Incomplete Dominance in Snapdragon (Pink Flower) Explained
Biology · Principles of Inheritance and Variation · NEET
Incomplete dominance is when neither allele is fully dominant, so the heterozygote shows a NEW blended trait. In snapdragon (Antirrhinum), red (RR) crossed with white (rr) gives pink (Rr) flowers in F1. When pink is selfed, F2 shows 1 red : 2 pink : 1 white. Memory hook: "One allele is not strong enough, so RR + rr = a mix = pink."
Snapdragon cross: red (RR) x white (rr) gives pink (Rr) in F1. Selfing pink gives F2 as 1 red : 2 pink : 1 white, where the phenotype ratio equals the genotype ratio because each genotype shows a different colour.
Your doubts, answered
Why is the F1 flower pink and not red like in Mendel's pea?
Because the R allele is not strong enough to fully hide the r allele. In a normal Mendelian cross, one allele (like round seed) makes enough working product to look dominant. In snapdragon, the R allele alone makes only HALF the red pigment. Half pigment looks pink, not full red. So the heterozygote Rr shows a new in-between colour. This is why it is called INCOMPLETE dominance.
Why is the F2 ratio 1:2:1 and not the usual 3:1?
The genotype (allele combination) ratio is always 1 RR : 2 Rr : 1 rr in a monohybrid cross. In Mendel's pea, both RR and Rr looked the same (round), so 3 looked dominant and 1 recessive (3:1). In snapdragon, each genotype has its OWN look: RR = red, Rr = pink, rr = white. Since all three phenotypes are different, the phenotype ratio equals the genotype ratio = 1 red : 2 pink : 1 white.
Does the Law of Segregation still apply in incomplete dominance?
Yes. This is a very common NEET trap. The alleles R and r still separate cleanly during gamete formation, exactly as Mendel's Law of Segregation says. The genotype ratio 1:2:1 proves the alleles are still purely segregating. Only the Law of DOMINANCE is broken, because R is not completely dominant over r. So segregation works, dominance does not.
Which genotype gives which colour in snapdragon?
RR = red flower, Rr = pink flower, rr = white flower. Two R alleles make full red pigment. One R and one r make half pigment = pink. Two r alleles make no pigment = white. Learn these three pairs and you can solve any snapdragon cross.
Is pink a brand new colour or just red mixed with white?
Pink is the heterozygote (Rr) phenotype caused by only half the amount of red pigment being made. There is no white pigment mixing in. The white parent simply makes no pigment. So pink comes from a REDUCED dose of red pigment, not from paint-style mixing of red and white.
How do I cross a pink plant with a red plant?
Pink is Rr and red is RR. Gametes from pink are R and r; gametes from red are R and R. Offspring: 1/2 RR (red) and 1/2 Rr (pink). No white is possible because neither parent can give two r alleles. This exact cross came in NEET 2024 and the answer was red and pink plants.
⚠️ The NEET trap ✗ Students think incomplete dominance breaks the Law of Segregation, so they mark that segregation does not apply. ✓ Segregation DOES apply. Alleles still separate purely (genotype ratio stays 1:2:1). Only the Law of Dominance is not followed because R is not completely dominant. 🧠 Incomplete dominance breaks DOMINANCE, never SEGREGATION. The 1:2:1 genotype ratio is proof segregation is intact.
Real NEET questions
NEET 2019
In Antirrhinum (Snapdragon), a red flower was crossed with a white flower and in F1 generation pink flowers were obtained. When pink flowers were selfed, the F2 generation showed white, red and pink flowers. Choose the incorrect statement from the following:
A · This experiment does not follow the Principle of Dominance.
B · Pink colour in F1 is due to incomplete dominance.
C · Ratio of F2 is 1/4 (Red) : 2/4 (Pink) : 1/4 (White)
D · Law of Segregation does not apply in this experiment ✓
Solution: Snapdragon flower colour is incomplete dominance: red (RR) x white (rr) gives pink (Rr) F1, and F2 shows 1 Red : 2 Pink : 1 White (i.e. 1/4 : 2/4 : 1/4). The genotype ratio is exactly like a normal monohybrid cross, so the Law of Segregation DOES apply. Only the Law of Dominance is not followed. Hence the incorrect statement is (D).
NEET 2024
A pink flowered Snapdragon plant was crossed with a red flowered Snapdragon plant. What type of phenotype/s is/are expected in the progeny?
A · Red flowered as well as pink flowered plants ✓
B · Only pink flowered plants
C · Red, Pink as well as white flowered plants
D · Only red flowered plants
Solution: Pink is Rr and red is RR. Cross Rr x RR gives 1/2 RR (red) and 1/2 Rr (pink). No white (rr) is possible because neither parent can donate two r alleles. So the progeny are red and pink. Answer (A).
NEET 2023 Phase 2
Match List-I (Type of cross) with List-II (Phenotypic ratio): A. Monohybrid Cross, B. Dihybrid Cross, C. Incomplete dominance, D. Test Cross | I. 1:1, II. 1:2:1, III. 3:1, IV. 9:3:3:1
A · (A)-(II), (B)-(III), (C)-(IV), (D)-(I)
B · (A)-(IV), (B)-(III), (C)-(I), (D)-(II)
C · (A)-(III), (B)-(IV), (C)-(II), (D)-(I) ✓
D · (A)-(II), (B)-(IV), (C)-(III), (D)-(I)
Solution: Monohybrid = 3:1 (III), Dihybrid = 9:3:3:1 (IV), Incomplete dominance = 1:2:1 (II) because all three genotypes show different phenotypes, Test cross = 1:1 (I). So A-III, B-IV, C-II, D-I, which is option (C).
Solved Principles of Inheritance and Variation NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the F1 phenotype in incomplete dominance in snapdragon?
The F1 is pink (Rr). It is a new intermediate colour between the red (RR) and white (rr) parents, because the R allele only makes half the amount of red pigment.
What is the F2 ratio in incomplete dominance?
The F2 phenotype ratio is 1 red : 2 pink : 1 white, which is the same as the genotype ratio 1 RR : 2 Rr : 1 rr. It is NOT the usual 3:1.
Which Mendel law is not followed in incomplete dominance?
The Law of Dominance is not followed, because R is not completely dominant over r. The Law of Segregation is still followed fully.
What is the scientific name of the snapdragon plant?
The snapdragon plant used in the NCERT example is Antirrhinum. It is the standard NEET example for incomplete dominance.
How is incomplete dominance different from codominance?
In incomplete dominance the heterozygote shows a NEW blended trait (pink). In codominance BOTH alleles show fully and separately at the same time, like the AB blood group where A and B are both expressed.
