The answer is two. In incomplete dominance, heterozygotes have a trait that is intermediary between dominant and recessive homozygote. Let allele A be dominant and allele a recessive allele. The genotypes and phenotypes are as following: genotype: AA - phenotype: dominant homozygote with red feathers, genotype: Aa - heterozygote with purple feathers, and genotype: aa - recessive homozygote with blue feathers. In the cross of two purple birds (Aa x Aa), we have 1 bird with genotype AA and red feathers, 2 birds with genotype Aa and purple feathers, and 1 bird with genotype aa with blue feathers.
