The approximate ratio of the progeny is about 1:2:1, which gives us the clue that they are heterozygous in nature since the parents are said to be formed of only one type of phenotype. The reason for the formation of a different type of phenotype that is diverged from the parent is as a result of incomplete dominance or codominance. In the progeny among the three, one of the erminette phenotype contains an intermediate finding itself between fully black and fully white, which shows that they are incompletely dominant. Also when then are both black and white feathers it proves the concept of codominance. The genetic pattern suggested would depend basically on whether the entire bird is looked upon (incomplete dominance) or only a part of it (codominant).
To make sure that the progeny are heterozygous, a cross with the either of the homozygous strains could be made, which would provide a progeny in the ratio of 1:1 in both the cases.