Co-dominance in Genetics: ABO Blood Group Example

Co-dominance is a pattern of inheritance in which both alleles in a heterozygote are fully expressed at the same time. The best-known human example is the AB blood group: a person with alleles I^A and I^B has blood group AB, showing features of both A and B, not a blend of them.

This topic matters because blood groups decide the safety of blood transfusions and organ transplants. It also introduces multiple alleles — the ABO gene has three alleles in the population, even though any one person carries only two.

Key Concepts

• Co-dominance: both alleles express themselves equally and simultaneously in the heterozygote.
• Multiple alleles: the ABO gene exists as three alleles — I^A, I^B and i — in the human population.
• Antigens: I^A produces antigen A on red blood cells, I^B produces antigen B; allele i produces neither and is recessive to both.
• Genotypes and phenotypes: I^A I^A or I^A i gives group A; I^B I^B or I^B i gives group B; I^A I^B gives group AB (both antigens); i i gives group O (neither antigen).
• Antibodies: group A blood has anti-B antibodies, group B has anti-A, AB has neither, and O has both — the basis of transfusion compatibility.

Formulas

Possible genotypes from three alleles: with n alleles, the number of possible genotypes is n(n+1)/2. Here n = 3, so 3 x 4 / 2 = 6 genotypes, which produce 4 phenotypes (A, B, AB, O). For a cross I^A i x I^B i, each parent gives two gamete types with probability 1/2 each. Offspring: I^A I^B = 1/4 (AB), I^A i = 1/4 (A), I^B i = 1/4 (B), i i = 1/4 (O). So all four blood groups can appear with equal probability 1/4 from just two parents.

Worked Example

A father has blood group A with genotype I^A i, and the mother has group B with genotype I^B i. Their possible children: I^A from father with I^B from mother = I^A I^B (AB); I^A with i = I^A i (A); i with I^B = I^B i (B); i with i = i i (O). Each has probability 1/4, so in 100 such children (statistically) about 25 would be expected in each group. This also shows why two parents of groups A and B can have an O child — both carry the hidden recessive i allele.

Common Misconceptions

• "AB blood is a blend or average of A and B." Wrong. Both A and B antigens are present fully and separately on the red cells — that is co-dominance, unlike the intermediate pink of incomplete dominance.
• "A child must have the same blood group as a parent." Recessive alleles hidden in parents can produce an O child from A and B parents, as the worked example shows.
• "Anyone can receive O blood only if they are group O." O red cells have no A or B antigens, so O (especially O negative) is the universal donor group for red cell transfusion.

Real-World Applications

Blood banks type donors and patients for ABO groups to prevent dangerous transfusion reactions. Co-dominance is also seen in roan coat colour in cattle, where red and white hairs appear side by side. ABO typing has been used as supporting evidence in paternity disputes, since certain parent combinations cannot produce certain child groups.

In co-dominance both alleles show fully together — as A and B antigens in AB blood — and with three alleles (I^A, I^B, i) the ABO system produces six genotypes and four blood group phenotypes.