Incomplete Dom Codominance

8 MCQs1 revision card9-step worked example
Source: NCERT Structural Organisation in AnimalsPYQ coverage: NEET 2020, 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

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.

Practice MCQs

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

In a cross between red-flowered (RR) and white-flowered (rr) snapdragons, the F₁ generation is pink. This pattern of inheritance is called:

MCQ 2Easy RecallPractice

How many alleles of the ABO blood group gene exist in the human population?

MCQ 3Easy RecallPractice

A person with blood group AB has the genotype IᴬIᴮ. The relationship between Iᴬ and Iᴮ alleles is an example of:

MCQ 4Direct ApplicationPractice

In snapdragons showing incomplete dominance, a cross between two pink-flowered plants (Rr × Rr) produces offspring in the phenotypic ratio:

MCQ 5Direct ApplicationPractice

Parents with blood groups A (IᴬIᴬ) and B (IᴮIᴮ) have a child. The child's blood group will be:

MCQ 6Direct ApplicationPractice

A man with blood group AB marries a woman with blood group O. Which of the following blood groups is NOT possible in their children?

MCQ 7Concept TrapPractice

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:

MCQ 8Concept TrapPractice

Which of the following correctly distinguishes incomplete dominance from codominance?

Quick recall before you leave

Worked Example

  1. 1

    Given

    A couple has the following blood groups: Father = AB, Mother = A (heterozygous, Iᴬi).

  2. 2

    Required

    What are the possible blood groups of their children and in what ratio?

  3. 3

    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.

  4. 4

    Formula / Rule

    Each parent contributes one allele. Father (IᴬIᴮ) contributes either Iᴬ or Iᴮ. Mother (Iᴬi) contributes either Iᴬ or i.

  5. 5

    Substitution / Punnett square

    | | Father: Iᴬ | Father: Iᴮ | |---|---|---| | Mother: Iᴬ | IᴬIᴬ (Group A) | IᴬIᴮ (Group AB) | | Mother: i | Iᴬi (Group A) | Iᴮi (Group B) |

  6. 6

    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).

  7. 7

    Final answer

    Possible blood groups: **A : AB : B = 2 : 1 : 1**. Blood group O is not possible (no ii combination).

  8. 8

    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.

  9. 9

    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.)* ---

Before solving, remember these

Key Fact

Inheritance patterns

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. 72

Formulas

Mendel's monohybrid ratio

F2 ratio in monohybrid cross — products of independent assortment of two alleles per locus.

SymbolQuantitySI Unit
ratioF2 progeny ratio-

Valid when

  • Single gene with complete dominance
  • Pure-bred parents

Mendel's dihybrid ratio

F2 ratio in dihybrid cross with two independently-segregating loci, complete dominance, no linkage.

SymbolQuantitySI Unit
ratioF2 phenotype ratio-

Valid when

  • Two unlinked loci
  • Complete dominance

Hardy-Weinberg equation

In an idealised population (no mutation, drift, selection, gene flow, random mating), allele and genotype frequencies remain constant.

SymbolQuantitySI Unit
pfreq of dominant allele A-
qfreq of recessive allele a-

Valid when

  • Idealised population
  • All five conditions met
  • Diploid, autosomal, biallelic locus

Recombination frequency (genetic mapping)

Proportion of recombinant offspring measures genetic distance between linked loci. Capped at 50% (independent assortment).

SymbolQuantitySI Unit
RFrecombination frequency%

Valid when

  • Linked loci on same chromosome

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

5' → 3' synthesis polarity

Category: Sign Convention

DNA polymerase synthesises only 5'→3'. Leading strand: continuous, same direction as fork. Lagging: discontinuous (Okazaki), opposite to fork.

When it triggers

Question on Okazaki, leading vs lagging, primer direction.

How to avoid

Reading template 3'→5'; synthesising 5'→3'. Lagging strand needs short fragments because it can't run continuously against fork direction.

Trap

Hardy-Weinberg disequilibrium causes

Category: Negative Marking

Five forces disturb HW: mutation, gene flow, drift, selection, non-random mating. ANY of these violates equilibrium.

When it triggers

Question asks which factor maintains/disturbs HW.

