The dominant allele is for black fur. the recessive allele is for white fur.
a mouse that is homozygous for the dominant trait has the genotype
it has
a mouse that is homozygous for the recessive trait has the genotype
it has

FF , Black fur.

ff, white fur.

Explanation:

Genotype may be defined as the genetic constitution of the organism whereas phenotype depends on the genotype for the physical expression of the trait.

Let the genotype of the dominant fur (black ) be D and the genotype of white fur (recessive)  is f. homozygous means the individual should have the same allele for the trait. A mouse with homozygous dominant has genotype FF . This mouse has black fur. A mouse homozygous for the recessive trait has the genotype f. This mouse has white fur.

Thus, the answer is FF , Black fur.

ff, white fur.