Question

A hornless bull is crossed with three cows, A, B, and C. Cow A is horned and produces calf A’, which is also horned. Cow B is hornless and produces calf B’, which is horned. Cow C is horned and produces calf C’, which is hornless. Which is the dominant trait? Explain your reasoning. Give the genotypes of all seven animals.

Answer 1

The dominant trait is hornless.

If this is correct, then the original bull is a heterozygote:
H = hornless
h = horned
Bull is Hh

Cross 1: Hh x hh (horned female) - offspring is hh (horned)

Cross 2: Hh x Hh (hornless female) - offspring is hh (horned)

Cross 3: Hh x hh (horned female) - offspring is Hh (hornless)

Cross 3 is the key to confirming this because the only way to get a hornless offspring would be if hornless is recessive.