Allele will be both heterozygous dominant and homozygous recessive in the progeny in the ratio of 1:1 ie 50 per-cent chance of both the alleles be expressed.
Phenotype: 50 per-cent chance of both black and gray colour mice
Genotype: Gg, gg 1:1
Explanation:
Dominant G
Recessive g
Baby mice produced from the cross between:
gg X Gg
Gg gg Gg gg
Genotype would be Gg gg in the ratio of 1:1
Phenotype would be Gray if dominant heterozygous gene having a dominant allele  appear or black if recessive homozygous appears in the progeny mice. It will be in the ratio of 2:2.