Practice question

Unlinked dihybrid loci segregating independently generate four distinct phenotypic classes when each dominant allele expresses fully without masking or modification by other locus. Genotypic combinations A_B_, A_bb, aaB_ and aabb appear in 9:3:3:1 proportion because gametes carry alleles randomly during meiosis and fertilisation restores combinations in expected frequencies. Any deviation such as 12:3:1, 9:7, 15:1 or 9:3:4 indicates non-allelic interaction where one gene alters expression of another through pathway. Consequently only 9:3:3:1 reliably denotes absence of interaction, confirming independent action.