Q.A child born to a couple has blood group O. One of the parents claims to have blood group AB. Using your knowledge of the genetics of the ABO system, examine whether this claim is genetically possible. Justify your answer.

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 …

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: …

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 (hom …

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 …

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 e …

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

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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.

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