How to avoid

Random mating + no other forces → equilibrium. ANY of mutation/migration/drift/selection/assortative mating → disequilibrium.

Trap

Genotype vs phenotype ratio confusion

Category: Similar Terms

Monohybrid: genotype 1:2:1 (AA:Aa:aa); phenotype 3:1.

When it triggers

Question asks for one ratio while presenting cross details.

How to avoid

Always note dominance: phenotype merges Aa + AA; genotype keeps them separate.

Trap

X-linked recessive: father→daughter expression

Category: Similar Terms

X-linked recessive (haemophilia, colour-blindness): affects males predominantly; carrier mother → 50% sons affected; affected father → all daughters carriers but not affected.

When it triggers

Pedigree question; carrier vs affected.

How to avoid

Sex chromosomes: XX vs XY. Recessive on X needs both copies (XaXa) in female, only one (XaY) in male.

Trap

Sympatric vs allopatric speciation

Category: Similar Terms

Allopatric: geographic isolation. Sympatric: same area, no physical barrier (e.g. polyploidy in plants, host-shift).

When it triggers

Question gives speciation scenario and asks which type.

How to avoid

If geographic barrier mentioned → allopatric. If population overlaps → sympatric.

Past Year Questions

51 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.

NEET 2025

Identify the statement that is NOT correct.

1Constant region of heavy and light chains are located at C-terminus of antibody molecules
2Each antibody has two light and two heavy chains.
3The heavy and light chains are held together by disulfide bonds.
4Antigen binding site is located at C-terminal region of antibody molecules.
NTA Answer: Option 4(final)
NEET 2025

Given below are two statements : Statement I : In the RNA world, RNA is considered the first genetic material evolved to carry out essential life processes. RNA acts as a genetic material and also as a catalyst for some important biochemical reactions in living systems. Being reactive, RNA is unstable. Statement II : DNA evolved from RNA and is a more stable genetic material. Its double helical strands being complementary, resist changes by evolving repairing mechanism. In the light of the above statements, choose the most appropriate answer from the options given below :

1Statement I is incorrect but statement II is correct
2Both statement I and statement II are correct
3Both statement I and statement II are incorrect
4Statement I is correct but statement II is incorrect
NTA Answer: Option 2(final)
NEET 2024

Which one of the following can be explained on the basis of Mendel's Law of Dominance? A. Out of one pair of factors one is dominant and the other is recessive. B. Alleles do not show any expression and both the characters appear as such in F generation. 2 C. Factors occur in pairs in normal diploid plants. D. The discrete unit controlling a particular character is called factor. E. The expression of only one of the parental characters is found in a monohybrid cross. Choose the correct answer from the options given below:

1A, B and C only
2A, C, D and E only
3B, C and D only
4A, B, C, D and E
NTA Answer: Option 2(final)
NEET 2024

Which of the following statement is correct regarding the process of replication in E.coli?

1The DNA dependent DNA polymerase catalyses polymerization in one direction that is 3’ → 5’
2The DNA dependent RNA polymerase catalyses polymerization in one direction, that is 5’ → 3’
3The DNA dependent DNA polymerase catalyses polymerization in 5’ → 3’ as well as 3’ → 5’ direction
4The DNA dependent DNA polymerase catalyses polymerization in 5’ → 3’ direction
NTA Answer: Option 4(final)
NEET 2023

The phenomenon of pleiotropism refers to

1Presence of two alleles, each of the two genes controlling a single trait
2A single gene affecting multiple phenotypic expression
3More than two genes affecting a single character
4Presence of several alleles of a single gene controlling a single crossover
NTA Answer: Option 2(final)
NEET 2023

Expressed Sequence Tags (ESTs) refers to

1All genes that are expressed as proteins.
2All genes whether expressed or unexpressed.
3Certain important expressed genes.
4All genes that are expressed as RNA.
NTA Answer: Option 4(final)
NEET 2023

Which of the following statements are correct about Klinefelter’s Syndrome? A. This disorder was first described by Langdon Down (1866). B. Such an individual has overall masculine development. However, the feminine developement is also expressed. C. The affected individual is short statured. D. Physical, psychomotor and mental development is retarded. E. Such individuals are sterile. Choose the correct answer from the options given below:

