Download Genetics Practice Problems

Sample Genetics Problems
1. You have crossed two pure breeding lines of silkmoths, one strain weaves thin yellow
fibers, and another strain produces thick white fibers. The offspring all weaved thin
yellow fibers. When this progeny was allowed to interbreed it result in progeny with the
following phenotypes:
904 thin yellow
289 thin white
327 thick yellow
92 thick white
Explain these results. Give the genotypes of the parents, their offspring, and list all the
possible genotypes which could account for the phenotypes observed among F2.
2. Two true-breeding stocks of garden peas are crossed. One parent had red, axial
flowers, and the other had white, terminal flowers; all F1 individuals had red, axial
flowers. Assuming these two genes are unlinked, what is the proportion of phenotypes
you would observe in the F2. What are the genotypes of the parents, F1 and F2
individuals? Which is the dominant trait?
3. Huntington’s chorea is a rare fatal disease that usually develops in middle age. It is
caused by a dominant allele. A phenotypically normal man in his early twenties learns
that his father has developed Huntington’s chorea.
a. What is the probability that he himself will develop the symtoms later on?
b. What is the probability that his son will develop the symptoms later in life?
4. Holstein cattle normally are black and white. A superb black and white bull, Charlie,
was purchased by a farmer for $100,000. The progeny sired by Charlie were all normal in
appearance. However, certain pairs of his progeny, when inbred, produced red and white
progeny at a frequency of about 25%. Charlie was soon removed from the stud list of
Holstein breeders. Explain precisely why, using symbols.
5. In humans, the disease galactosemia is inherited as a monogenic recessive trait in a
simple Mendelian manner. A woman whose father had galactosemia intends to marry a
man whose grandfather was galactosemic. They are worried about having a galactosemic
child. What is the probability of this outcome?
6. Two black guinea pigs were mated and over several years produced 29 black and 9
white offspring. Explain these results, giving the genotypes of parents and progeny.
7. When a pea plant of genotype AaBb produces gametes, what proportion will be A b?
(Assume these genes are not linked).
8. You have crossed two pure-breeding plants varieties, one with tall stems and white
fruit pulp, the other with short stems and pink fruit pulp. All the progeny of this cross had
tall stems and pink fruit pulp. When these progeny were selfed, the resulting F2
generation showed the following phenotypes:
tall, white 500
tall, pink 1033
short, pink 498
a. Which phenotypes are dominant and recessive.
b. Explain these results by labeling the genotypes of the parents, F1 and F2.
9. Suppose that a husband and wife are both heterozygous for a recessive gene for
albinism. If they have dizygotic twins (two-egg), what is the probability that both of the
twins will have the same phenotype with respect to pigmentation.
10. Consider three yellow round peas, labeled A, B, and C. Each was grown into a plant
and crossed with a plant grown from a green wrinkled pea. Exactly 100 peas issuing from
each cross were sorted into phenotypic classes as follows:
A: 51 yellow round; 49 green round
B: 100 yellow round
C: 24 yellow round; 26 yellow wrinkled; 25 green round; 25 green wrinkled
What were the genotypes of A, B, and C? (use gene symbols of your own choosing; be
sure to define each one)
11. Daphne’s sister died of cystic fibrosis as a child. Daphne does not have the disease,
and neither do her parents. Daphne is pregnant with her first child. If you were a genetic
counselor, what would you tell her about the probability that her child will have cystic
fibrosis?
12. Your instructor presents you with a Drosophila with red eyes, as well as a stock of
purebreeding white-eyed flies and another stock of pure-breeding red-eyed flies. You
know that the presence of white eyes in Drosophila is caused by homozygosity for a
recessive allele. How would you determine whether the single red-eyed fly was
heterozygous for the white-eyed allele?
13. What is the mode of inheritance for the pedigrees below? Write the genotypes of as
many individuals as possible.