Q.What is incomplete dominance? Explain this phenomenon using the example of flower colour inheritance in the four-o'clock plant, Mirabilis jalapa.

Showing the 12 most recent of 16 on this concept.

1. CBSE 2026Set ANNUAL1 markMCQ
   Q.In incomplete dominance, the expected phenotypic and genotypic ratios are

   (a) 1 : 1 : 2 and 2 : 1 : 1

   (b) 1 : 2 : 1 and 1 : 2 : 1

   (c) 1 : 1 : 1 and 1 : 1 : 1

   (d) 1 : 1 and 1 : 1

   ›Reveal solutionSolution

   In incomplete dominance, the F2 phenotypic and genotypic ratios coincide exactly, both being 1:2:1, because the heterozygote's phenotype is unique and distinguishable.

   In incomplete dominance, when a heterozygote is self-crossed (e.g., F1 pink Rr × F1 pink Rr in Mirabilis jalapa), the F2 generation shows genotypes in the ratio 1 RR : 2 Rr : 1 rr — and because the heterozygous genotype (Rr) produces its own visibly distinct, intermediate phenotype (pink), rather than resembling either homozygous parent, the phenotypic ratio observed (1 red : 2 pink : 1 white) is exactly the same as the genotypic ratio, i.e., 1:2:1 for both.

   ✓Final answer

   (b) 1 : 2 : 1 and 1 : 2 : 1

2. CBSE 2026Set SEM31 markMCQ
   Q.In Mirabilis sp. the appearance of pink hybrid (Rr) between cross of a red (RR) and white (rr) flower plant parent, indicates

   (a) Incomplete dominance

   (b) Segregation

   (c) Co-dominance

   (d) Independent assortment

   ›Reveal solutionSolution

   In Mirabilis jalapa (4 o'clock plant / snapdragon-type inheritance), RR × rr gives a pink (Rr) F1 — an intermediate phenotype — which is the hallmark of incomplete dominance.

   In incomplete dominance, the F1 heterozygote shows a phenotype intermediate between the two homozygous parents because neither allele is fully dominant. Here:

   • Red (RR) × White (rr) → Pink (Rr).

   The pink colour is a blend, produced because a single R allele makes only enough pigment for an intermediate (pink) shade. This differs from co-dominance, where both alleles are fully and independently expressed together (e.g. AB blood group), and from simple dominance where the heterozygote resembles one parent.

   A classic NCERT/CBSE Class 12 Biology inheritance question distinguishing incomplete dominance from co-dominance.

   ✓Final answer

   (a) Incomplete dominance.

3. CBSE 2025Set ANNUAL1 markMCQ
   Q.The phenotypic and genotypic ratio is same i.e. 1 : 2 : 1 in

   (a) Monohybrid cross

   (b) Dihybrid cross

   (c) Incomplete dominance

   (d) Test cross

   ›Reveal solutionSolution

   In complete dominance the F2 phenotypic ratio (3:1) differs from the genotypic ratio (1:2:1); incomplete dominance is the exception where they coincide because the heterozygote has its own distinct, intermediate phenotype.

   In a monohybrid cross with complete dominance (e.g. Mendel's pea plants), the F1 heterozygote (Tt) looks exactly like the dominant homozygote (TT). So in the F2 generation the genotypes are 1 TT : 2 Tt : 1 tt, but the PHENOTYPES only fall into two classes - tall (TT + Tt) and dwarf (tt) - giving a 3:1 phenotypic ratio, different from the 1:2:1 genotypic ratio.

   In incomplete dominance, neither allele is completely dominant over the other, so the heterozygote (Rr) has its own intermediate phenotype, distinct from both homozygotes. The classic example is the flower colour in Mirabilis jalapa (four o'clock plant): RR = red, Rr = pink, rr = white. Because each genotype now corresponds to its own unique phenotype, the F2 phenotypic ratio (1 red : 2 pink : 1 white) becomes numerically identical to the genotypic ratio (1 RR : 2 Rr : 1 rr) - i.e. 1:2:1.

