Incomplete dominance

Incomplete dominance is a pattern of inheritance in which a heterozygote's phenotype is intermediate between the phenotypes of the two homozygotes. Neither allele completely dominates the other, so the result is a blended phenotype in the heterozygous individual.

• A classic example is flower color in snapdragons: one parent with red flowers and one with white flowers will produce heterozygous offspring with pink flowers. The pink phenotype is intermediate, showing incomplete dominance of the red and white alleles.
• In incomplete dominance, heterozygotes have their own phenotype (distinct from either homozygote). This often leads to phenotype ratios like 1:2:1 in a monohybrid cross. For instance, crossing two pink snapdragons (Rr x Rr) yields 25% red (RR), 50% pink (Rr), and 25% white (rr) offspring.
• Key difference from co-dominance: incomplete dominance results in a blend (intermediate trait), whereas co-dominance results in both traits being displayed separately. If you see "blended" or intermediate characteristics in a heterozygote, it's incomplete dominance.

• If a question describes offspring with a phenotype that is a mix or intermediate of the parents' phenotypes (e.g., two true-breeding plants - one with long leaves and one with short leaves - producing all medium-leaf offspring), that indicates incomplete dominance.
• Exams may emphasize the contrast with complete dominance: in incomplete dominance, you won't see a 3:1 phenotype ratio. Instead, heterozygotes form an additional category. Recognizing a 1:2:1 phenotypic ratio in F2 generation or an intermediate trait in F1 is a strong clue for incomplete dominance.