This is the ratio of phenotypes in a dihybrid cross.
Explanation:
If both parents are heterogeneous for both traits the ratio of phenotypes is the ratio of 9:3:3:1.
One trait is dominant and the other trait is recessive. Of the 16 possible offsprings only 1 will have both recessive genes. Only with double recessives will the phenotype show both recessives. Nine will have at least one gene for both dominate genes. With one or more genes for the dominate gene the phenotype will show both dominate phenotypes.
Three possible off springs will have a double recessive for one trait therefore that recessive trait will be expressed while the other trait will be the dominate form. Three possible offspring will have a double recessive for the other trait.
Hence the ratio of 9:3:3:1 of phenotypes.
