Rationale
A dog with a tan fur phenotype has the genotype bb.
In this scenario, the black allele (B) is completely dominant over the tan allele (b). A tan fur phenotype indicates that the dog does not possess the dominant black allele, which means it must have two copies of the recessive allele, resulting in the genotype bb.
A) BB
The genotype BB represents a dog that carries two copies of the dominant black allele. Such a dog would display a black fur phenotype, not a tan one. Therefore, BB cannot correspond to a dog with a tan fur phenotype, as it does not include any recessive alleles.
B) Bb
The genotype Bb indicates that the dog carries one dominant black allele and one recessive tan allele. In this case, the presence of the dominant allele would result in a black fur phenotype, not tan. Thus, this genotype also cannot account for a tan fur phenotype.
C) bb
This is the correct genotype for a dog with a tan fur phenotype. The only way a dog can exhibit tan fur is if it has two copies of the recessive allele (b), represented as bb. This genotype confirms the absence of the dominant black allele.
D) Not enough information
This choice suggests that there isn't sufficient data to determine the genotype. However, the question clearly states that the dog has a tan fur phenotype, which directly indicates the presence of two recessive alleles (bb). Thus, there is indeed enough information to conclude the genotype.
Conclusion
In genetic scenarios involving complete dominance, phenotypes directly inform us about genotypes. A tan fur phenotype in dogs, governed by the alleles black (B) and tan (b), conclusively indicates a genotype of bb, as only the recessive alleles can manifest as tan fur. Understanding these relationships is fundamental in genetics, allowing for accurate predictions of phenotypic expression based on allelic combinations.
