Final answer:
Yellow fur color in Labrador retrievers is expressed when the dog has two copies of the recessive e allele in its genotype (ee_). This is an example of epistasis, not a simple dominant-recessive relationship, since an interaction between two gene loci occurs. In Mendelian genetics, the Law of Dominance explains how dominant alleles mask the expression of recessive alleles.
Step-by-step explanation:
In genetics, certain traits are controlled by alleles, where one may be dominant and the other recessive. Yellow fur color in Labrador retrievers is due to a recessive allele. Specifically, Labrador retrievers with the genotype ee_ (where _ can be either allele B or b) will be yellow, indicating that e is the recessive allele when compared to E. Contrastingly, B_E_ dogs are black and bbE_ dogs are chocolate, illustrating an epistasis interaction where one gene locus masks or modifies the effect of a second locus.
The Law of Dominance was proposed by Gregor Mendel and suggests that when two different alleles are present in a pair, the dominant allele is expressed in the phenotype while the recessive allele is only expressed when it is present in both copies of the gene pair.
For example in pea plants, which were studied by Mendel, the dominant yellow allele Y is expressed over the recessive green allele y. Therefore, plants with YY or Yy genotypes will have yellow peas, while only the yy genotype will result in green peas.