Allele for gray fur is A (dominant)
Allele for white fur is a (recessive)
Genotype of heterozygous is Aa (heterozygous is gray, because its allele is dominant)
If we cross two heterozygous
Parents: Aa x Aa
F1: AA Aa Aa aa
The ratio between phenotypes is 3:1 (gray:white)
AA Aa Aa are with gray fur
aa is with white fur