The Family with a Recessive Trait
A couple, both with normal skin pigmentation, have a child with albinism, a condition caused by a recessive allele (a). The couple's second child has normal pigmentation. The couple wants to know the chance that their third child will have albinism. The mother's brother has albinism, but the father has no family history of albinism. The couple also has a dog with a similar condition, but the dog is not related to them. A genetic counsellor explains that albinism is autosomal recessive and that both parents must be carriers (Aa). The couple asks: 'If our second child is normal, does that mean the third child is more likely to be affected?'
1. Explain why both parents must be carriers (Aa) even though they have normal pigmentation. Use a genetic diagram to show the possible genotypes of their children.
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Albinism is caused by a recessive allele, so a person must have two copies (aa) to show the condition. The parents both have normal pigmentation, so they must have at least one dominant allele (A). Since they have a child with albinism (aa), each parent must have passed on an 'a' allele. Therefore, both parents are heterozygous (Aa). A genetic diagram (Punnett square) shows the cross Aa × Aa. The possible gametes are A and a from each parent. The offspring genotypes are AA (25%), Aa (50%), and aa (25%). The phenotypes are normal (AA and Aa) 75% and albino (aa) 25%. So each child has a 1 in 4 chance of albinism, regardless of the previous child's phenotype.
2. The couple asks: 'If our second child is normal, does that mean the third child is more likely to be affected?' Answer their question and explain the probability for the third child.
Read the worked answer
No, the second child's normal pigmentation does not change the chance for the third child. Each pregnancy is an independent event. The parents are both Aa, so each child has a 25% chance of being aa (albinism) and a 75% chance of being normal (AA or Aa), regardless of previous children. The second child being normal is consistent with the 75% chance. Some people mistakenly think that after a normal child, an affected child is 'due', but this is not true. The probability remains 1 in 4 for each child. The mother's brother having albinism supports that the mother is a carrier, but the father's carrier status is confirmed by having an affected child. So the third child has a 25% chance of albinism.