The trap: NEET questions on incomplete dominance and codominance both involve heterozygotes that don't look like the dominant parent — but the underlying mechanism is different, and confusing one for the other costs marks. A common confusion is reading "intermediate phenotype" and selecting codominance, or reading "both alleles expressed" and selecting incomplete dominance.
Incomplete dominance occurs when the heterozygote shows a phenotype intermediate between the two homozygotes. The classic NCERT example (Class 12 Biology, Chapter 4, page 72): in snapdragon (Antirrhinum), crossing red-flowered (RR) × white-flowered (rr) produces pink-flowered F₁ (Rr). The F₂ phenotype ratio is 1 red : 2 pink : 1 white — identical to the genotype ratio 1:2:1, because no allele fully masks the other.
Codominance occurs when both alleles are fully expressed simultaneously — not blended, but independently visible. The NCERT example: ABO blood group alleles Iᴬ and Iᴮ are codominant. A person with genotype IᴬIᴮ expresses both A and B antigens on red blood cells, giving blood group AB. Neither antigen is intermediate; both are fully present.
Multiple alleles refers to more than two allelic forms of a gene existing in a population (though any individual carries at most two). ABO blood groups illustrate this: three alleles (Iᴬ, Iᴮ, i) produce six genotypes and four phenotypes.
Key distinction for NEET: incomplete dominance → blending/intermediate phenotype, F₂ ratio 1:2:1 phenotypically. Codominance → both parental traits appear side by side, no blending. Multiple alleles → a population-level concept (more than two alleles); ABO is the standard example combining codominance (Iᴬ with Iᴮ) and dominance (Iᴬ and Iᴮ each dominant over i).
Watch out: when a question gives an F₂ ratio of 1:2:1 for phenotype, the cross involves incomplete dominance (not complete dominance, which gives 3:1). This is the genotype-vs-phenotype ratio trap applied in a non-Mendelian context.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
In a cross between red-flowered (RR) and white-flowered (rr) snapdragons, the F₁ generation is pink. This pattern of inheritance is called:
Answer: D. D is correct. When the heterozygote shows an intermediate phenotype (pink between red and white), the pattern is incomplete dominance (NCERT Class 12 Biology, Chapter 4, page 72).
Why A is wrong: A is wrong because codominance produces BOTH parental phenotypes simultaneously (e.g., AB blood group), not a blended intermediate (trap: confusing 'both alleles contributing' with 'intermediate phenotype').
Why B is wrong: B is wrong because epistasis involves one gene masking the expression of another gene at a different locus, not allelic interaction at a single locus.
Why C is wrong: C is wrong because over-dominance (heterosis) refers to the heterozygote being MORE fit than either homozygote — not simply intermediate in appearance.
How many alleles of the ABO blood group gene exist in the human population?
Answer: B. B is correct. The ABO locus has three alleles in the population: Iᴬ, Iᴮ, and i (NCERT Class 12 Biology, Chapter 4, page 72).
Why A is wrong: A is wrong because two alleles per individual is correct, but the question asks about the population, which has three alleles — this is the individual-vs-population confusion in multiple alleles.
Why C is wrong: C is wrong because four is the number of blood group phenotypes (A, B, AB, O), not the number of alleles.
Why D is wrong: D is wrong because six is the number of possible genotypes (IᴬIᴬ, IᴬIᴮ, Iᴬi, IᴮIᴮ, Iᴮi, ii), not the number of alleles.
A person with blood group AB has the genotype IᴬIᴮ. The relationship between Iᴬ and Iᴮ alleles is an example of:
Answer: B. B is correct. In blood group AB, both A and B antigens are independently and fully expressed on red blood cells — the hallmark of codominance (NCERT Class 12 Biology, Chapter 4, page 72).
Why A is wrong: A is wrong because incomplete dominance would produce a blended intermediate phenotype, not the simultaneous full expression of both A and B antigens (trap: confusing intermediate blending with dual expression).
