Question
In snapdragon flowers (Antirrhinum majus), a red-flowered plant (RR) is crossed with a white-flowered plant (rr). The F₁ generation shows pink flowers (Rr). When F₁ pink plants are self-pollinated:
- What phenotypic ratio appears in F₂?
- Why does the standard 3:1 Mendelian ratio not apply here?
Solution — Step by Step
We start with two pure-breeding parents: red (RR) × white (rr).
Since both are homozygous, every F₁ offspring is Rr — only one possible genotype. The surprise is that Rr plants are pink, not red. That single observation tells us R is not fully dominant over r.
In complete dominance, one allele completely masks the other. Here, R only produces enough pigment for a full red colour when two copies are present (RR).
One copy of R (in Rr) produces roughly half the pigment — the result is pink. This “blending” effect is called incomplete dominance. The r allele isn’t recessive in the traditional sense; R simply cannot compensate on its own.
F₁ cross: Rr × Rr
| R | r | |
|---|---|---|
| R | RR | Rr |
| r | Rr | rr |
Genotypic ratio: 1 RR : 2 Rr : 1 rr
- RR → Red (two doses of R = full pigment)
- Rr → Pink (one dose of R = partial pigment)
- rr → White (no R = no pigment)
Phenotypic ratio: 1 Red : 2 Pink : 1 White
Here the genotypic ratio and phenotypic ratio are identical — that’s the hallmark of incomplete dominance.
Seeing white flowers reappear in F₂ is critical. The white trait vanished in F₁ but came back in F₂. This proves rr plants haven’t lost the r allele — it was just masked (partially) in the Rr heterozygote.
This is how Mendel’s particulate inheritance still holds even with incomplete dominance.
Why This Works
The enzyme coded by the R allele produces red pigment. When you have RR, the enzyme output is high enough to give full red. In the Rr heterozygote, you only have one functioning copy — half the enzyme, roughly half the pigment concentration — so the flower appears pink.
This is gene dosage sensitivity. The phenotype is literally a function of how much functional protein is made. It is not that the alleles are “blending” permanently — the F₂ results prove that. The alleles remain distinct particles that separate during gamete formation (Mendel’s Law of Segregation still operates).
That’s why the 3:1 ratio breaks down here. In complete dominance, Rr and RR look identical (both red), so you count 3 red : 1 white. In incomplete dominance, Rr has its own distinct phenotype (pink), so you see all three genotype classes as separate phenotypic classes. Phenotypic ratio equals genotypic ratio.
Alternative Method — Working Backwards from F₂
NEET questions sometimes give you the F₂ ratio and ask you to identify the type of dominance.
If you see 1 : 2 : 1 with three distinct phenotypes in F₂ from an F₁ × F₁ cross, you know immediately:
- Incomplete dominance is at work
- F₁ was heterozygous
- The “intermediate” phenotype is the heterozygote
The shortcut to remember: When genotypic ratio = phenotypic ratio in F₂, it’s incomplete dominance (or codominance). In complete dominance, they differ (1:2:1 genotypic but 3:1 phenotypic).
Common Mistake
Many students write the F₂ ratio as 3 pink : 1 white — copying the standard 3:1 pattern by reflex. That’s the complete dominance answer. In incomplete dominance, red comes back in F₂. The correct answer is 1 red : 2 pink : 1 white. In your NEET answer, always specify all three phenotypes — leaving out red costs you the mark.
A related error: students confuse incomplete dominance with codominance. In codominance, both alleles express fully and simultaneously (like AB blood group, where both A and B antigens appear). In incomplete dominance, neither allele fully expresses — you get an intermediate. Snapdragon pink is intermediate, not a mixture of red patches and white patches.
This question has appeared directly in NEET and CBSE Class 12 board exams multiple times. NCERT Chapter 5 (Principles of Inheritance and Variation) treats it as a standard example. Expect it in one-mark MCQs asking for the F₂ ratio, and occasionally in three-mark short-answer questions asking you to compare incomplete dominance with Mendelian dominance.