of his own
animals, he obtained plaid tiggers. Interbreeding among the plaid tiggers produced litters of a
majority of plaid members, but some crosswise- and lengthwise-striped animals were also
produced. Diagram the crosses made Pooh, showing the genotypes of the tiggers that account
for the coat patterns observed.
10. In some flowers, a true-breeding , red flowered strain gives all pink flowers when crossed with
a white flowered-strain: RR (red) x R’R’ (white)  RR’ (pink). Flower position is determined
by the dominant axial (base of the branch) gene (A) and the recessive terminal (tip of the
branch) gene (a).
a. What will the genotypic and phenotypic ratios of the offspring from the following cross
be: Homozygous axial-red x terminal-white ?
b. What will the ratios be in the F2 generation?
11. In cattle, the gene for straight coats (H) is dominant to the allele for curly coats (h). The gene
pairs for red (RR) or white (R’R’) coat color show an absence of dominance; heterozygotes have
a roan coat (RR’), which is red lightened by intermixing with white hairs.
a. If a curly, red cow is mated to a homozygous straight haired, white bull, what will the
genotype and phenotype of the calf be?
b. If the calf is mated to a roan animal with curly hair, what are the possible phenotypes?
Multiple Alleles
12. When looking at blood type, assume that type A and B is inherited over O, but A and B
incompletely dominate over each other. Genotypes AA and AO are phenotypically type A,
while genotypes BB and BO are phenotypically type B. Genotype AB is considered blood type
AB and genotype OO is considered blood type O. Blood typing is often used as evidence in
paternity cases, when the blood type of the mother and child may indicate that a man alleged to
be the father could not possibly have fathered the child. For the following mother and child
combinations, indicate which blood groups of potential fathers would be exonerated.
Blood Group of
Mother
AB
Blood Group of Child
O
B
O
O
B
A
Exonerated Blood Type Groups
A
13. A man whose father is type B and whose mother is type A has blood type A. he marries a type
A woman, whose parents had the same blood types as his parents. What are the genotypes of
the man and woman, and what is the probability that their first child will be blood type A?
Their second child?
Epistasis
14. In guinea pigs, the gene for production of melanin is epistatic to the gene for the deposition of
melanin. The dominant allele, M, causes melanin to be produced; therefore, mm individuals
cannot produce the pigment at all. The dominant allele, B, cause the deposition of a lot of
pigment and produces a black guinea pig. In animals with genotype bb, only a small amount of
pigment is deposited, resulting in a light brown color. Without an M allele, no pigment is
produced, so the allele for color (B) has no effect, resulting in a white guinea pig. If a
homozygous black pig is cross with a homozygous recessive white guinea pig (MMBB x mmbb),
what are the genotype and phenotypes expected for F1 and F2 generations?
Polygenes
15. The base height of a weed is 10 cm. However, the height of the weed is the result of polgenic
inheritance involving three sets of alleles (A, B, C). Each dominant gene can contribute 5cm to
the height of the plant. Thus, the homozygous state for a dominant allele can contribute 10cm
to the base height of the plant. This means that a plant that is genotype AABBCC will have a
height of 40cm, while a plant having the genotype aabbcc will be 10cm in height. If a plant
with the genotype AABBCC is crossed with a plant having genotype aabbcc, what will be the
genotype and phenotype (height) of the offspring? Use the product and sum rule to determine
the ratio of genotypes and phenotypes if these offspring were to crossbreed.
16. In cats, short hair is dominant to long hair, white spotted coat is dominant to nonspotted, and
diluted color is dominant o non-diluted color. Given the cross between two cats with the
genotypes llSsdd and LlSsDd, what is the probability of a cat having the genotype llssdd?
What is the probability of a cat having the short hair, white spotted, non-diluted phenotype?
Sex-Linkage
17. Red-green color blindness is inherited as a sex-linked, recessive trait. If a color-blind woman
marries a man with normal vision, what would be the expected phenotypes of their children?
18. A man and his wife both have normal color vision, but a daughter has red-green color
blindness. The man sues his wife for divorce on grounds of infidelity. Can genetics provide
evidence to support his case?