USABO - USA Biology Olympiad Classical and Mendelian Genetics Questions and Answers — 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)?
- 20%
- 40%
- 80% (Correct answer)
- 50%
Correct answer: 80%
A recombination frequency of 20% (20 map units) means that 20% of the gametes produced by the heterozygous parent will be recombinant (Tp and tP) and 80% will be parental (TP and tp). Since the test cross parent (ttpp) only produces one type of gamete (tp), the phenotypes of the offspring directly reflect the gametes from the heterozygous parent. Therefore, 80% of the offspring will exhibit the parental phenotypes.
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?
- 3 green : 1 blue
- 1 blue : 2 green : 1 yellow (Correct answer)
- 1 blue : 1 yellow
- All green
Correct answer: 1 blue : 2 green : 1 yellow
This is a classic example of incomplete dominance. The cross between two heterozygous F1 individuals (BY x BY) results in a genotypic ratio of 1 BB : 2 BY : 1 YY. Because of incomplete dominance, each genotype has a unique phenotype: BB is blue, BY is green (a blend), and YY is yellow. Therefore, the phenotypic ratio is 1 blue : 2 green : 1 yellow.
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?
- 100%
- 0%
- 50%
- 25% (Correct answer)
Correct answer: 25%
The woman's genotype is X(C)X(c) and the man's is X(C)Y. The probability of having a son is 1/2. Of the sons, there is a 1/2 chance they will inherit the mother's X(c) chromosome. The overall probability is the product of these two independent probabilities: (1/2 chance of a son) * (1/2 chance the son gets the recessive allele) = 1/4 or 25%.
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?
- 9 black : 3 brown : 4 yellow (Correct answer)
- 9 black : 3 yellow : 3 brown : 1 golden
- 1 black : 2 brown : 1 yellow
- 9 black : 7 yellow
Correct answer: 9 black : 3 brown : 4 yellow
A dihybrid cross of BbEe x BbEe produces offspring with genotypes in a 9:3:3:1 ratio (B_E_ : bbE_ : B_ee : bbee). The phenotypes are: B_E_ = Black; bbE_ = Brown; B_ee = Yellow; bbee = Yellow. Because the 'ee' genotype masks the B/b locus, both B_ee and bbee genotypes result in a yellow coat. Therefore, the phenotypic ratio is 9 Black : 3 Brown : (3+1) Yellow, which simplifies to 9:3:4.
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?
- X-linked recessive
- Autosomal recessive
- Y-linked
- Autosomal dominant (Correct answer)
Correct answer: Autosomal dominant
The key features described—trait appearing in every generation, affected individuals having an affected parent, and equal prevalence in both sexes—are hallmarks of autosomal dominant inheritance. The observation that affected fathers can pass the trait to their sons rules out X-linked inheritance (fathers pass a Y to their sons) and confirms it is autosomal. Autosomal recessive traits often skip generations.
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)?
- 1/16
- 9/16
- 1/4 (Correct answer)
- 1/8
Correct answer: 1/4
In a dihybrid cross (RrYy x RrYy), we can consider each trait separately. The probability of getting Rr from a Rr x Rr cross is 1/2. Similarly, the probability of getting Yy from a Yy x Yy cross is 1/2. According to the law of independent assortment, the probability of both events occurring together is the product of their individual probabilities: (1/2) * (1/2) = 1/4.
USABO - USA Biology Olympiad Classical and Mendelian Genetics Questions and Answers
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)?