AP Syllabus focus:
‘In incomplete dominance, neither allele masks the other, producing an intermediate blended phenotype in heterozygous individuals.’
Incomplete dominance is a classic inheritance pattern where heterozygotes show an intermediate phenotype. Understanding how allele products interact links Mendelian genetics to molecular function and helps interpret offspring ratios that differ from simple dominant–recessive traits.
Core idea: why phenotypes “blend”
What incomplete dominance means
In incomplete dominance, neither allele fully determines the phenotype in a heterozygote; instead, the heterozygote expresses an intermediate (blended) phenotype relative to the two homozygotes.
Practice Questions
FAQ
Often it is due to dosage: one functional allele produces about half the normal amount of product.
It can also reflect intermediate enzyme activity or partial loss-of-function alleles.
Look for three reproducible phenotype categories that correlate with genotype.
Using quantitative measurements (e.g., pigment intensity) can clarify whether heterozygotes cluster in-between.
Yes. A gene may have multiple alleles across a population, while any given individual still carries two.
Different allele pairs can show different degrees of intermediacy.
Not always. The heterozygote can be closer to one homozygote if allele products are not strictly additive.
Dominance can be partial rather than perfectly intermediate.
They genotype individuals and test whether heterozygotes consistently show a distinct intermediate phenotype.
They also check whether offspring ratios match expectations for a single gene with two alleles.
