In humans, smooth chin and straight hairline are each inherited as autosomal recessive traits - VCE - SSCE Biology - Question 18 - 2009 - Paper 1

To determine the father's genotype, we need to analyze the traits of the family members carefully. The mother and son both exhibit a smooth chin and a straight hairline. Smooth chin is recessive (represented by the allele 's'), indicating that both the mother and son must have the genotype 'ss' for the chin trait.

For the hairline, the son has a straight hairline, which is also recessive (represented by the allele 'w'). Therefore, the son’s genotype for hairline must also be 'ww', leading us to conclude that the mother has to have contributed 'w' alleles as well.

We know that the father has a cleft chin (which is dominant, represented as 'S') and a widow's peak (which is also dominant, represented as 'W'). This indicates that the father must have a genotype including at least one capital 'S' and one capital 'W'.

Since the son has a smooth chin ('ss'), the father must carry one recessive allele 's' to pass this trait to his son. Thus, his genotype for chin must be 'Ss'.

For the hairline, the son has 'ww', indicating the father must carry a recessive 'w' allele too. Thus, the father's genotype for hairline must also be 'Ww'.

Putting all this together, the father's genotype is 'Ss Ww', corresponding with option A.