Albinism is a recessive trait. A man and woman who both have normal pigmentation have one child out of three who has albinism (without melanin pigmentation). What are the genotypes of this child's parents?

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The question describes albinism as an autosomal recessive trait. When both parents have normal pigmentation but have a child with albinism, it implies both parents carry the recessive allele (they are heterozygous). In genetic terms, Aa x Aa can produce three possible genotypes among offspring: AA (normal, non-carrier) 1/4, Aa (normal pigmentation but carrier) 1/2, and aa (albino) 1/4. The observation of an affected child is consistent with this cross, and the proportion (one affected out of three) aligns with expected probabilities, considering small family size.

Option 1: 'One parent must be homozygous dominant; the other parent must be heterozygous.' If one parent is AA and the other Aa, all offspring would be either AA or Aa, with no possibility of any aa affected child. This makes the scenario impossible, so this option is incorrect.

Option 2: 'One parent must be homozygous for the recessive allele; the other parent can be homozygous dominant, homozygous recessive, or heterozygous.' If one parent were aa, and the other were AA, Aa, or aa, there would still be no way to produce an albino child unless both contributed a recessive allele. The only consistent pairing that yields an affected child is Aa x Aa. Therefore this option is incorrect as stated.

Option 3: 'Both parents must be heterozygous.' This exactly matches the autosomal recessive inheritance pattern needed to produce an albino child when both parents have normal pigmentation. Aa x Aa yields the possibility of an affected child (aa), making this option correct.

Option 4: 'One parent must be heterozygous; the other parent can be homozygous dominant, homozygous recessive, or heterozygous.' As noted for Option 2, if the other parent is not also a carrier (Aa), the chance of an affected child would be zero or inconsistent with the observation. Since the other parent being AA or Aa or aa changes the probabilities and, in many cases, eliminates the possibility of an affected child, this option is incorrect.

Bio 93 Section CDE 2025 Final Exam Practice, Part 1

Albinism is a recessive trait. A man and woman who both have normal pigmentation have one child out of three who has albinism (without melanin pigmentation). What are the genotypes of this child's parents?

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