Many genes have more than two possible versions in a population, and these versions are called multiple alleles. The ABO blood type system is a classic human example because the gene has three common alleles: IA, IB, and i. Your blood type matters in blood transfusions because immune cells can react to unfamiliar antigens on red blood cells.
Understanding ABO inheritance helps connect genetics, cell surfaces, and real medical decisions.
The IA allele makes A antigens, the IB allele makes B antigens, and the i allele makes no A or B antigen. IA and IB are codominant, so a person with IAIB has type AB blood and shows both antigens. The i allele is recessive, so type O blood appears only when a person has ii.
Punnett squares can predict possible offspring blood types by combining one allele from each parent.
Key Facts
- ABO alleles: IA, IB, and i
- IA and IB are codominant: IAIB = type AB
- i is recessive: ii = type O
- Type A genotypes: IAIA or IAi
- Type B genotypes: IBIB or IBi
- Phenotype ratio = number of each visible blood type divided by total offspring outcomes
Vocabulary
- Allele
- An allele is a version of a gene that can produce a particular trait variation.
- Multiple alleles
- Multiple alleles means that more than two allele forms of a gene exist in a population.
- Codominance
- Codominance occurs when two different alleles are both fully expressed in the heterozygous phenotype.
- Antigen
- An antigen is a molecule on a cell surface that can be recognized by the immune system.
- Genotype
- A genotype is the allele combination an organism has for a specific gene.
Common Mistakes to Avoid
- Calling type O dominant is wrong because O appears only when both alleles are i, making genotype ii.
- Writing AB as a blended blood type is wrong because IA and IB are codominant, so both A and B antigens are present rather than mixed into a new antigen.
- Assuming a type A parent must be IAIA is wrong because type A can be IAIA or IAi, and the hidden i allele changes offspring predictions.
- Using phenotypes instead of alleles in a Punnett square is wrong because offspring inherit alleles, not whole blood type labels.
Practice Questions
- 1 A parent with genotype IAi has a child with a parent with genotype IBi. Make a Punnett square and list the probability of each blood type.
- 2 Two parents have genotypes IAIB and ii. What percent of their children are expected to have type A blood, type B blood, type AB blood, and type O blood?
- 3 A child has type O blood. Explain what allele each parent must have contributed and what this implies about the possible genotypes of the parents.