Biology 22
Mendelian Genetics Problems
1. Consider the following two unlinked genes in the poisonous plant, Jimson
Weed. The dominant allele V produces violet petals while the recessive
allele v produces white petals. The dominant allele P produces prickly
capsules while the recessive allele p produces smooth capsules. A truebreeding violet, smooth plant is crossed to a true-breeding white, prickly
plnat. What is the genotype and phenotype of the F1 individuals? The F1
individuals are crossed to each other to produce an F2 generation. What
hare the phenotypes and frequencies of the offspring in the F2
generation?
2. Retinitis pigmentosum, a progressive degeneration of the retina, is an Xlinked recessive trait in humans. Adam does not have retinitis
pigmentosum. He marries Barbara, who also does not have the disease.
Barbara’s father, however, does have retinitis pigmentosum. What are the
genotypes, phenotypes and frequencies of the expected offspring for
Adam and Barbara?
3. Phenylketonuria, PKU, is a neurodegenerative disorder resulting from the
inability to metabolize phenylalanine. PKU is inherited as an autosomal
recessive trait. Jack does not have PKU but he has a sister who is afflicted
with the condition. Jill also does not have PKU but she has a brother with
the disease.
a. What is the probability that Jack and Jill will have a daughter with
PKU?
b. What is the probability that Jack and Jill’s first child will have PKU,
their second child will not have PKU and their third child will have
PKU?
c. Out of four children born to Jack and Jill, what is the probability that
two children will have PKU?
d. Suppose Jack and Jill’s first child is born with PKU. Using this
information, what is the probability that their next child will be a son
with PKU?
e. Suppose Jack and Jill’s first child is born with PKU. Using this
information, what is the probability that out of three children, two will
have PKU?
4. In rabbits, coat color is affected by a series of alleles with a hierarchy of
dominance such that C+>Cch>Ch>C. Each allele is dominant over all the
alleles listed to its right. C+ codes for full color, Cch codes for chinchilla, Ch
codes for himalayan (white with black extremities) and C codes for albino.
A chinchilla rabbit is mated to a himalayan rabbit. Each of these rabbits
had one albino parent. What are the phenotypes of the offspring and in
what frequencies will they be produced?
5. The autosomal dominant allele ML specifies a tailless phenotype in cats.
This allele is lethal in the homozygous state. A tailless female cat is mated
to a tailless male cat. What are the phenotypes of the offspring and in
what frequencies will they be produced?
6. In chickens, two genes affect the shape of the comb. Chickens with at least
one copyof both dominant alleles, R_P_, have a walnut comb. Chickens
with at least one copy of the P allele, rrP_, have a pea comb. Chickens
with at least one copy of the R allele, R_pp, have a rose comb. Chickens
with no dominant alleles, rrpp, have a single comb. A walnut-combed
rooster is mated to a rose-combed hen. Each of these chickens had one
parent with a single comb. What are the phenotypes of the offspring and in
what frequencies will they be produced?