Conditional Probability in Lethal Allele Inheritance

Steps:

• Cross two heterozygotes (Ss × Ss): offspring ratios are ¼ SS (thick), ½ Ss (thin), ¼ ss (non-surviving).
• Surviving offspring total ¾ (SS + Ss).
• Heterozygotes (Ss) among survivors: (½) / (¾) = ⅔.
• Thus, probability a surviving snail is heterozygous is 0.67.

Why D is correct:

• In lethal recessive inheritance, the conditional probability formula—P(heterozygote | survival) = P(heterozygote and survival) / P(survival)—yields ⅔ for Ss among viable offspring.

Why the others are wrong:

• A: Incorrect, as heterozygotes survive (ss is lethal).
• B: Matches frequency of SS overall, not conditional on survival.
• C: Matches unconditional heterozygote frequency (½), ignoring lethality.

Final answer: D