Essential Genetics 6th Edition by Daniel Hartl

Exercise 17

In Drosophila pseudoobscura, the alleles P and p determine red versus pink eyes, and Na and na determine wide (wildtype) versus narrow wings. A dihybrid cross was carried out to produce flies homozygous for both p and na. The following phenotypes were obtained in the F2 generation:
red eyes, wide wings 583
red eyes, narrow wings 193
pink eyes, wide wings 168
pink eyes, narrow wings 56
Test these data for agreement with the 9 : 3 : 3 : 1 ratio expected under the hypothesis that the two pairs of alleles undergo independent assortment.