3.14 - Inheritance of Blood Groups

Multiple alleles and the ABO blood groups

• Most genes have just two alleles (two versions) - one dominant and one recessive.
• A gene with more than two possible alleles is said to have multiple alleles.
• Any one individual still carries only two alleles (one on each chromosome of the pair), but across a population there can be three or more different alleles to choose from.

The three alleles of the blood-group gene

• The gene controlling human ABO blood group is the standard example - it has three alleles: I^A, I^B, and I^o (also written I^O).
• The combination of the two alleles a person inherits decides their blood group: A, B, AB, or O.
• These four blood groups are the phenotypes; the pairs of alleles behind them are the genotypes.

How the alleles interact

• I^A and I^B are codominant - if a person inherits one of each, neither overrules the other and both are expressed, so both contribute to the phenotype.
• This is what codominance means: in a heterozygous individual both alleles are expressed and together determine the phenotype, rather than one dominant allele masking a recessive one.
• I^o is recessive - it is masked whenever it is paired with I^A or I^B, so its effect only shows when there is no codominant allele present.

Genotypes and their phenotypes

• I^A I^A or I^A I^o - Blood group A
• I^B I^B or I^B I^o - Blood group B
• I^A I^B - Blood group AB (both alleles expressed - codominance)
• I^o I^o - Blood group O (the only group needing two recessive alleles)

Predicting blood groups with a genetic cross

A blood-group cross is set out as an ordinary genetic diagram (a Punnett square). Work through it in four steps:

1. Write each parent's two alleles.
2. Work out the alleles their gametes can carry.
3. Put one parent's gametes along the top, the other's down the side.
4. Combine them in the cells to give the offspring genotypes.

The one extra step compared with a simple dominant-recessive cross is reading each offspring genotype back to a blood group, using the rule that I^A and I^B are codominant and I^o is recessive.

Example - Group A × group B

Parent 1 is group A (I^A I^o); parent 2 is group B (I^B I^o).

• I^A I^B - Group AB (both alleles expressed)
• I^A I^o - Group A (the recessive I^o is masked)
• I^B I^o - Group B (the recessive I^o is masked)
• I^o I^o - Group O (two recessive alleles)

So the child could be any of the four groups - AB, A, B or O - each a 1 in 4 (25%) chance, a 1:1:1:1 ratio. This surprises people, because neither parent is group AB or group O.

The ratio depends on the parents' genotypes

• A cross between two parents who are both group AB (each I^A I^B) gives a different pattern.
• Each parent's gametes carry either I^A or I^B, so the offspring genotypes are:
  • I^A I^A - Group A
  • I^A I^B - Group AB
  • I^A I^B - Group AB
  • I^B I^B - Group B
• That's a 1:2:1 ratio of group A to group AB to group B, with no group O possible - neither parent carries an I^o allele to pass on.

As with every genetic diagram, the square gives only the probability of each outcome, not a certainty of what any individual child will be.