   ✓Final answer

   (c) Incomplete dominance.

4. CBSE 2025Set ANNUAL1 markMCQ
   Q.Incomplete dominance is observed in

   (a) Pea plant

   (b) Snapdragon

   (c) Dragonfly

   (d) Human blood group

   ›Reveal solutionSolution

   Incomplete dominance, where the heterozygote shows an intermediate phenotype, is classically demonstrated by flower colour in snapdragon (Antirrhinum majus), as well as in Mirabilis jalapa.

   In snapdragon, a cross between red-flowered (RR) and white-flowered (rr) plants produces pink-flowered (Rr) F1 offspring - an intermediate blend rather than one colour dominating completely. This differs from the pea plant (Mendel's classic complete-dominance model), the ABO human blood group system (which shows codominance - both IA and IB are fully expressed together in AB blood type), and dragonfly (not a standard genetics example for dominance patterns).

   ✓Final answer

   (b) Snapdragon.

5. CBSE 2024Set E1 markMCQ
   Q.In a cross between true breeding red flowered and true breeding white flowered plants of Antirrhinum the F1 generation was pink flowered which is an example of

   (a) Dominance

   (b) Codominance

   (c) Incomplete dominance

   (d) Blending of contrasting forms of a character

   ›Reveal solutionSolution

   A pink F1 from a red × white cross in Antirrhinum is the textbook example of INCOMPLETE DOMINANCE.

   In Antirrhinum (snapdragon/dog flower), a cross between true-breeding red (RR) and true-breeding white (rr) flowers gives an F1 (Rr) that is PINK — a blend of the two:

   • Neither allele is completely dominant, so the heterozygote shows an INTERMEDIATE phenotype (pink), not red.

   • This is incomplete dominance, distinct from codominance (where BOTH alleles express fully and separately, e.g. AB blood group).

   • The alleles do NOT truly blend at the genetic level: selfing the pink F1 restores red, pink and white in a 1:2:1 ratio, proving the alleles stay intact.

   ✓Final answer

   (c) Incomplete dominance.

6. CBSE 2024Set ANNUAL1 mark
   Q.Law of dominance is not universally applicable. Give reason.

   ›Reveal solutionSolution

   Exceptions like incomplete dominance and co-dominance, where the heterozygote does not resemble either homozygous parent exclusively, show that dominance is not always complete.

   Mendel's law of dominance states that in a heterozygote, one (dominant) allele masks the expression of the other (recessive), so the heterozygote resembles the dominant homozygote. This is not universally true. In incomplete dominance (e.g., flower colour in Mirabilis jalapa or Antirrhinum/snapdragon), the heterozygote (Rr) shows an intermediate phenotype (pink flowers, between red RR and white rr), because neither allele is completely dominant. In co-dominance (e.g., the ABO blood group system, where I^A and I^B are co-dominant), the heterozygote (I^A I^B) expresses the phenotypes of both alleles simultaneously (AB blood group), rather than one masking the other.

   ✓Final answer

   Incomplete dominance and co-dominance are real exceptions where the heterozygote's phenotype is not simply that of the dominant allele.

7. CBSE 2023Set ANNUAL1 mark
   Q.Give an example of deviation from Mendelism. OR Mention the sequence of nucleotides of Pribnow Box.

   ›Reveal solutionSolution

   Incomplete dominance (e.g., flower colour in Mirabilis jalapa) is a well-known deviation from simple Mendelian dominance.

   Mendel's laws are based on the assumption of complete dominance, where the heterozygote shows the same phenotype as the dominant homozygote. However, several inheritance patterns deviate from this classic model. One clear example is incomplete dominance:

   In the 4 o'clock plant (Mirabilis jalapa):

   • A cross between a red-flowered plant (RR) and a white-flowered plant (rr) produces an F1 generation that is pink-flowered (Rr) — an intermediate phenotype, rather than all-red as classic Mendelian dominance would predict.
   • Self-pollinating the F1 pink plants gives an F2 generation in the ratio 1 Red : 2 Pink : 1 White, which happens to match the genotypic ratio (1:2:1) rather than the Mendelian phenotypic 3:1 ratio, because neither allele is fully dominant over the other — the heterozygote shows a blended, intermediate phenotype.

