In a species of beetle, body colour is determined by two unlinked genes, E and M. The dominant allele E produces melanin; the recessive allele e results in no melanin, giving a pale body. The dominant allele M causes even distribution of melanin, resulting in a dark body; the recessive allele m causes clumpy distribution, resulting in a patchy body. However, if no melanin is produced (genotype ee), the effect of the M gene is not visible and the beetle is pale.
A dihybrid cross was carried out between two beetles heterozygous for both genes (EeMm × EeMm). The offspring were scored for body colour. The results are shown in the table.
| Phenotype | Number of offspring |
|---|---|
| Dark | 560 |
| Patchy | 190 |
| Pale | 250 |
Explain why the observed numbers differ from the expected 9:3:3:1 ratio for a dihybrid cross with independent assortment of two genes.