Q.Explain the inheritance of the ABO blood group system in humans. Name the three alleles of the gene $I$ involved, and state clearly which relationships among the alleles are dominant-recessive and which are co-dominant.

AB blood group is produced by co-dominance of I^A and I^B, giving the genotype I^A I^B.

The ABO blood group system is controlled by a single gene (I) with three alleles: I^A, I^B and i. I^A and I^B are each dominant over i, but I^A and I^B are co-dominant with each other. When both are present, both antigens A and B are expressed on the red cells, giving blood group AB. This is only possible with the genotype I^A I^B. Genotypes I^A i and I^B i give groups A and B respectively, and ii gives group O.

While a diploid individual carries only two alleles of a gene, a population can have more than two alternative forms at that locus — this is multiple allelism.

Multiple alleles occur when a gene exists in more than two allelic forms within a population, even though any individual diploid organism can carry only two of them at once. The classic NCERT example is the ABO blood group system in humans, controlled by gene I with three alleles — I^A, I^B, and i.

This is distinct from:

• Epistatic genes — where one gene's expression masks/suppresses the expression of another gene at a different locus.

• Supplementary genes — genes that interact so that both together produce a phenotype different from either alone.

• Linked genes — genes located close together on the same chromosome that tend to be inherited together.

Codominance gives an F2 phenotypic ratio of 1 : 2 : 1 (same as the genotypic ratio).

In codominance, both alleles of a heterozygote are expressed together and fully (for example the AB blood group, where both A and B antigens appear). Because the heterozygote has its own distinct phenotype, each genotype corresponds to a separate phenotype. On crossing two heterozygotes, the F2 genotypic ratio 1 (homozygous) : 2 (heterozygous) : 1 (homozygous) is directly reflected as the phenotypic ratio 1 : 2 : 1 — unlike complete dominance, which gives 3 : 1.

ABO blood group is controlled by multiple alleles (I^A, I^B, i) with I^A and I^B codominant to each other and both dominant over i; an AB × O cross always yields A or B offspring, never AB or O.

Genotypes: mother is AB → genotype I^A I^B (produces gametes carrying either I^A or I^B). Father is O → genotype ii (produces only gametes carrying i).

Cross: I^A I^B (mother) × ii (father)

Possible offspring genotypes:

• I^A i → phenotype A
• I^B i → phenotype B

Each type occurs with equal (1:1) probability. Since the father contributes only the recessive 'i' allele and never I^A or I^B, no child can inherit two dominant alleles together (so AB is impossible), and since every child gets at least one dominant allele (I^A or I^B) from the mother, O (ii) is also impossible. Hence all children will be either blood group A or blood group B.

Blood group A can arise from two genotypes — I^A I^A (homozygous) or I^A i (heterozygous) — because allele I^A is dominant over i.

Human ABO blood grouping is controlled by a single gene with three alleles: I^A, I^B, and i. Both I^A and I^B are dominant over i, and I^A/I^B show co-dominance with each other. A person with two copies of I^A (I^A I^A) or one I^A and one recessive i (I^A i) will both show phenotype 'Group A', since i is recessive and does not express its antigen. Since the question specifically asks for the heterozygous genotype, only I^A i qualifies (I^A I^A is homozygous, I^A I^B would be Group AB, and ii is Group O).

Multiple allelism describes a gene locus that has several allelic forms distributed across a population, illustrated best by the human ABO blood-group system.

Concept

In simple Mendelian inheritance, a gene is often considered to have just two alleles (dominant and recessive). However, for many genes, more than two alternative forms can exist at the same locus in the population's gene pool - this is called multiple allelism. Any single diploid individual, however, can possess at most two of these alleles (one inherited from each parent), since only two homologous chromosomes carry that locus.

Example - ABO blood grouping in humans

The gene controlling ABO blood groups exists as three alleles: I^A and I^B (both fully expressed and codominant with each other) and i (recessive to both). This gives six possible genotypes (I^A I^A, I^A i, I^B I^B, I^B i, I^A I^B, ii) but only four phenotypes (blood groups A, B, AB, and O), illustrating both multiple allelism and codominance.

The ABO blood group gene has three alternative alleles ($I^A$, $I^B$, $i$) in the human population, even though any one individual carries only two — this is the textbook example of multiple allelism in humans.

Reasoning

Multiple allelism refers to a single gene locus having more than two allelic forms within a population (though a diploid individual can carry only two of them). The ABO blood group system is controlled by the gene I, which has three alleles: $I^A$ and $I^B$ (both dominant to $i$, and codominant to each other), and $i$ (recessive). The six possible genotypes ($I^AI^A$, $I^Ai$, $I^BI^B$, $I^Bi$, $I^AI^B$, $ii$) produce the four phenotypes A, B, AB and O.

• The AB phenotype ($I^AI^B$) additionally illustrates codominance (both alleles express fully and simultaneously), but the question specifically asks about the number of alleles at the locus, which is the defining feature of multiple allelism.

• Polyploidy (b) refers to possessing more than two complete chromosome sets, and polygyny (a) is a mating system — neither describes allelic inheritance.

• Incomplete dominance (d) is not seen in ABO inheritance (there, $I^A$/$I^B$ show codominance, not blending).