Based on this information, what is Alvin's phenotype for the fur color trait?

 - light fur
 - dark fur
Alvin's genotype for the fur color gene is ff. Alvin's genotype of ff has only f alleles. The f allele is for light fur. So, Alvin's phenotype for the fur color trait must be light fur.
To check this answer, consider whether Alvin's alleles are dominant or recessive. The allele for light fur (f) is recessive to the allele for dark fur (F). This means F is a dominant allele, and f is a recessive allele.
Alvin's genotype of ff has only recessive alleles. An organism with only recessive alleles for a gene will have the recessive allele's version of the trait. So, Alvin's phenotype for the fur color trait must be light fur.