Incomplete dominance
With incomplete dominance neither allele wins: the heterozygote shows an in-between form, such as pink flowers from red and white. So each genotype has its own phenotype.
Rules
- RR red, WW white, RW pink (intermediate form)
- F1 from RR × WW: all pink
- F2 from RW × RW: red : pink : white = 1 : 2 : 1
- Genotypic and phenotypic ratios are the same
Your turn
Three problems on this topic. Type probabilities as a percentage number, e.g. 18.75.
Task
In snapdragons, flower colour shows incomplete dominance: RR is red, WW white and RW pink. A red plant is crossed with a pink one (RR × RW). What percentage of the offspring is red, pink and white?
Red (RR) in %
Pink (RW) in %
White (WW) in %
Probabilities as a percentage number (25 or 18.75), rounded to 2 decimal places; allele frequencies as a decimal (0.02).
Worked example
In four o’clock flowers, flower colour shows incomplete dominance: RR is red, WW white and RW pink. A pink plant is crossed with a pink one (RW × RW). What percentage of the offspring is red, pink and white?
Worked solution
- 1.Gametes: RW → R, W; RW → R, W
- 2.Combinations: RR 1/4, RW 1/2, WW 1/4
- 3.Incomplete dominance: each genotype has its own phenotype, red 25%, pink 50%, white 25%
Typical mistake: Counting RW as red, as in dominant-recessive inheritance. With incomplete dominance the heterozygote is its own intermediate form; RW × RW gives 1 : 2 : 1.
Common questions
What is the difference between incomplete dominance and codominance?
Incomplete dominance: a blend (pink). Codominance: both traits show side by side, as in blood group AB.