This inheritance is termed as Incomplete Dominance.When a red bird and blue bird mate the result is a purple offspring.This is because neither of the two colors are expressed.This type of a cross results in formation of a new colored offspring here it is purple.Incomplete dominance is a trait in which an intermediate offspring is formed because one allele is not completely expressed over the other paired allele.The resulting offspring is formed with characters that is a combination of both of their parents character.