MCAT Biology Question 169: Answer and Explanation

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Question: 169

4. If a test cross on a species of plants reveals the appearance of a recessive phenotype in the offspring, what must be true of the phenotypically dominant parent?

  • A. It must be genotypically heterozygous.
  • B. It must be genotypically homozygous.
  • C. It could be either genotypically heterozygous or homozygous.
  • D. It must have the same genotype as the test cross control parent.

Correct Answer: A

Explanation:

The control parent in a test cross is always recessive. Therefore, if the test parent is phenotypically dominant, yet can provide a recessive allele (as evidenced by the presence of recessive children), then the parent must have both a dominant and recessive allele. Therefore, this test parent must by heterozygous.

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