3.17B Describe the inheritance of the ABO blood groups with reference to…

The idea

The ABO blood group gene has three alleles in the population: Iᴬ, Iᴮ and i. Any one person carries only two of these, so the gene shows multiple alleles because more than two exist across the population, while each individual still has just two. Iᴬ and Iᴮ are codominant: a person with both shows both antigens, giving blood group AB. Both Iᴬ and Iᴮ are dominant to i, so Iᴬi gives group A and Iᴮi gives group B, while ii gives group O. To describe inheritance, set out a genetic cross: write each parent's genotype, separate the alleles into gametes, then combine them in a Punnett square to give the offspring genotypes and phenotypes.

For example: A group AB parent (IᴬIᴮ) crossed with a group O parent (ii) gives Iᴬi (group A) and Iᴮi (group B) offspring in equal proportion.

What earns the marks

• States that the ABO gene has three alleles (Iᴬ, Iᴮ and i) present in the population, which is what is meant by multiple alleles.
• Explains that any individual still inherits only two alleles, one from each parent.
• Identifies Iᴬ and Iᴮ as codominant because a heterozygote IᴬIᴮ expresses both antigens and has blood group AB.
• States that Iᴬ and Iᴮ are each dominant to i, so Iᴬi is group A and Iᴮi is group B, while ii is group O.
• Uses a correct genetic cross (parent genotypes, gametes, Punnett square) to work out offspring genotypes and phenotypes.

Common mistakes and how to put them right

• Thinking multiple alleles means one person carries three alleles.

  Put it right by stressing that the three alleles exist in the population but each person has only two.

• Treating Iᴬ and Iᴮ as showing simple dominance so that AB is impossible.

  Put it right by explaining codominance gives both antigens in the heterozygote.

• Writing group O as having a dominant allele.

  Put it right by showing O is the homozygous recessive genotype ii.

Exam tips

• Always show the parental genotypes and the gametes before completing a Punnett square, so the working is visible.
• Label offspring with both genotype and blood group phenotype, since questions often ask for both.
• Check that each parent contributes exactly one allele to every offspring box.