Non-Mendelian patterns
A single gene may have more than two alleles in a population even though a diploid individual usually carries only two at that locus. In incomplete dominance and codominance, the heterozygote is distinguishable from either homozygote, so a heterozygote cross can produce a 1:2:1 phenotypic ratio. Polygenic traits often vary continuously. Epistasis occurs when one gene affects how another locus appears in the phenotype. Environmental effects can change phenotype without changing the inherited allele sequence.
In a pigment pathway, a homozygous recessive mutation that prevents pigment production can hide the effect of a separate gene determining pigment color.
An intermediate phenotype does not mean the parental alleles permanently blended. They can segregate unchanged into the next generation.