Awhite rooster mates with a black hen. the offspring is gray in color. what is this phenomenon called?

This phenomenon is called Co-dominance. This occurs when the alleles for a certain characteristic shared equal contribution to the offspring. The alleles became neither recessive nor dominant. In the example, the phenotype White (genotype WW) mated with the phenotype Black (genotype BB). With co-dominance happening, the offspring will become a phenotype Grey (genotype BW).

Its probably the possible factors tt, tt, tt,