   This shows that dominance is not always complete, which is a genuine deviation from (an extension of) the principles Mendel originally described.

   ✓Final answer

   Incomplete dominance in Mirabilis jalapa (Rr = pink, intermediate) — F2 ratio 1:2:1 instead of Mendel's 3:1 — is a classic example of deviation from Mendelism.

8. CBSE 2021Set TERM11 markMCQ
   Q.Assertion : In Mirabilis, selfing of F1 pink flower plants produces same phenotypic & genotypic ratio. Reason : Flower colour gene shows the incomplete dominance.

   (a) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.

   (b) If both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.

   (c) If Assertion is true, but Reason is false.

   (d) If both Assertion and Reason are false.

   ›Reveal solutionSolution

   Because Mirabilis flower colour shows incomplete dominance, the F2 phenotypic ratio (1 red : 2 pink : 1 white) exactly mirrors the genotypic ratio (1 RR : 2 Rr : 1 rr) -- so selfing F1 pink plants does give matching phenotypic and genotypic ratios.

   Assertion: In Mirabilis jalapa, selfing the F1 pink-flowered plants (genotype Rr) does produce F2 offspring whose phenotypic ratio equals the genotypic ratio.

   Reason: This happens precisely because flower colour in Mirabilis shows incomplete dominance -- neither the red (R) nor white (r) allele is fully dominant, so the heterozygote (Rr) shows a distinct, intermediate pink phenotype rather than resembling either homozygous parent. Because each genotype (RR, Rr, rr) therefore maps to its own unique phenotype (red, pink, white), the F2 genotypic ratio of 1 RR : 2 Rr : 1 rr and phenotypic ratio of 1 red : 2 pink : 1 white come out numerically identical (1:2:1).

   Since the Reason correctly explains why the Assertion is true, option (a) is correct.

   ✓Final answer

   (a).

9. CBSE 2021Set NC1 markMCQ
   Q.In incomplete dominance, the genotypic ratio is

   (a) 3 : 1

   (b) 9 : 3 : 3 : 1

   (c) 9 : 7

   (d) 1 : 2 : 1

   ›Reveal solutionSolution

   Incomplete dominance does not change the underlying Mendelian genotypic ratio of a monohybrid cross (still 1:2:1); what changes is that the phenotypic ratio also becomes 1:2:1 instead of 3:1, since the heterozygote has its own distinct blended phenotype.

   Incomplete dominance explained (classic example: Mirabilis jalapa, the four o'clock plant)

   When a cross is made between red-flowered (RR) and white-flowered (rr) plants, the F1 hybrids (Rr) are neither red nor white but pink — an intermediate blend, because neither allele completely masks the other.

   When these F1 pink (Rr) plants are self-pollinated (Rr × Rr), the F2 generation segregates as:

   • RR (red) : Rr (pink) : rr (white) = 1 : 2 : 1 (genotypic ratio, as in any standard monohybrid cross)

   Because the heterozygote (Rr) has its own unique, intermediate phenotype (pink) rather than resembling one of the parents, the phenotypic ratio also becomes 1:2:1 — unlike complete dominance, where Rr would look identical to RR, giving a 3:1 phenotypic ratio despite the same 1:2:1 genotypic ratio underneath.

   Why not the others:

   • (a) 3:1 — this is the phenotypic ratio under complete dominance, not incomplete dominance.

   • (b) 9:3:3:1 — this is the dihybrid cross ratio (two genes), not relevant to a single-gene (monohybrid) incomplete dominance cross.

   • (c) 9:7 — this is the ratio seen in complementary gene interaction (two genes), not incomplete dominance of a single gene.

