Question

Explain incomplete dominance, using snapdragon flowers as an example.

Answer 1

Incomplete dominance is a form of inheritance in which the heterozygote has a phenotype intermediate between the two homozygotes — neither allele completely masks the other.

Snapdragon example (Antirrhinum majus)
- Alleles: let R = allele that produces red pigment, r = allele that produces no/less pigment.
- Phenotypes:
- RR = red flower
- rr = white flower
- Rr = pink flower (intermediate)

Crosses and expected ratios
- Red (RR) × white (rr) → all offspring Rr → all pink.
- Pink (Rr) × pink (Rr):
- Punnett square: RR, Rr, Rr, rr → genotype ratio 1:2:1
- Phenotype ratio: 1 red : 2 pink : 1 white

Why this happens (mechanism)
- Typically it’s a dosage or quantitative effect: RR produces a full complement of pigment-producing enzyme → red; rr produces little or none → white; Rr makes about half the enzyme → less pigment, so flowers appear pink.
- The heterozygote shows a blended/intermediate phenotype rather than showing both parental traits side-by-side (which would be codominance).

How to tell incomplete dominance from simple dominance
- In simple (complete) dominance a heterozygote shows the dominant phenotype and a cross of two heterozygotes gives a 3:1 phenotype ratio. In incomplete dominance the heterozygotes are intermediate and heterozygote crosses give a 1:2:1 phenotype ratio — the classic test used with snapdragons.