MCAT Biology Question 167: Answer and Explanation
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Question: 167
2. Suppose that in humans, the allele for black hair (B) is dominant to the allele for brown hair (b), and the allele for curly hair (C) is dominant to the allele for straight hair (c). When a person of unknown genotype is crossed against a person with straight, brown hair, the phenotypic ratio is as follows:
25% curly black hair
25% straight black hair
25% curly brown hair
25% straight brown hair
What is the genotype of the unknown parent?
- A. BbCC
- B. bbCc
- C. Bbcc
- D. BbCc
Correct Answer: D
Explanation:
In this dihybrid problem, a doubly recessive individual is crossed with an individual of unknown genotype; this is known as a test cross. The straight- and brown-haired individual has the genotype bbcc and can thus only produce gametes carrying bc. Looking at the F1 offspring, there is a 1:1:1:1 phenotypic ratio. The fact that both the dominant and recessive traits are present in the offspring means that the unknown parental genotype must contain both dominant and recessive alleles for each trait. The unknown parental genotype must therefore be BbCc. If you want to double-check the answer, you can work out the Punnett square for the cross BbCc × bbcc:
