IGA 8/e Chapter 2

Chapter Two 47they are expecting a child. They find out that they are distantly related. Part ofthe family tree is shown here.How would you advise the parents about the probability of their child being adeaf boy, a deaf girl, a normal boy, or a normal girl? Be sure to state anyassumptions that you make.Answer: Because the disorder is X-linked recessive, the affected male had tohave received the allele, a, from the female common ancestor in the firstgeneration. The probability that the affected man’s wife also carries the a alleleis the probability that she also received it from the female common ancestor.That probability is 1 / 8 .The probability that the couple will have an affected boy is:p(father donates Y) p(the mother has a) p(mother donates a)1/ 2 1 / 8 1 / 2 = 1 / 32The probability that the couple will have an affected girl is:p(father donates X a ) p(the mother has a) p(mother donates a)1/ 2 1 / 8 1 / 2 = 1 / 32The probability of normal children is:= 1 – p(affected children)= 1 – p(affected male) – p(affected female)= 1 – 1 / 32 – 1 / 32 = 30 / 32 = 15 / 16Half the normal children will be boys, with a probability of 15 / 32 , and half thenormal children will be girls, with a probability of 15 / 32 .80. The accompanying pedigree shows a very unusual inheritance pattern thatactually did exist. All progeny are shown, but the fathers in each mating havebeen omitted to draw attention to the remarkable pattern.