Black color in horses is governed primarily by a recessive allele (a) at the A l
ID: 146405 • 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. A- 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, so you just won't see black horses very often. C. The speaker is incorrect, because recessive alleles tend to be under positive selection, so black horses are expected to be common, not rare D.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. O F. The speaker's answer is only partially correct, because they did not mention that a recessive allele will decrease in frequency through time simply througlh the repeated process of meiosis and fertilization in a population at Hardy- Weinberg equilibrium.Explanation / Answer
Recessive phenotype occurs only in homozygous recessive phenotype whereas the dominant phenotype occurs in both homozygous and heterozygous condition. The frequencies of heterozygous phenotypes will increase from generation to generations because of positive selection. Because of the above mentioned two factors, it would be rare to see the homozygous recessive trait after few generations. So, the answer for this question is option B.
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