Pedigree Chart

Let's symbolize S the dominant allele for the straight thumb and s the recessive allele for the  curved “hitchhiker's thumb”.

IV 1 and IV 3 must have 2 s alleles (cause they have the phenotype of the recessive allele) so their parents must have at least one s each. Also their parents have the dominant phenotype so they must have at least one S each.
So, III 1 and III 2 are Ss.
VI 2 and IV 4 could be either Ss or SS (because they have the dominant phenotype)
Now, lets start from I generation.
I 1 has genotype ss (recessive phenotype)
I 2 can be either SS or Ss (dominant phenotype)
So, I 1 has passed 1 s allele to II 1, II 2 and II 3. But II 1, II 2 and II 3 has the dominant phenotype so they have inherted one S allele from I 2.
So II 1, II 2 and II 3 are Ss.
II 4 could either be SS or Ss (dominant phenotype)
III 3 could be either SS or Ss (dominant phenotype)

Now lets spent some more time explaining why this allele is not x-linked.
Let's suppose that the allele is x-linked and the recessive allele is Xa.
Then IV3 would have 2 recessive alleles XaXa.
But that means that her father would have at least one ressecive allele Xa so his genotype would be XaY, and he must have been expressing the recessive phenotype. But that doesn't happen (since III 1 is expressing the dominant phenotype), so the allele can't be x-linked.


(Whose genotypes can you not be certain of? It's the people's that genotype can be either SS or Ss)

Thank you so much.