Multiple Alleles

What multiple alleles means

Most genes discussed so far have two alleles, e.g. T and t. But some genes have more than two alleles present across a population.

This is called multiple alleles. Any single individual can still only carry two of these alleles (one on each of its two homologous chromosomes), but across the whole population, three or more versions of the gene exist.

The ABO blood group gene

The human ABO blood group gene has three alleles: I^A, I^B and i. I^A and I^B are codominant with each other, and both are dominant over i (which is recessive).

Why this is not the same as codominance alone

Multiple alleles describes how many versions of a gene exist in a population; codominance and dominance describe how those alleles interact within a genotype. The ABO system combines both ideas: it has multiple alleles (three), and among those alleles there is a dominance hierarchy, I^A and I^B are codominant, but both dominate over i.

How it is examined

SPM questions may ask you to state how many alleles exist for the ABO blood group gene, to list all possible genotypes for a given blood group, or to determine possible blood groups of children from parents of known blood groups.

Worked problem 1 (easy): group A crossed with group O

A person of blood group A with genotype I^A i has children with a person of blood group O (genotype i i). Work out the possible blood groups of the children and their ratio.

The group A parent forms I^A and i gametes in equal numbers, while the group O parent forms only i gametes. Half the children are I^A i (group A) and half are i i (group O), giving a ratio of 1 group A : 1 group O.

Worked problem 2 (medium): group A crossed with group B

A group A parent with genotype I^A i has children with a group B parent of genotype I^B i. Work out the possible blood groups of the children.

The group A parent forms I^A and i gametes; the group B parent forms I^B and i gametes. The four combinations give I^A I^B (group AB), I^A i (group A), I^B i (group B) and i i (group O), a ratio of 1 AB : 1 A : 1 B : 1 O.

All four ABO groups are possible from this single cross.

Worked problem 3 (SPM-level): deduce the parents' genotypes

A woman of blood group A and a man of blood group B have four children, one of whom is blood group O. Explain how two parents who are not group O can have a group O child, state the genotype of each parent, and list the blood groups their children could have.

A group O child has genotype i i, so the child received an i allele from each parent. The mother is group A but passed on i, so she must be I^A i; the father is group B but passed on i, so he must be I^B i.

Both parents are heterozygous and each carries a hidden recessive i allele.

The cross I^A i x I^B i can give I^A I^B (AB), I^A i (A), I^B i (B) and i i (O), so the children could be any of the four ABO groups: A, B, AB or O.

Practice problems

1. A man of blood group AB (I^A I^B) has children with a woman of blood group O (i i). List the possible blood groups of their children and the expected ratio.
2. State all the possible genotypes of a person with blood group B.
3. Two parents both have blood group A, yet one of their children has blood group O. Deduce the genotype of each parent.

Common mistakes

Source:SRC-DSKP-EN

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