Final answer:
The two possible genotypes for a man without a hitchhiker's thumb are homozygous dominant (HH) or heterozygous (Hh), as hitchhiker's thumb is a dominant trait.
Step-by-step explanation:
If a man does not have a hitchhiker's thumb, the two possible genotypes are HH (homozygous dominant) or Hh (heterozygous). Since hitchhiker's thumb is a dominant trait, not having it means the man must have two non-hitchhiker's thumb alleles, which would be hh. However, if a person does not have a hitchhiker's thumb, it means they do not possess the recessive allele in homozygous form, hence they cannot have hh. Instead, they could either have two dominant alleles (HH) or one dominant and one recessive allele (Hh), meaning they express the dominant non-hitchhiker's thumb trait.
To clarify, a dominant allele (represented by 'H') masks the presence of a recessive allele (represented by 'h'). In autosomal traits like the hitchhiker's thumb, both men and women have an equal chance of inheriting the trait, since it is not linked to sex chromosomes but instead comes from the autosomes, or non-sex chromosomes. A person with the genotype HH has two copies of the dominant allele, while a person with genotype Hh has one dominant and one recessive allele.