NCERT grounding
NCERT Class 12 Biology places this subtopic in Section 4.2.2.2, immediately after incomplete dominance. The text states that "in the case of co-dominance the F1 generation resembles both parents" and then uses the ABO blood group as its single worked example. The chapter is explicit that "ABO blood groups are controlled by the gene I" and that "the gene (I) has three alleles IA, IB and i." The same section notes that ABO grouping "also provides a good example of multiple alleles," since three alleles govern one character.
The NIOS supplement (Chapter 22, Principles of Genetics) reinforces this. It groups "multiple alleles and codominance" under deviations from Mendel's laws and tabulates the genotypes and phenotypes of human blood groups directly. Both sources agree on the central fact set, so every number and ratio on this page is anchored to the prescribed syllabus rather than inferred.
"But when IA and IB are present together they both express their own types of sugars: this is because of co-dominance. Hence red blood cells have both A and B types of sugars." — NCERT Class 12 Biology, Section 4.2.2.2
Co-dominance and multiple alleles in the ABO system
Co-dominance is a pattern of inheritance in which the heterozygote expresses both alleles fully and independently, so the hybrid carries the distinct contribution of each parent simultaneously. It is not a blend. Where incomplete dominance averages two alleles into a single intermediate trait, co-dominance keeps both products visible side by side. The textbook frame is precise: in dominance the F1 resembles one parent; in incomplete dominance it is in-between; in co-dominance it resembles both. The ABO blood group is the standard NCERT illustration of this third pattern.
Multiple alleles is a separate but linked idea. It means that a single gene exists in more than two allelic forms in the population. Mendel's pea genes — T/t, R/r, Y/y — each had only two alleles. The I gene that determines ABO blood group has three. Because humans are diploid, any one person can carry only two copies of the gene, one on each homologous chromosome. The third allele is therefore never visible in a single individual; the complete set of three is recovered only when the whole population is surveyed. The ABO system is unusual and exam-friendly because it demonstrates both concepts at once.
Alleles of the I gene
IA, IB and i — more than two forms, so the gene is multiple-allelic at the population level.
Alleles per person
A diploid individual holds only two of the three; the third allele exists elsewhere in the population.
The molecular basis: sugars on the red blood cell surface
NCERT grounds the abstract genetics in a concrete cell-surface fact. The plasma membrane of a red blood cell carries sugar polymers that protrude from its surface, and the I gene controls which sugar is added. Allele IA directs production of one form of the sugar (the A antigen); allele IB directs a slightly different form (the B antigen); allele i produces no sugar at all. This single mechanistic detail explains the entire dominance hierarchy.
Because i makes no product, it cannot contribute a phenotype of its own — it is functionally recessive. Both IA and IB make a working enzyme that adds a sugar, so each is completely dominant over i. When IA and i are together, only the A sugar appears, and the cell is group A. When IB and i are together, only the B sugar appears, and the cell is group B. When IA and IB are together, there is no allele to suppress: both enzymes act, both sugars are added, and the red blood cell displays A and B antigens together. That simultaneous, undiminished expression of both alleles is the definition of co-dominance, and the AB phenotype is its physical signature.
Figure 1. IA adds the A sugar (circles), IB adds the B sugar (squares), and i adds none. In an IAIB cell both enzymes act, so both sugars decorate the surface — the visible mark of co-dominance.
Six genotypes, four phenotypes
With three alleles taken two at a time, the I gene yields exactly six genotype combinations. NCERT lists them all in Table 4.2. Three of these combinations are homozygous-like pairings of identical or different alleles, and three involve the recessive i allele. The crucial step for NEET is the collapse from six genotypes to four phenotypes: this happens because complete dominance of IA and IB over i merges two genotype pairs into single phenotypes.
| Genotype | Blood group (phenotype) | Antigens on RBC surface | Allelic relationship |
|---|---|---|---|
| IAIA | A | A sugar | Homozygous |
| IAi | A | A sugar | IA dominant over i |
| IBIB | B | B sugar | Homozygous |
| IBi | B | B sugar | IB dominant over i |
| IAIB | AB | A and B sugars | IA and IB co-dominant |
| ii | O | No sugar | Recessive homozygote |
Reading the table downwards makes the pattern obvious. Group A arises from two genotypes; group B arises from two genotypes; group AB and group O each arise from exactly one. So a person with blood group A or B may be homozygous or heterozygous, and that ambiguity cannot be resolved by a blood test alone — only a family pedigree or a test cross can. A person with group AB must be IAIB, and a person with group O must be ii; their genotypes are read off directly from the phenotype.
The dominance hierarchy in one line: IA = IB > i. The first two are equal partners; both outrank i.
IA over i
IAi shows only the A sugar. The i allele makes nothing, so IA alone determines the phenotype — complete dominance.
IB over i
IBi shows only the B sugar — the same logic. IB is completely dominant over the silent i allele.
IA with IB
IAIB shows both sugars. Neither masks the other — this is co-dominance, giving the AB phenotype.
What "multiple alleles" means at the population level
NCERT makes one subtle point that NEET examiners enjoy testing. Multiple alleles cannot be observed in a single individual. A diploid person has only two slots for the I gene, so a person can never simultaneously carry IA, IB and i. The third allele is always present somewhere else in the population. To see all three you must move from the individual to the population — multiple alleles are a population-level phenomenon, not an individual-level one.
