For shorthorn cattle, the mating of a red bull (R) and a white cow (W) produces a calf that is described as a roan (RW). Roan results from intermingled red and white hairs. What would be the expected results for the mating of a red bull and a white cow?

The question asked is wrong, it ought to be

What would be the expected results for the mating of a roan bull and a roan cow?

The answer is a phenotypic and genotypic ratio 1:2:1 consisting of 1 red: 2 roan : 1 white

Explanation:

The genetic scenario described in this question is called INCOMPLETE DOMINANCE, a case where an allele does not completely mask the expression of its allele pair, instead an intermediate phenotype is produced. In this case, the red (R) and white (W) alleles are incomplety dominant over one another. They produce the ROAN (RW) phenotype.

In a cross between a roan bull (RW) and a roan cow (RW) i.e. heterozygous, each allele will separate into gametes according to the LAW OF SEGREGATION, which then combines randomly to form four possible offsprings. The offsprings will possess the genotypes: RR, RW, RW, and WW.

RR (1) Red offspring

RW (2) Roan offspring

WW (1) White offspring

The cross will produce a genotypic ratio 1:2:1 i.e. 1 Red, 2 Roans, 1 White.

R. W

R RR RW

W RW WW

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