Why is the genotype of the guinea pig with black fur expressed as (B)?

The genotype of the guinea pig with black fur is expressed as (B) because it represents the dominant allele for fur color. In genetic notation, uppercase letters indicate dominant traits, while lowercase letters represent recessive traits. Since black fur is a dominant characteristic, the presence of at least one (B) allele will result in the expression of that trait, regardless of the other allele's state. Thus, a guinea pig with the genotype (BB) or (Bb) will both display black fur.