Question

Two cousins marry and their first child has a rare autosomal recessive trait conferring high musical ability. If they plan to have 6 children, compute the expected chance that exactly four of the six children will inherit this trait. Please show me how you got it so I can understand. Also is the chance that they inherit this trait referring to them being homozygous recessive or heterozygous?

Answer 1

As the trait is recessive it needs both the recessive allele to be present for expressing the trait,Let 'a' is the recessive allele for the trait and 'A' is the dominant one. Now for the first child , it needs to have the genotype 'aa' to express the trait.For that both parents of that child must have genotype 'Aa'.No other combination would give the required result.As if mating occurs between two parents having 'Aa' that is Aa×Aa ,it results as AA , 2Aa and aa. so it is 1 among 4 that results 0.25 or 25%.So all the children they will have , have 25% chance to get that trait.

As it is a recessive disorder both the recessive allele must be present so that it will always be homozygous recessive and never be heterozygous as in heterozygous the domint allele will suppress the recessive one from being expressed.