Partial Dominance Calculators

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Partial dominance, more commonly called incomplete dominance, occurs when neither allele in a heterozygous individual is fully dominant over the other, producing an intermediate phenotype that is a blend of both parental traits. Unlike complete dominance where one allele masks the other, incomplete dominance results in an F1 phenotype distinctly different from both parents. The classic example is snapdragon flower color — red (RR) crossed with white (WW) produces pink (RW) offspring. Understanding incomplete dominance is fundamental to genetics and explains many real-world phenotypic patterns in plants, animals, and humans.

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Incomplete Dominance vs. Complete Dominance

In complete dominance: Aa genotype shows the same phenotype as AA. In incomplete dominance: Aa shows an intermediate phenotype between AA and aa. Neither allele is dominant — both contribute partially to the phenotype, typically because one functional copy of a gene produces less protein than two copies, resulting in an intermediate trait.

Classic Example: Snapdragon Flowers

Red (RR) × White (WW) → All Pink (RW) F1. F2 cross RW × RW → 1 RR (Red) : 2 RW (Pink) : 1 WW (White). Phenotype ratio = 1:2:1 (unlike complete dominance which gives 3:1). The genotype ratio is the same — 1:2:1 — but in incomplete dominance, each genotype produces a distinct phenotype.

Other Examples

  • Carnation flowers: Red × white → pink hybrids
  • Andalusian chickens: Black × white → blue plumage
  • Human hair texture: Curly × straight → wavy in some populations
  • Familial hypercholesterolemia: Heterozygous carriers have intermediate LDL levels

Molecular Basis

Incomplete dominance usually results from gene dosage effects. One functional allele produces 50% of the normal protein level, which is insufficient for the full phenotype but sufficient for an intermediate effect. This is common in pigmentation pathways where enzyme activity is proportional to gene copy number.

Glossary

Incomplete Dominance
A pattern of inheritance where heterozygotes (Aa) show a phenotype intermediate between the two homozygous phenotypes; F2 phenotype ratio is 1:2:1.
Gene Dosage Effect
Phenotype proportional to the number of functional gene copies; underlies incomplete dominance when one copy produces insufficient protein for the full phenotype.
Haploinsufficiency
When a single functional gene copy produces insufficient protein for normal phenotype; causes incomplete dominance or disease in heterozygous carriers.

Frequently Asked Questions

Incomplete dominance occurs when a heterozygous individual (Aa) shows a phenotype intermediate between the two homozygous phenotypes (AA and aa). Neither allele fully dominates. Example: in snapdragons, red (RR) × white (WW) gives all pink (RW) — neither red nor white, but a blend. The F2 ratio from RW × RW is 1 red : 2 pink : 1 white (1:2:1 phenotype ratio), unlike complete dominance which gives 3:1.

In incomplete dominance, crossing two F1 heterozygotes (Rr × Rr) gives: 1/4 RR (parental phenotype 1), 2/4 Rr (intermediate phenotype), 1/4 rr (parental phenotype 2). Phenotype ratio = 1:2:1. This contrasts with complete dominance (3:1 ratio), where Rr looks identical to RR. In incomplete dominance, all three genotypes produce distinguishable phenotypes, making it easier to determine genotype ratios from phenotypic observation.

Incomplete dominance produces a blended intermediate — pink from red and white parents, where neither original color is visible. Codominance shows both parental traits simultaneously and distinctly — like ABO AB blood type expressing both A and B antigens, or roan cattle showing both red and white hairs on the same animal. Incomplete dominance: new intermediate phenotype. Codominance: both originals present at once.

Incomplete dominance usually reflects gene dosage. If one allele produces a functional protein and the other is a null allele, heterozygotes produce 50% of normal protein. If the phenotype scales directly with protein concentration — as in pigment synthesis pathways — heterozygotes show an intermediate phenotype. Complete dominance occurs when 50% protein is enough for the full phenotype. Haploinsufficiency describes cases where a single gene copy is insufficient for normal function, causing intermediate or disease phenotypes.