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Chapter 4: Problem 12

In Shorthorn cattle, coat color is controlled by a single gene that can exist as a red allele \((R)\) or a white allele \((r)\). The heterozygotes \((R r)\) have a color called roan that looks less red than the \(R R\) homozygotes. However, when examined carefully, the roan phenotype in cattle is actually due to a mixture of completely red hairs and completely white hairs. Should this be called incomplete dominance, codominance, or something else? Explain your reasoning.

Short Answer

Expert verified
The color pattern of the Shorthorn cattle should be considered as codominance. This is because both the red and white alleles appear simultaneously in the phenotype, rather than a blending of the two, which would be characterized as incomplete dominance.

Step by step solution

01

Understanding Incomplete dominance

Incomplete dominance is when the phenotype of the heterozygous genotype is an intermediate of the phenotypes of the homozygous genotypes. In the context of this problem, that would mean roan cattle would have a uniformly 'medium' red coat, neither as red as the homozygous red cattle nor as white as white cattle.
02

Understanding Codominance

Codominance is when both alleles for a gene are expressed fully and simultaneously. In context of this problem, that would mean roan cattle would have patches of red fur and patches of white fur - which is what we observe.
03

Comparing with Observations

Observations show that roan cattle have a mixture of completely red hairs and completely white hairs. This implies that both the red allele and the white allele are expressed fully and simultaneously. Therefore, this is not a case of incomplete dominance, but rather of codominance.

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