Question

In cattle, the allele for a black-haired coat (B) is dominant over the allele for a red-haired coat (b). A cross between two cattle results in a calf with a black-haired coat.
What is a possible genotype of the calf?

A. (B)
B. (b)
C. (Bb)
D. (bb)

Answer 1

Answer:

C. Bb

General Formulas and Concepts:

Genetic I

Punnett Squares

Alleles

• Genotypes
• Phenotypes

Mendelian Genetics

• Complete dominance

Explanation:

We are given that the allele B (black-haired coat) is dominant over the allele b (red-haired coat). That means it is a complete dominance and we do not have to worry about incomplete or co-dominance.

We know from Mendelian genetics that if there is a dominant allele present, then the genotype has to include the dominant allele, in this case B. The phenotype must also match the dominant allele, in this case a black-haired coat.

We are given a calf with a black-haired coat. This is the phenotype for the dominant allele B. Therefore we can eliminate answer choices B and D since they are both recessive alleles. We are left with answer choices A and C.

Since alleles come from parents, you will receive 1 allele from each parent (think about cell division and gamete production). So, if you are an offspring, you will have 2 parents and therefore 2 alleles. Therefore, C is the correct answer.