Fur color in mice is affected by a single gene. The gene for fur color has two alleles: B that causes dark brown fur, and b that causes light brown fur. In a mouse with dark brown fur, the genotype could be BB or:

The genotype could be BB or Bb

Explanation:

The genotype could be Bb which means it has heterozygous alleles.

The B which is allele for dark brown fur is dorminant and b for light brown is recessive. In this case, the dorminant allele masked the effect of the recessive allele b which make the dorminant allele B to be expressed phenotypically.

small, big, or no change could occur in the phenotype. it could cause something as simple as a cats ears to curl backwards or something as frighting as death. it could also occur in the protien-coding area and have an effect on that but no effect on the amino acids. what has to occur is a changing in the hox gene or the control gene.