AP Biology Block: ____________
Genetic Probabilities and Outcomes Worksheet
1. In humans, dimples is inherited as a dominant trait. A man heterozygous for dimples marries a woman
A. What are the genotypes of the couple?
B. What proportion of their children will have dimples?
2. In guinea pigs, the allele for black fur (B) is dominant over the allele for white fur (b). A back male guinea pig
is mated to a white female guinea pig. The offspring were 7 black and 6 white guinea pigs. What are the
genotypes of the parents?
i. Father: Mother:
3. A brown mouse is mated to a white mouse and all of their offspring are brown.
A. Give a name to the gene responsible for this trait, and assign symbols for the possible alleles. Which
allele is dominant?
B. What are the genotypes of the mice that were crossed?
C. If two of the brown offspring were mated together, what percentage of the F2 mice would be brown?
4. In tomatoes, red fruit (R) is dominant to yellow fruit (r). A true breeding red-fruited plant is bred to a true-
breeding yellow-fruited plant.
A. What phenotypic ratio would you expect to see in the F1 offspring?
B. If two of the F1 plants were bred, what phenotypic ratio would you expect to see in the F2 offspring?
C. What genotypic ratio would you expect to see in the F2 offspring?
5. In fruit flies, curly wings (c) is recessive to straight wings (C). Ebony body (e) is recessive to tan body (E).
Suppose you had a male fly that had curly wings and was heterozygous for body color and a female fly that
was heterozygous for wing type and had had an ebony body.
A. What is the genotype for the male fly?
B. What type of gametes could the male fly produce?
C. What is the genotype for the female fly?
D. What type of gametes could the female fly produce?
E. Construct a Punnett Square to determine the PHENOTYPIC ratio that you would expect to find in the
offspring of these two flies.
6. A recently married couple are both heterozygous for cystic fibrosis, a recessive genetic disorder. They would
like to start a family and would like to have two children.
A. What is the probability that their first pregnancy would result in a child with cystic fibrosis?
B. What is the probability that their second pregnancy would result in a child with cystic fibrosis?
C. What is the probability that both their first and second child would have cystic fibrosis?
7. Tim and Jan both have freckles, but their son Michael does not. Show with a Punnett square how this is
possible. If Tim and Jan have another child, what is the probability that it will have freckles?
8. Both Tim and Jan (from problem 9) have a widow’s peak, but Michael has a straight hairline.
A. What are Tim, Jan, and Michael’s genotype for hairline type?
B. If Tim and Jan had another child, what would be the probability of that child having both freckles and
a widow’s peak?
9. In shorthorn cattle, the heterozygous condition of the alleles for red coat color (CR) and white coat color (CW) is
a sandy color called roan. If two cattle with roan coats are mated, what phenotypic ratio of offspring will be
10. The following pedigree shows the transmission of a rare, but not fatal, genetic disorder in a family:
A. Identify the mode of inheritance suggested by the pedigree.
B. Indicate the genotypes of all the individuals in the pedigree above.
11. In the following pedigrees, state by answering yes or no in the appropriate blank space, whether transmission
of the trait could be accounted for on the basis of each of the listed simple modes of inheritance:
Pedigree A Pedigree B
Autosomal recessive __________ __________
Autosomal dominant __________ __________
X-linked recessive __________ __________
12. Both John and Cathy have normal colorvision. After 10 years of marriage, Cathy gives birth to a colorblind son. From which
grandparent could the son have inherited his X chromosome?
Cathy’s Mom: _________ Cathy’s Dad: _________
John’s Mom: __________ John’s Dad: __________
13. Marfan Syndrome is a genetic disorder with pleiotropic effects. Affected people are often very tall and thin,
with double-jointed fingers. The spine curves abnormally. Arms, fingers, and lower limbs are
disproportionately long. Eye lenses dislocate easily. Thin tissue flaps in the heart that act like valves to direct
blood flow may billow the wrong way when the heart contracts. The heart beats irregularly. The main artery
from the heart is fragile. Its diameter widens; its wall may tear.
14. Genetic analysis shows the disorder follows a pattern of autosomal dominant inheritance. What is the chance
any child will inherit the allele if one parent is homozygous for it?
15. In poultry the genes for rose comb (R) and pea comb (P), if present together, give a walnut comb. The recessive
alleles of each gene, when present together in a homozygous state, give single comb. What will be the
characteristics of the offspring of the following crosses?
A. RR Pp x rr Pp
B. rr PP x Rr Pp
C. Rr pp x rr Pp
D. Rr Pp x Rr Pp
E. Rr pp x Rr pp
16. For the following crosses involving the comb character in poultry, determine the genotypes of the two parents:
A. A walnut crossed with a single produces offspring ¼ of which are walnut, ¼ rose, ¼ pea, and ¼
B. A rose crossed with a walnut produces offspring 3/8 of which are walnut, 1/8 pea, and 1/8 single.
C. A rose crossed with a pea produces five walnut and six rose offspring.
D. A walnut crossed with a walnut produces one rose, two walnut, and one single offspring.
17. Refer to the traits below. What are the chances of a mother heterozygous for all of the following traits
and a father heterozygous for eyes, ears, hairline, and dimples, homozygous dominant for thumb and hair,
and homozygous recessive for tongue and pinky to have a child that has blue eyes, unattached ear lobes,
straight hairline, dimples, bent back thumb, light hair, cannot roll tongue, and bent pinky?
Non-blue eyes (B) Bent Back Thumb (T)
Blue eyes (b) Straight Thumb (t)
Unattached Ear Lobes (E) Dark Hair (H)
Attached Ear Lobes (e) Light Hair (h)
Widow’s Peak (W) Can Roll Tongue (R)
Straight Hairline (w) Cannot Roll Tongue (r)
Dimples (2 or 1) (D) Bent Pinky (P)
No Dimples (d) Straight Pinky (p)