Albinism in a Family: Recessive Inheritance and Sex Linkage
In a family, both parents have normal skin, hair and eye pigmentation. They have three children: a daughter with albinism, a son with albinism, and a daughter with normal pigmentation. The mother's brother has albinism; the father has no known family history of albinism. Albinism is caused by a recessive allele. A genetic counsellor explains that the parents are both carriers. The parents ask about the chance that their next child will have albinism and whether the condition is sex-linked. They also ask why neither parent is affected. The counsellor draws a genetic diagram using A for the normal allele and a for the albinism allele. The parents want to know whether their unaffected daughter could pass albinism to her children.
1. Using A for the normal allele and a for the albinism allele, write the genotypes of the parents and each child. Explain why neither parent has albinism but two children do.
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Both parents must be Aa because they are unaffected but have affected children. The affected daughter and affected son are aa; the unaffected daughter is A_ (AA or Aa, and is a carrier with probability 2/3 if she is unaffected). Each affected child receives one a from each parent. Albinism is recessive, so a person needs two a alleles to show it. The parents each have one A, which masks the a allele and allows normal pigment. They are carriers, not affected.
2. Calculate the probability that the next child will have albinism and the probability that the next child will be an unaffected carrier. Show your reasoning.
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Aa × Aa gives a 1:2:1 ratio: 1 AA : 2 Aa : 1 aa. So P(albinism, aa) = 1/4 = 25%. P(unaffected carrier, Aa) = 2/4 = 1/2 = 50%. P(unaffected, AA or Aa) = 3/4 = 75%. Each child is an independent event, so the previous two affected children do not change these probabilities for the next child.
3. Explain why the evidence does not support sex-linked inheritance. Include what you would expect if albinism were X-linked recessive and the mother were a carrier.
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If albinism were X-linked recessive, the mother could be X^A X^a and the father X^A Y. Daughters would receive X^A from the father, so all daughters would be unaffected; an affected daughter would need X^a from her father, meaning he would be X^a Y and affected. Here the father is normal, yet a daughter is affected. That is impossible for X-linked recessive inheritance. The affected son is possible under X-linked inheritance, but the affected daughter rules it out. Autosomal recessive fits both sexes and the carrier parents.