Incomplete Dominance Calculators

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Incomplete dominance is a pattern of inheritance in which neither allele is fully dominant over the other, resulting in a heterozygous offspring that shows an intermediate or blended phenotype — different from either parent. The classic example is snapdragon flowers: crossing red-flowered (RR) and white-flowered (WW) plants produces pink-flowered (RW) offspring rather than red or white. Incomplete dominance challenged early Mendelian genetics because it appeared to support blending inheritance, but the original phenotypes re-emerge in predictable F₂ ratios, confirming particulate inheritance.

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What Is Incomplete Dominance?

In complete dominance, one allele fully masks the other in heterozygotes. In incomplete dominance, neither allele is fully dominant — the heterozygote shows an intermediate phenotype that is a blend of the two homozygous phenotypes.

Genotype-phenotype mapping:

  • RR → Red flower (fully pigmented)
  • WW → White flower (no pigment)
  • RW → Pink flower (half pigment — intermediate)

One R allele produces enough pigment for a pink, but not full red, because the R allele produces pigment protein and WW produces none — one copy of R makes enough for intermediate color, not full red.

F₁ and F₂ Crosses

F₁ cross: RR × WW → all RW (all pink) — no dominance

F₂ cross: RW × RW → 1 RR : 2 RW : 1 WW = 1 red : 2 pink : 1 white

Phenotype ratio (1:2:1) differs from the typical 3:1 of complete dominance, but genotype ratio is still 1:2:1 — confirming Mendelian inheritance.

Examples of Incomplete Dominance

  • Snapdragon flowers: Red × white → pink
  • Camellia flowers: Red × white → pink
  • Andalusian chickens: Black × white → blue (blue Andalusian)
  • Human familial hypercholesterolemia: LDL receptor heterozygotes have intermediate cholesterol levels between normal and affected homozygotes
  • Wavy hair: Curly × straight → wavy hair in some populations

Incomplete Dominance vs. Codominance

  • Incomplete dominance: Heterozygote shows a NEW, blended intermediate phenotype (pink — neither red nor white)
  • Codominance: Heterozygote expresses BOTH parental phenotypes simultaneously (e.g., AB blood type shows both A and B antigens, not an intermediate)

Glossary

Incomplete Dominance
A pattern of inheritance where neither allele is fully dominant — heterozygotes show an intermediate phenotype between the two homozygous phenotypes. F₁ phenotype ratio is 1:2:1 (unlike 3:1 for complete dominance).
Codominance
Both alleles are fully expressed simultaneously in the heterozygote — neither is dominant. Example: AB blood type expresses both A and B antigens. Contrasts with incomplete dominance where a blended intermediate phenotype appears.
Gene Dosage Effect
The phenotypic effect of having one vs. two functional copies of a gene. In incomplete dominance, one functional allele produces a submaximal level of gene product — enough for an intermediate phenotype but not the full homozygous dominant effect.

Frequently Asked Questions

In complete dominance, one allele fully masks the other in heterozygotes — AA and Aa look identical (both show the dominant phenotype). In incomplete dominance, neither allele is fully dominant — heterozygotes (RW) show an intermediate phenotype between the two homozygotes. Example: red (RR) × white (WW) snapdragons → pink (RW) F₁ offspring, not red. The F₂ ratio is 1 red : 2 pink : 1 white — phenotype ratio differs from the 3:1 of complete dominance.

Classic examples: (1) Snapdragon flowers — red (RR) × white (WW) → pink (RW) heterozygotes. (2) Camellia and four o'clock flowers show the same pattern. (3) Andalusian chickens — black × white → blue-gray feathering. (4) Familial hypercholesterolemia in humans — LDL receptor heterozygotes have intermediate plasma LDL cholesterol levels between affected homozygotes and unaffected individuals.

In red snapdragons (RR), two copies of the R allele produce full-intensity red pigment. In white (WW), the pigment enzyme gene is non-functional — no pigment. In pink heterozygotes (RW), only one functional R allele is present, producing half the normal pigment amount — enough for pink, not full red. This is gene dosage effect — one copy of R produces a detectable but submaximal amount of the pigment protein, resulting in an intermediate phenotype.

In incomplete dominance, the heterozygote shows a new blended intermediate phenotype that is different from either parent (pink — not red or white). In codominance, both parental phenotypes are simultaneously and fully expressed in the heterozygote — there is no blending. The AB blood type is codominant: type A (IA IA) × type B (IB IB) → type AB (IA IB), which shows both A and B antigens on red blood cells simultaneously.