[Campbell Biology P.333] In the pedigree for widow's peak Figure 14.15a , the grandparents 1... | Practice Question

In the pedigree for widow's peak Figure 14.15a , the grandparents 1st generation with widow's peaks must have the genotype Ww. Why can't they be WW?

  • A: Because they have offspring who are homozygous recessive (ww).
  • B: Because widow's peak is a recessive trait.
  • C: Because all individuals in the second generation have a widow's peak.
  • D: Because their parents (not shown) did not have widow's peaks.

Explanation

A parent with a dominant phenotype who has an offspring with a recessive phenotype must be heterozygous. If a grandparent were homozygous dominant WW , they could only pass on a W allele, and all of their children would have a widow's peak. Since they have children with the ww genotype, they must have been able to pass on a w allele.