Why C is wrong: C is wrong because in complete dominance one allele entirely masks the other — here both Iᴬ and Iᴮ are equally visible.
Why D is wrong: D is wrong because pleiotropy refers to a single gene affecting multiple traits, not two alleles being expressed simultaneously.
In snapdragons showing incomplete dominance, a cross between two pink-flowered plants (Rr × Rr) produces offspring in the phenotypic ratio:
Answer: C. C is correct. With incomplete dominance the heterozygote (Rr) is phenotypically distinct (pink), so the phenotypic ratio mirrors the genotypic ratio: 1 RR (red) : 2 Rr (pink) : 1 rr (white) (trap: genotype-vs-phenotype ratio confusion — in complete dominance this would be 3:1, but here heterozygotes are distinguishable).
Why A is wrong: A is wrong because 3:1 is the phenotypic ratio under complete dominance where Rr looks identical to RR. In incomplete dominance Rr is visibly pink, not red (trap: applying Mendel's 3:1 ratio to a non-Mendelian cross).
Why B is wrong: B is wrong because 1:1 would result from a testcross (Rr × rr), not from crossing two heterozygotes.
Why D is wrong: D is wrong because all-pink offspring come from crossing the two homozygotes (RR × rr); Rr × Rr segregates into red, pink and white.
Parents with blood groups A (IᴬIᴬ) and B (IᴮIᴮ) have a child. The child's blood group will be:
Answer: A. A is correct. IᴬIᴬ × IᴮIᴮ → all offspring are IᴬIᴮ. Since Iᴬ and Iᴮ are codominant, all children express both A and B antigens → blood group AB.
Why B is wrong: B is wrong because for blood group B the child would need IᴮIᴮ or Iᴮi, but the mother can only contribute Iᴬ — the child always carries both alleles.
Why C is wrong: C is wrong because for blood group A the child would need genotype IᴬIᴬ or Iᴬi, but the father can only contribute Iᴮ — every child gets one Iᴬ and one Iᴮ.
Why D is wrong: D is wrong because blood group O requires genotype ii, but neither parent carries the i allele.
A man with blood group AB marries a woman with blood group O. Which of the following is the complete set of blood groups possible in their children?
Answer: A. A is correct. The father is IᴬIᴮ, so each child receives either Iᴬ or Iᴮ from him — never both, and never neither. The mother is ii, so every child receives an i from her. The only two genotypes available are therefore Iᴬi, which is group A, and Iᴮi, which is group B, in a 1:1 ratio. Note what this rules out at BOTH ends: a child cannot be AB, because that needs Iᴬ and Iᴮ from the two different parents and the mother has neither; and a child cannot be O, because that needs an i from each parent and the father has none.
Why B is wrong: B is wrong because it lists every group indiscriminately. AB and O are each impossible here for opposite reasons — AB needs an Iᴬ or Iᴮ from the mother, who has neither, and O needs an i from the father, who has none.
Why C is wrong: C is wrong because AB is the one outcome the father cannot produce on his own: he passes ONE of his two alleles, not both. A child of an AB parent is never AB unless the other parent also supplies Iᴬ or Iᴮ.
Why D is wrong: D is wrong because group O requires the genotype ii, meaning an i allele from each parent. The father is IᴬIᴮ and carries no i at all.
In a plant species, crossing a red-flowered plant with a white-flowered plant produces offspring that show red and white patches (not a blended pink colour). This is an example of:
Answer: C. C is correct. When heterozygotes display both parental phenotypes simultaneously in distinct patches (not blended), this is codominance — both alleles are independently expressed in different cell patches.
Why A is wrong: A is wrong because incomplete dominance would produce a BLENDED intermediate (e.g., uniform pink), not discrete red and white patches (trap: assuming any non-Mendelian heterozygote pattern = incomplete dominance).
