In cattle, black hair color is dominant to red hair color. Which of the following would a farmer use to mate with a black haired cow to determine the black cow’s genotype?
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Correct Answer: C. a homozygous red haired cow
In genetics, a test cross is used to determine the genotype of an organism that expresses the dominant phenotype. The black cow could be BB or Bb for the black hair color. If you were to mate the cow with a homozygous black cow BB, all of the offspring would be black haired and this would not help the farmer determine the genotype of the original black cow. Likewise using a heterozygous black cow (Bb), could result in all black offspring, since there is only a 25% chance of getting a red haired cow if the original cow was heterozygous (Bb). A white cow nor an albino cow are not mentioned in the original question and would not be relevant in determining the genotype of the black cow. A test cross, uses a homozygous recessive individual, since this genotype is easily physically identified. If the 50% of the offspring are black and 50% are red, then the black cow was heterozygous (Bb). If all black offspring result, then the black cow was homozygous dominant (BB).
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