Incomplete Dominance

Incomplete dominance occurs when neither allele is completely dominant, resulting in a heterozygous phenotype that blends both parents' traits. Unlike complete dominance where only the dominant phenotype appears in F1 offspring, incomplete dominance produces an intermediate phenotype. For example, snapdragon plants with red and white flower alleles produce pink flowers in the F1 generation, demonstrating this blended inheritance pattern.

In incomplete dominance, both alleles are partially expressed in heterozygous organisms because neither allele is fully dominant over the other. Each allele contributes to the phenotype, creating an intermediate trait. This differs from complete dominance where one allele masks the other. The result is a phenotype that represents a blend of both parental traits rather than displaying only one parent's characteristic.

In incomplete dominance, the phenotypic ratio in the F2 generation is 1:2:1, identical to the genotypic ratio. This occurs because all three possible genotypes produce distinct phenotypes. For example, crossing two F1 pink snapdragons yields one red-flowered plant, two pink-flowered plants, and one white-flowered plant, reflecting the equal expression of both alleles in heterozygotes.

Tay-Sachs disease illustrates incomplete dominance through enzyme production levels. Homozygotes with normal alleles produce full amounts of beta-hexosaminidase A enzyme, while homozygotes with recessive alleles produce none, causing lipid accumulation in the brain. Heterozygotes produce only half the functional enzyme, showing both alleles are expressed but at reduced capacity, resulting in an intermediate phenotype between the two homozygous states.

In Mendelian inheritance, one dominant allele masks the recessive allele, so heterozygotes display the dominant phenotype. In incomplete dominance, both alleles influence the phenotype equally, creating an intermediate trait. The combination of alleles determines the phenotype in both cases, but incomplete dominance represents a non-Mendelian pattern where the interaction between genotype and environmental factors may also influence trait expression.

Heterozygous carriers produce approximately half the normal amount of functional enzyme compared to homozygotes with dominant alleles. This reduced enzyme production occurs because only one allele produces functional protein while the other does not. In conditions like Tay-Sachs disease, this intermediate enzyme level allows heterozygotes to avoid severe symptoms while still showing some biochemical effects of the recessive allele.

In incomplete dominance, the phenotypic ratio equals the genotypic ratio because each genotype produces a unique, distinguishable phenotype. All three possible genotypes—homozygous dominant, heterozygous, and homozygous recessive—display different traits. This contrasts with complete dominance, where two different genotypes produce the same phenotype, making phenotypic and genotypic ratios unequal.