Why B is wrong: B is wrong because multiple allelism refers to more than two alleles in the population — the scenario describes only two alleles (red and white) interacting in a heterozygote.
Why D is wrong: D is wrong because polygenic inheritance involves multiple genes contributing additively to a trait (e.g., skin colour, height), not two alleles of one gene producing patchy expression.
Which of the following correctly distinguishes incomplete dominance from codominance?
Answer: A. A is correct. The defining difference: incomplete dominance → intermediate/blended phenotype in the heterozygote; codominance → both parental phenotypes independently and fully expressed. This is the core conceptual distinction tested in NEET (NCERT Class 12 Biology, Chapter 4, page 72).
Why B is wrong: B is wrong because BOTH incomplete dominance and codominance show an F₂ phenotype ratio of 1:2:1 (the heterozygote is distinguishable from both homozygotes in both cases). The 3:1 ratio belongs to complete dominance only (trap: genotype-vs-phenotype ratio confusion).
Why C is wrong: C is wrong because the number of alleles is unrelated to the distinction — incomplete dominance and codominance both involve two alleles at one locus. Multiple alleles is a separate concept (e.g., ABO with three alleles).
Why D is wrong: D is wrong because both phenomena occur across kingdoms — incomplete dominance in snapdragons (plant) and codominance in ABO blood groups (animal) are the standard NCERT examples, but neither is kingdom-restricted.
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Given
A couple has the following blood groups: Father = AB, Mother = A (heterozygous, Iᴬi).
Required
What are the possible blood groups of their children and in what ratio?
Concept
ABO blood group inheritance involves three alleles (Iᴬ, Iᴮ, i) — a case of multiple allelism. Iᴬ and Iᴮ are codominant with each other; both are dominant over i.
Formula / Rule
Each parent contributes one allele. Father (IᴬIᴮ) contributes either Iᴬ or Iᴮ. Mother (Iᴬi) contributes either Iᴬ or i.
Substitution / Punnett square
| | Father: Iᴬ | Father: Iᴮ |
|---|---|---|
| Mother: Iᴬ | IᴬIᴬ (Group A) | IᴬIᴮ (Group AB) |
| Mother: i | Iᴬi (Group A) | Iᴮi (Group B) |
Calculation
Offspring genotypes: IᴬIᴬ (1/4), IᴬIᴮ (1/4), Iᴬi (1/4), Iᴮi (1/4).
Offspring phenotypes: Group A (IᴬIᴬ + Iᴬi = 2/4), Group AB (IᴬIᴮ = 1/4), Group B (Iᴮi = 1/4).
Final answer
Possible blood groups: A : AB : B = 2 : 1 : 1. Blood group O is not possible (no ii combination).
Common trap
Students sometimes expect blood group O from an AB father. An AB parent MUST contribute either Iᴬ or Iᴮ — the child always gets at least one dominant allele, making O impossible from an AB parent. This is a high-frequency distractor in NEET.
Similar NEET-style question
A woman with blood group B (heterozygous) marries a man with blood group A (heterozygous). What proportion of their children can have blood group O?
*(Answer: Father Iᴬi × Mother Iᴮi → ii = 1/4, so 25% can be group O.)*
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Incomplete dominance: snapdragon RR (red), Rr (pink), rr (white). Codominance: ABO blood — IA, IB co-expressed. Multiple alleles: ABO has 3 alleles (IA, IB, i). Pleiotropy: one gene → multiple traits (sickle cell, PKU).
-- NCERT Class 12 Biology, Ch. 4, p. 60More in Genetics and Evolution: 21 exam traps and mistakes · 4 formulas · 1 question pattern from its other lessons.
6 questions from NEET 2020, 2023, 2024, 2025, 2026. Answers verified against NTA official keys.
What is the pattern of inheritance for polygenic trait?
The phenomenon of pleiotropism refers to
Identify the wrong statement with reference to the gene ‘I’ that controls ABO blood groups.
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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