Discrete variation within genes gives rise to altered type of gene product or altered level of gene product, and thus to a measurable human phenotype. Since both alleles of a diploid genotype in humans are usually active, a combined outcome is observed. The effects of the two alleles may interact in a nonlinear fashion (genetical dominance).
| R305W | R419Q | Blue | Green/Hazel | Brown |
|---|---|---|---|---|
| R/R | R/R | 48.9% (517) | 29.3% (310) | 21.8% (230) |
| R/R | R/Q | 26.5% (54) | 45.1% (92) | 28.4% (58) |
| R/R | Q/Q | 0% (0) | 57.1% (4) | 42.9% (3) |
| R/W | R/R | 47.6% (60) | 17.5% (22) | 34.9% (44) |
| R/W | R/Q | 30.0% (3) | 50.0% (5) | 20.0% (2) |
In the usual case where diploid genotypes are measured, haplotypes must be inferred via statistical modelling.