In humans, a widow’s peak is caused by a dominant allele (W), and a continuous hairline is caused by a recessive allele (w). Short fingers are caused by a dominant allele (S), and long fingers are caused by a recessive allele (s). Suppose a woman with a continuous hairline and short fingers and a man with a widow’s peak and long fingers have three children. One child has short fingers and a widow’s peak, one has long fingers and a widow’s peak, and one has long fingers and a continuous hairline. What are the genotypes of the parents?

Question

BlackTom AI · Accepted Answer

We start by restating what the problem gives and what we know about the alleles: for hairline, widow’s peak (W) is dominant over continuous hairline (w); for fingers, short fingers (S) are dominant over long fingers (s). The woman has a continuous hairline and short fingers, so her phenotype is ww for hairline and S? for fingers. Because short fingers are dominant, she must have at least one S allele; thus her possible genotypes for fingers are SS or Ss. The man has a widow’s peak and long fingers, so his phenotype is W? for hairline and ss for fingers. Since long fingers are recessive, his genotype for fingers is ss; for hairline, he could be WW or Ww.

Option by option analysis:
- Option A: Female WwSs, male WwSs
  This contradicts the given phenotypes. The woman’s phenotype is continuous hairline, which requires ww, not W_. Therefore this option is inconsistent with the problem statement, making it impossible.
  Additionally, a mother with W and w alleles would not produce a continuous-hairline offspring with ww phenotype. The parent genotypes do not align with the stated traits.

- Option B: Female wwSs, male Wwss
  This matches the described phenotypes: the woman is ww for hairline (continuous) and Ss for fingers (short fingers since S is dominant), and the man is Ww for hairline (widow’s peak) and ss for fingers (long fingers). When we cross wwSs × Wwss, the possible gametes are:
  - mother: wS or ws
  - father: Ws or ws
  Offspring combinations include:
  - WwSs (short fingers, widow’s peak)
  - Wwss (long fingers, widow’s peak)
  - wwss (long fingers, continuous hairline)
  This reproduces all three observed offspring phenotypes: short fingers with widow’s peak, long fingers with widow’s peak, and long fingers with continuous hairline. Therefore this option is internally consistent and feasible.

- Option C: Female wwSs, male WWss
  Here the father is WW for hairline and ss for fingers, the mother is wwSs. The cross would yield offspring with hairline WW or Ww (both W-bearing) and fingers either Ss or ss depending on the maternal S allele. Because the father provides only s alleles for fingers, all children would inherit at least one s, giving at least some long-finger phenotypes, but the widow’s-peak genotype would always be W_ from the father, and the combinations would not necessarily produce the exact observed trio (one with short fingers and widow’s peak, one with long fingers and widow’s peak, and one with long fingers and continuous hairline) in a way that matches a WW parent contributing only W alleles for hairline. The specific arrangement becomes inconsistent with the precise three-child phenotype set.

- Option D: Female wwSS, male WWss
  The mother is ww for hairline and SS for fingers, the father is WW for hairline and ss for fingers. Offspring would all be W_ for hairline and Ss for fingers, yielding all children with widow’s peak and short fingers only. This cannot produce the observed variety of phenotypes (including long fingers and continuous hairline), so this option is incompatible with the data.

In summary, evaluating each option against the given parental phenotypes and the observed offspring phenotypes shows that Option B is the only one that simultaneously satisfies all constraints: the cross wwSs × Wwss can produce the three observed child phenotypes (short fingers with widow’s peak; long fingers with widow’s peak; long fingers with continuous hairline). The other options fail to align with the specified parental traits or with the exact set of offspring outcomes.

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