1C and D only
2B and E only
3A and E only
4A and B only
NTA Answer: Option 2(final)
NEET 2022

Given below are two statements : Statement I : Mendel studied seven pairs of contrasting traits in pea plants and proposed the Laws of Inheritance. Statement II : Seven characters examined by Mendel in his experiment on pea plants were seed shape and colour, flower colour, pod shape and colour, flower position and stem height. In the light of the above statements, choose the correct answer from the options given below :

1Statement I is incorrect but Statement II is correct
2Both Statement I and Statement II are correct
3Both Statement I and Statement II are incorrect
4Statement I is correct but Statement II is incorrect
NTA Answer: Option 2(final)
NEET 2022

Statements related to human Insulin are given below. Which statement(s) is/are correct about genetically engineered Insulin? (a) Pro-hormone insulin contain extra stretch of C-peptide (b) A-peptide and B-peptide chains of insulin were produced separately in E.coli, extracted and combined by creating disulphide bond between them. (c) Insulin used for treating Diabetes was extracted from Cattles and Pigs. (d) Pro-hormone Insulin needs to be processed for converting into a mature and functional hormone. (e) Some patients develop allergic reactions to the foreign insulin. Choose the most appropria

1(c), (d) and (e) only
2(a), (b) and (d) only
3(b) only
4(c) and (d) only
NTA Answer: Option 3(final)
NEET 2021

Match List-I with List-II. (2) (a)-Replication; (b)-Transcription; (c)-Transduction; (d)-Protein (3) (a)-Translation; (b)-Replication; (c)-Transcription;(d)-Transduction (4) (a)-Replication; (b)-Transcription; (c)-Translation; (d)-Protein Answer (4) Choose the correct answer from the options given 127. Which of the following are not secondary below. metabolites in plants? (a) (b) (c) (d)

1Rubber, gums (1) (ii) (i) (iv) (iii)
2Morphine, codeine (2) (ii) (iv) (i) (iii)
3Amino acids, glucose (3) (iv) (iii) (ii) (i)
4(iii) (i) (iv) (ii) (4) Vinblastin, curcumin
NTA Answer: Option 2(final)
NEET 2020

Which of the following refer to correct example(s) of organisms which have evolved due to changes 97. ÁŸêŸ ◊ ∑§ÊÒŸ, ∞ ‚ ¡ËflÊ ∑ § ‚„Ë ©ŒÊ„⁄UáÊÊ ∑§Ê ‚ ŒÁ÷¸Ã ∑§⁄UÃÊ „Ò in environment brought about by anthropogenic ¡Ê ◊ÊŸfl ∑§Ë Á∑˝§ÿÊ•Ê mÊ⁄UÊ flÊÃÊfl⁄UáÊ ◊ ’Œ‹Êfl ∑ § ∑§Ê⁄UáÊ action ? Áfl∑§Á‚à „È∞ „Ò? (a) Darwin’s Finches of Galapagos islands. (a) ªÒ‹Ê¬ÒªÊ mˬ ◊ «UÊÁfl¸Ÿ ∑§Ë Á» §ø (b) Herbicide resistant weeds. (b) π⁄U¬ÃflÊ⁄UÊ ◊ ‡ÊÊ∑§ŸÊ‡ÊË ∑§Ê ¬˝ÁÃ⁄UÊ œ (c) Drug resistant eukaryotes. (c) ‚‚Ë◊∑ §ãŒ˝∑§Ê ◊ ŒflÊßÿÊ ∑§Ê ¬˝ÁÃ⁄UÊ œ (d) Man-created breeds of domesticated animal

1∑ §fl‹ (d) (1) only (d)
2∑ §fl‹ (a) (2) only (a)
3(a) ∞fl (c) (3) (a) and (c)
4(b), (c) ∞fl (d) (4) (b), (c) and (d) Hindi+English 23 H3
NTA Answer: Option 4(final)
NEET 2020