This is why NCERT writes that "multiple alleles can be found only when population studies are made." It also explains why the I gene generates six genotypes rather than the three a two-allele gene would produce. The number of genotypes for n alleles is n(n+1)/2; for three alleles that is 3 × 4 / 2 = 6, exactly the count in Table 4.2. The formula is a fast cross-check in the exam hall.
Three alleles, six genotypes, four phenotypes — the ABO system is the cleanest demonstration in the syllabus of multiple alleles and co-dominance acting together.
ABO blood group · core idea
How blood-group crosses behave
Because co-dominance and dominance both operate in the I gene, blood-group crosses can be worked with a standard Punnett square as long as the allele labels are kept exact. The process below traces a single cross from parental genotypes to offspring phenotypes; the same four steps handle every blood-group problem NEET sets.
Working a blood-group cross
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Step 1
Fix parent genotypes
Parent 1 is AB → IAIB. Parent 2 is heterozygous A → IAi.
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Step 2
List the gametes
Parent 1 gives IA or IB. Parent 2 gives IA or i.
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Step 3
Fill the Punnett square
Offspring: IAIA, IAi, IAIB, IBi — four genotypes.
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Step 4
Translate to phenotypes
Groups A, A, AB, B — three phenotypes from four genotypes.
Figure 2. The cross IAIB × IAi gives four genotypes but only three phenotypes — the standard 2017 NEET counting problem in diagram form.
The pea-starch footnote: dominance is not absolute
NCERT closes the section with a caution worth carrying into the exam. Starch synthesis in pea seeds is controlled by one gene with two alleles, B and b. The BB homozygote makes large starch grains and round seeds; the bb homozygote makes small grains and wrinkled seeds. The heterozygote Bb produces round seeds — so against the seed-shape phenotype, B looks completely dominant. But the starch grains of Bb seeds are of intermediate size — so against the grain-size phenotype, the same alleles show incomplete dominance. Dominance, NCERT concludes, "is not an autonomous feature of a gene": it depends on which phenotype you choose to examine. This is the basis of the 2018 NEET "wrongly matched" question, where starch synthesis in pea was paired with multiple alleles — an incorrect pairing, because pea starch has only two alleles.
Worked examples
A husband has genotype IAIB and his wife has genotype IAi. How many different genotypes and phenotypes are possible among their children?
The husband produces gametes IA and IB; the wife produces IA and i. The Punnett square gives four offspring genotypes: IAIA, IAIB, IAi and IBi. Translating to phenotypes: IAIA and IAi are both group A, IAIB is group AB, and IBi is group B. So the answer is 4 genotypes and 3 phenotypes. The collapse happens because two of the four genotypes both code for group A.
A child has blood group O. The father is group A and the mother is group B. Work out the genotypes of the parents and the possible genotypes of their other children.
A group O child is ii, so it received an i allele from each parent. The father is group A but must carry i, so his genotype is IAi; the mother is group B but must carry i, so hers is IBi. The cross IAi × IBi gives four equally likely offspring genotypes: IAIB (AB), IAi (A), IBi (B) and ii (O). So this couple can have children of all four blood groups — A, B, AB and O — in a 1:1:1:1 ratio.
Match each term with its description: (a) Dominance, (b) Co-dominance, (c) Pleiotropy, (d) Polygenic inheritance — against (i) many genes govern a single character, (ii) in a heterozygote only one allele expresses itself, (iii) in a heterozygote both alleles express themselves fully, (iv) a single gene influences many characters.
Dominance is the case where only one allele of a heterozygote shows, so (a)–(ii). Co-dominance is the case where both alleles of a heterozygote express fully, so (b)–(iii). Pleiotropy is one gene affecting several traits, so (c)–(iv). Polygenic inheritance is several genes shaping one trait, so (d)–(i). The correct matching is (a)-ii, (b)-iii, (c)-iv, (d)-i — exactly the 2016 NEET answer.
Two parents have blood groups AB and O. Can any of their children be group AB or group O?
The AB parent is IAIB and gives gametes IA or IB. The O parent is ii and can give only i. The offspring are therefore IAi or IBi — that is, group A or group B in equal proportion. Neither IAIB nor ii can appear, so a child of an AB and an O parent can be neither AB nor O. This counter-intuitive result is a recurring NEET trap.
Common confusion & NEET traps
The single biggest error on this subtopic is treating co-dominance and incomplete dominance as the same thing because both deviate from Mendel and both give a 1:2:1 phenotypic ratio in F2. They are genuinely different mechanisms, and NEET writes options that punish a vague memory of either. The side-by-side below is the distinction you must hold.
Co-dominance
Both show
heterozygote resembles both parents
- Both alleles express fully and separately
- No new intermediate trait is created
- Example: AB blood group carries A and B sugars together
- Heterozygote phenotype = sum of both parental products
Incomplete dominance
Blend
heterozygote resembles neither parent
- Neither allele fully dominates the other
- A new, intermediate phenotype appears
- Example: pink snapdragon flower from red × white
- Heterozygote phenotype = in-between average