USABO - USA Biology Olympiad Classical and Mendelian Genetics Questions and Answers

Question 1

In pea plants, tall (T) is dominant to dwarf (t), and purple flowers (P) are dominant to white flowers (p).
A test cross is performed by mating a plant that is heterozygous for both traits with a plant that is homozygous recessive for both traits.
If the two genes are linked and 20 map units apart, what percentage of the offspring are expected to have a parental phenotype (tall with purple flowers or dwarf with white flowers)?

Accepted Answer

80%

Answer

20%

Answer

40%

Answer

50%

Question 2

In a certain species of fish, the alleles for scale color exhibit incomplete dominance. A blue-scaled fish (BB) is crossed with a yellow-scaled fish (YY). The F1 generation are all green-scaled (BY). If two green-scaled F1 fish are crossed, what is the expected phenotypic ratio in the F2 generation?

Accepted Answer

1 blue : 2 green : 1 yellow

Answer

3 green : 1 blue

Answer

1 blue : 1 yellow

Answer

All green

Question 3

Red-green color blindness is an X-linked recessive trait. A woman who is a carrier for color blindness has children with a man who has normal vision. What is the probability that they will have a son who is color blind?

Accepted Answer

25%

Answer

100%

Answer

0%

Answer

50%

Question 4

In Labrador retrievers, coat color is determined by recessive epistasis. The B/b locus determines pigment color (B=black, b=brown), while the E/e locus determines pigment deposition (E=deposit, e=no deposit). A dog with genotype 'ee' will be yellow regardless of its B/b genotype. If two dogs with the genotype BbEe are crossed, what is the expected phenotypic ratio of their offspring?

Accepted Answer

9 black : 3 brown : 4 yellow

Answer

9 black : 3 yellow : 3 brown : 1 golden

Answer

1 black : 2 brown : 1 yellow

Answer

9 black : 7 yellow

Question 5

A student is analyzing a human pedigree for a rare genetic disorder. She observes that the trait appears in every generation, affects both males and females equally, and that all affected individuals have at least one affected parent. Furthermore, she notes that affected fathers can pass the trait to their sons. Which of the following is the most likely mode of inheritance?

Accepted Answer

Autosomal dominant

Answer

X-linked recessive

Answer

Autosomal recessive

Answer

Y-linked

Question 6

In Mendel's dihybrid cross experiments with pea plants (RrYy x RrYy), he observed that the traits for seed shape (Round/wrinkled) and seed color (Yellow/green) assorted independently. What is the probability of obtaining an F2 offspring that is heterozygous for both traits (RrYy)?

Accepted Answer

1/4

Answer

1/16

Answer

9/16

Answer

1/8