Question

The pedigree shown above is based on a family with freckles (filled circles or squares) or lacking freckles (empty circles or squares). Freckles are determined by one gene locus. Which of the following statements about freckles is most likely to be correct?

Answer 1

The possible answers are 
A. Freckles are a sex-linked recessive allele.
B. Freckles are an autosomal recessive allele.
C. Freckles are a sex-linked dominant allele.
D. Freckles are an autosomal dominant allele.

The correct answer is D Freckles are an autosomal dominant allele.

From what we can see on the pedigree, in the first generation the father has freckles and the mother doesn't. All of their children have freckles. This excludes the possibility of sex-linked alleles because the father gives different sex chromosomes to his children ( X to his daughters and Y to his sons) and they all have freckles. Therefore, this must be an autosomal allele.
Since all of their offspring have the freckles, the trait must be dominant and the only possible genotypes of the parents are FF  and ff. The father has two dominant alleles for freckles ( FF a double dominant homozygote) and the mother has two recessive alleles (ff a double recessive homozygote). All of their children then must have the Ff genotype and have freckles because of the dominant expression of the F allele.