Q.(a) Explain incomplete dominance and codominance with suitable example.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Incomplete Dominance
Incomplete dominance is an intragenic (allelic) interaction in which neither allele of a heterozygous pair is completely dominant over the other, so the heterozygote shows a single, new phenotype intermediate between the two homozygous parental phenotypes - genuinely different from both parents, not identical to either. German botanist Carl Correns demonstrated this in 1905 in the four o'clock plant (Mirabilis jalapa): crossing pure-breeding red (R1R1) with pure-breeding white (R2R2) gives an entirely pink F1 (R1R2); selfing the pink F1 gives an F2 in the ratio 1 red (R1R1) : 2 pink (R1R2) : 1 white (R2R2), the identical numerical pattern for genotype and phenotype. Because the original red and white phenotypes reappear intact in F2 rather than every F2 plant being some shade of pink, incomplete dominance is proof that the alleles themselves stayed discrete …
Heterozygotes don't always show one allele's effect to the complete exclusion of the other — some traits reveal a genuine blend of both parental phenotypes, while others display both parental phenotypes together, unblended, and each pattern is illustrated by a classic genetic example with its own Punnett-square ratios. …
Incomplete dominance gives an intermediate blended phenotype (e.g.
Incomplete dominance blends the phenotype; codominance expresses both alleles fully and separately.
(a) In incomplete dominance, neither of the two contrasting alleles is completely dominant over the other, so the heterozygote displays a phenotype intermediate between the two homozygous parental phenotypes — a genuine "blending." The classic example is flower colour in the four o'clock plant, Mirabilis jalapa: a cross between red-flowered (RR) and white-flowered (rr) plants produces pink-flowered (Rr) offspring in the F1, and self-pollinating F1 pink plants gives a 1 Red : 2 Pink : 1 White ratio in F2 (genotypic and phenotypic ratios coincide, unlike simple Mendelian dominance). In codominance, by contrast, both alleles of a heterozygous pair are fully expressed, distinctly and simultaneously, without blending — the heterozygote shows both parental traits side by side. The standard example is the human ABO blood group system, where the alleles IA and IB are codominant: an individual with genotype IAIB (blood group AB) expresses bot …
Showing the 12 most recent of 16 on this concept.
- CBSE 2026Set ANNUAL1 markMCQQ.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 t …
- CBSE 2026Set SEM31 markMCQQ.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). …
- CBSE 2025Set ANNUAL1 markMCQQ.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.
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- CBSE 2025Set ANNUAL1 markMCQQ.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 bl …
- CBSE 2024Set E1 markMCQQ.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. …
- CBSE 2024Set ANNUAL1 markQ.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 …
- CBSE 2023Set ANNUAL1 markQ.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. …
- CBSE 2021Set TERM11 markMCQQ.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.
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- CBSE 2021Set NC1 markMCQQ.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) …
- CBSE 2021Set ANNUAL1 markMCQQ.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) F …
- CBSE 2019Set ANNUAL1 markMCQQ.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.
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- CBSE 2019Set ANNUAL1 markMCQQ.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. …
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