Question
What is incomplete dominance? Explain with the example of flower colour in snapdragon (Antirrhinum majus). What are the F1 and F2 phenotypic and genotypic ratios?
(NCERT Class 12, commonly asked in NEET)
Solution — Step by Step
In incomplete dominance, the heterozygote shows a phenotype that is intermediate between the two homozygous phenotypes. Neither allele is completely dominant over the other — the dominant allele cannot fully mask the recessive allele.
This is different from Mendel’s complete dominance, where the heterozygote looks identical to the homozygous dominant.
Parents: Red flower (RR) x White flower (rr)
R = allele for red pigment, r = allele for no pigment (white)
Parent 1 produces only R gametes. Parent 2 produces only r gametes.
All F1 offspring are Rr.
But instead of being red (as in complete dominance), the F1 flowers are pink. Why? The single R allele produces only half the amount of red pigment compared to RR. This half-dose of pigment gives an intermediate pink colour.
F1 phenotype: All Pink
| R | r | |
|---|---|---|
| R | RR | Rr |
| r | Rr | rr |
F2 Genotypic ratio: 1 RR : 2 Rr : 1 rr
F2 Phenotypic ratio: 1 Red : 2 Pink : 1 White = 1:2:1
In incomplete dominance, the phenotypic ratio equals the genotypic ratio — because each genotype produces a distinguishable phenotype.
Why This Works
In complete dominance, one allele produces enough protein/enzyme to give the full phenotype even in single dose (one copy is enough). In incomplete dominance, gene expression is dose-dependent — two copies of R produce full red, one copy produces half the pigment (pink), and zero copies produce no pigment (white).
This is sometimes called “quantitative” or “dosage” effect. The underlying genetics (allele segregation) is exactly the same as Mendel’s laws — it is only the phenotypic expression that differs.
Key distinction for NEET: in incomplete dominance, the F2 ratio is 1:2:1 (both phenotypic and genotypic). In complete dominance, the phenotypic ratio is 3:1 but genotypic ratio is still 1:2:1. If a question gives a 1:2:1 phenotypic ratio, think incomplete dominance or codominance.
Common Mistake
Students confuse incomplete dominance with codominance. In incomplete dominance, the heterozygote shows a blended intermediate phenotype (pink = blend of red and white). In codominance, both alleles are fully expressed side by side — like in ABO blood groups where individuals express both A and B antigens (not a blend, but both present).
Another error: writing that incomplete dominance violates Mendel’s laws. It does not. The law of segregation still holds perfectly — alleles separate during gamete formation. The only difference is in how the phenotype is expressed in the heterozygote.