Black color in horses is governed primarily by a recessive allele (a) at the A l
ID: 143336 • Letter: B
Question
Black color in horses is governed primarily by a recessive allele (a) at the A locus. Horses with AA and Aa genotypes are nonblack colors, such as bay, while the aa homozygotes are black all over. In an online conversation, one person asked why there are relatively few black horses of the Arabian breed. One response was "Black is a rare color because it is recessive. More Arabians are bay or gray because those colors are dominant." How would you respond to this speaker's comment? Choose the ONE best answer. The speaker's answer is only partially correct; black is rare not only because allele a is recessive, but also because recessive traits are usually selected against. The speaker is correct. If the a allele responsible for black color is recessive, it will be masked by other alleles, OC.The speaker is incorrect, because recessive alleles tend to be under positive selection, so black horses are OD so you just won't see black horses very often. expected to be common, not rare. The speaker is incorrect, because whether an allele is dominant or recessive does not tell us anything about how common it is. E. The speaker is correct, because by definition recessive alleles are rarer than dominant alleles. OF The speaker's answer is only partially correct, because they did not mention that a recessive allele will decrease in frequency through time simply through the repeated process of meiosis and fertilization in a population at Hardy-Weinberg equilibrium.Explanation / Answer
Answer: Option F is correct.
Explanation:
Parental cross: AA X aa
Progeny: Aa
F1 cross: Aa X Aa
Progeny: AA Aa Aa aa
Phenotypic ratio = 3:1 (3 gray and one black)
Since the black coat color is a recessive trait, the genotype must be homozygous recessive (aa).
On the other hand, gray phenotype can be conferred by homozygous (AA) or heterozygous (Aa) genotypes.
So, the relative proportion of black horses is low as compared to gray horses.
Option A is incorrect because recessive alleles are not always under negative selection pressure.
Option B is incorrect because allele a is recessive only to allele A and not to any allele of other genes.
Option C is incorrect because recessive alleles are not always under positive selection pressure. Recessive traits are less common as compared to dominant traits.
Option D is incorrect because the frequency of an allele in a population depends upon the selection pressure as well as its dominance/recessive nature.
Option E is incorrect because the frequency of a recessive allele can be greater than a dominant allele if it is strongly selected.
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