This question involves a gene coding for feather colour in some birds in which the black allele (B) and white allele (W) is involved. According to the question the gene coding for feather colour exhibits co-dominance i.e. both the black (B) and white (W) allele are expressed in a heterozygous state (BW) to form an erminette phenotype.
Hence if the neither alleles is dominant or recessive over the other. The black phenotype will only be expressed when the genotype is BB.
The white phenotype will only be expressed when the genotype is WW
The combination of both alleles which results in the simultaneous expression of both in the erminette phenotype will be BW genotype.