•ŸÈ‹ πŸ ∑ § ‚◊ÿ «UË.∞Ÿ.∞. ∑§Ë ∑È §«U‹Ë ∑§Ê πÊ ‹Ÿ ◊ ∑§ÊÒŸ‚Ê

1RNA polymerase ∞ ¡Êß◊ ◊ŒŒ ∑§⁄UÃÊ „Ò?
2DNA ligase (1) •Ê⁄U.∞Ÿ.∞. ¬ÊÚÁ‹◊⁄ U$¡
3DNA helicase
4DNA polymerase (2) «UË.∞Ÿ.∞. ‹Êߪ $¡ (3) «UË.∞Ÿ.∞. „Ò‹Ë∑ §$¡ 117. Snow-blindness in Antarctic region is due to : (4) «UË.∞Ÿ.∞. ¬ÊÚ‹Ë◊⁄ U$¡ (1) Damage to retina caused by infra-red rays (2) Freezing of fluids in the eye by low
NTA Answer: Option 1(final)
NEET 2020

¡ËŸ ‘I’ ¡Ê ABO ⁄UÄà flª¸ ∑§Ê ÁŸÿ òÊáÊ ∑§⁄UÃÊ „Ò ©‚∑ § ‚ Œ÷¸ 121. Identify the wrong statement with reference to the gene ‘I’ that controls ABO blood groups. ◊ ª‹Ã ∑§ÕŸ ∑§Ê ¬„øÊÁŸ∞–

1Allele ‘i’ does not produce any sugar. (1) ‘i’ ∞ ‹Ë‹ ∑§Ê ߸ ÷Ë ‡Ê∑¸§⁄UÊ ©à¬ÛÊ Ÿ„Ë ∑§⁄UÃÊ–
2The gene (I) has three alleles. (2) ¡ËŸ (I) ∑ § ÃËŸ ∞ ‹Ë‹ „Ê Ã „Ò –
3A person will have only two of the three (3) ∞∑§ √ÿÁÄà ◊ ÃËŸ ◊ ‚ ∑ §fl‹ ŒÊ ∞ ‹Ë‹ „Ê ª – alleles.
4¡’ IA ∞fl IB ŒÊ ŸÊ ß∑§_ „Ê Ã „Ò , ÿ ∞∑§ ¬˝∑§Ê⁄U ∑§Ë (4) When IA and IB are present together, they ‡Ê∑¸§⁄UÊ •Á÷√ÿÄà ∑§⁄Uà „Ò – express same type of sugar.
NTA Answer: Option 4(final)
NEET 2020

≈˛UÊ ‚‹ ‡ÊŸ (•ŸÈflÊŒŸ/SÕÊŸÊ Ã⁄UáÊ) ∑§Ë ¬˝Õ◊ •flSÕÊ ∑§ÊÒŸ ‚Ë 123. The first phase of translation is : „Ê ÃË „Ò?

1Recognition of an anti-codon (1) ∞∑§ ∞ ≈UË-∑§Ê «UÊÚŸ ∑§Ë ¬„øÊŸ
2Binding of mRNA to ribosome (2) ⁄UÊß’Ê ‚Ê ◊ ‚ mRNA ∑§Ê ’㜟
3Recognition of DNA molecule (3) «UË.∞Ÿ.∞. •áÊÈ ∑§Ë ¬„øÊŸ
4Aminoacylation of tRNA (4) tRNA ∑§Ê ∞ ◊ËŸÊ ∞‚Ë‹ ‡ÊŸ
NTA Answer: Option 4(final)
NEET 2020

◊ «U‹ Ÿ Sflà òÊ M§¬ ‚ ¬˝¡ŸŸ ∑§⁄UŸ flÊ‹Ë ◊≈U⁄U ∑ § ¬ÊÒœ ∑§Ë 130. How many true breeding pea plant varieties did Mendel select as pairs, which were similar except Á∑§ÃŸË Á∑§S◊Ê ∑§Ê ÿÈÇ◊Ê ∑ § M§¬ ◊ øÈŸÊ ¡Ê Áfl¬⁄UËà Áfl‡Ê ∑§Ê in one character with contrasting traits ? flÊ‹ ∞∑§ ‹ˇÊáÊ ∑ § •‹ÊflÊ ∞∑§ ‚◊ÊŸ ÕË?

18 (1) 8
24 (2) 4
32 (3) 2
414 (4) 14 H3 30 Hindi+English
NTA Answer: Option 4(final)

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