[Campbell Biology P.337] Suppose the couple, Tyler and Lily, have a first child who is born ... | Practice Question

Suppose the couple, Tyler and Lily, have a first child who is born with the recessive disease. What is the probability that their second child will also have the disease?

  • A: 1/9
  • B: 1/4
  • C: 2/3
  • D: 1/16

Explanation

The birth of an affected child genotype aa definitively proves that both parents must be heterozygous carriers Aa . Once this is known, the probability for any subsequent child having the disease is simply the result of an Aa x Aa cross, which is 1/4. Each birth is an independent event.