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