2Fe2(SO4)3
Iron:
Sulfur:
Oxygen.

If the mouse with black fur is heterozygous, this indicates the black is the phenotype formed by the dominant alleles. Let B be the dominant allele for black fur and b be the recessive allele for white fur. If the mouse with black fur is heteozygous, it would have the genotype Bb and the mouse with white fur would have the genotype bb. Bb x bb would result in offspring genotypes of Bb (black) and bb (white) in equal ratio. Comparatively, if the black fur mouse was homozygous dominant BB, BBx bb would result in all offspring genotype of Bb (black)

Hence the presence of offsprings with white fur would indicate that the mouse with black fur was heterozygous.