[Campbell Biology P.337] Tyler and Lily each had a brother who died from a recessive lethal ... | Practice Question

Tyler and Lily each had a brother who died from a recessive lethal disease. Neither Tyler nor Lily has the disease. What is the probability that their first child will have the disease?

  • A: 1/4
  • B: 2/3
  • C: 1/9
  • D: 1/8

Explanation

First, determine the probability that Tyler is a carrier. Since his parents must be heterozygous Aa and he is not affected not aa , his possible genotypes are AA or Aa. The probability of being a carrier Aa is 2/3. The same logic applies to Lily. The probability of two carriers having an affected child is 1/4. Using the multiplication rule, the total probability is P Tyler is Aa P Lily is Aa P child is aa = 2/3 2/3 1/4 = 4/36 = 1/9.