The phenotypic ratio of incomplete dominance is 1:2:1.
Incomplete dominance is a fascinating form of intermediate inheritance where one allele for a specific trait isn't entirely dominant over the other paired allele. This results in a third phenotype that is a blend or mixture of the parental phenotypes. Unlike standard Mendelian dominance where the dominant allele completely masks the recessive one, incomplete dominance creates a new, intermediate expression in heterozygous individuals.
Understanding the 1:2:1 Phenotypic Ratio
The distinctive 1:2:1 phenotypic ratio arises from a cross between two heterozygous individuals in incomplete dominance. This ratio directly mirrors the genotypic ratio because each genotype corresponds to a unique phenotype.
Let's break down the ratio:
- 1: Represents the phenotype of individuals homozygous for one allele (e.g., red flowers).
- 2: Represents the phenotype of heterozygous individuals, showing the intermediate trait (e.g., pink flowers).
- 1: Represents the phenotype of individuals homozygous for the other allele (e.g., white flowers).
This is a key characteristic that distinguishes incomplete dominance from complete dominance, where the phenotypic ratio of a heterozygous cross is typically 3:1 (three dominant phenotypes to one recessive phenotype).
Example: Flower Color in Snapdragons
A classic example illustrating incomplete dominance and the 1:2:1 ratio is flower color in snapdragons (Antirrhinum majus).
Consider a cross between a purebred red-flowered snapdragon (genotype RR) and a purebred white-flowered snapdragon (genotype WW).
| W | W | |
|---|---|---|
| R | RW | RW |
| R | RW | RW |
The offspring (F1 generation) will all be heterozygous (RW). However, instead of being red or white, they will have pink flowers, demonstrating the intermediate phenotype.
Now, let's cross two of these pink-flowered heterozygotes (RW x RW):
| R | W | |
|---|---|---|
| R | RR | RW |
| W | RW | WW |
Looking at the possible genotypes and their corresponding phenotypes in the F2 generation:
- RR: 1 individual (Red flowers)
- RW: 2 individuals (Pink flowers)
- WW: 1 individual (White flowers)
This cross yields a genotypic ratio of 1:2:1 (RR : RW : WW).
Crucially, because each genotype produces a distinct phenotype in incomplete dominance, the phenotypic ratio is also 1:2:1 (Red : Pink : White).
This example clearly shows how the intermediate phenotype in heterozygotes leads to the characteristic 1:2:1 phenotypic ratio observed in incomplete dominance crosses between heterozygotes.