   ✓Final answer

   The ratio in incomplete dominance is 1 : 2 : 1 — option (d).

10. CBSE 2021Set ANNUAL1 markMCQ
    Q.In case of incomplete dominance, what will be the phenotypic ratio of F₂ generation?

    (a) 3:1

    (b) 1:2:1

    (c) 1:1:1:1

    (d) 2:2

    ›Reveal solutionSolution

    Incomplete dominance gives an F₂ phenotypic ratio of 1:2:1 (identical to the genotypic ratio) because heterozygotes show an intermediate phenotype.

    In complete dominance, F₂ shows a 3:1 phenotypic ratio because the heterozygote (Aa) looks the same as the homozygous dominant (AA). In incomplete dominance, however, neither allele is completely dominant over the other, so the heterozygote shows a distinct, intermediate phenotype. The classic example is flower colour in Mirabilis jalapa (four o'clock plant) and snapdragon: red (RR) × white (rr) gives all pink (Rr) F₁; selfing F₁ (Rr × Rr) gives 1 Red (RR) : 2 Pink (Rr) : 1 White (rr) in F₂ — a 1:2:1 phenotypic ratio, which exactly mirrors the underlying genotypic ratio, because each genotype now corresponds to a visibly distinct phenotype.

    ✓Final answer

    (b) 1:2:1

11. CBSE 2019Set ANNUAL1 markMCQ
    Q.The inheritance of flower colour (pink) in Mirabilis jalapa is the example of

    (a) incomplete dominance

    (b) codominance

    (c) linkage

    (d) complete dominance

    ›Reveal solutionSolution

    The pink flower colour in Mirabilis jalapa is a textbook example of incomplete dominance.

    In Mirabilis jalapa, when a homozygous red-flowered plant (RR) is crossed with a homozygous white-flowered plant (rr), the F1 offspring are pink (Rr) — a phenotype intermediate between the two parents, rather than resembling either parent fully. This happens because neither allele is completely dominant over the other; the heterozygote shows a blended phenotype. Selfing the pink F1 gives an F2 ratio of 1 Red : 2 Pink : 1 White (1:2:1), which matches the genotypic ratio exactly, unlike Mendelian complete dominance where phenotypic ratio (3:1) differs from genotypic ratio.

    This is distinct from codominance (b), where both alleles are fully and separately expressed together (e.g., AB blood group), from linkage (c), which concerns genes on the same chromosome being inherited together, and from complete dominance (d), where the heterozygote resembles one parent fully.

    ✓Final answer

    (a) Incomplete dominance.

12. CBSE 2019Set ANNUAL1 markMCQ
    Q.F₂ phenotypic ratio of 1 : 2 : 1 in monohybrid cross indicates the presence of which one of the following phenomena?

    (a) Incomplete dominance

    (b) Multiple alleles

    (c) Pleiotropy

    (d) None of these

    ›Reveal solutionSolution

    A monohybrid F₂ ratio of 1:2:1 (phenotype = genotype) is the hallmark of incomplete dominance — the answer is (a).

    In a normal monohybrid cross with complete dominance, the F₂ phenotypic ratio is 3 : 1 while the genotypic ratio is 1 : 2 : 1. But when the heterozygote has its own distinct (intermediate) appearance, each genotype is a separate phenotype, so the phenotypic ratio becomes the same 1 : 2 : 1 as the genotypic ratio.

    • (a) Incomplete dominance — heterozygote shows a blended intermediate phenotype (e.g. pink in Mirabilis), giving F₂ = 1 red : 2 pink : 1 white = 1 : 2 : 1. ✓

    • (b) Multiple alleles, (c) pleiotropy — do not give this classic 1:2:1 monohybrid pattern.

    ✓Final answer

    (a) Incomplete dominance — the 1:2:1 F₂ phenotypic ratio (equal to the genotypic ratio) indicates the heterozygote has a distinct intermediate phenotype.