Zoology · Ch 10 — Principles of Inheritance and Variation
Co-dominance
Co-dominance
Co-dominance
So far, we have seen crosses where the F₁ generation either looked like one of the two parents (dominance) or was a blend between them (incomplete dominance). Co-dominance is different: here, the F₁ generation resembles both parents at the same time.
A classic example is the ABO blood grouping system in humans. This system is controlled by a single gene called I. The surface of red blood cells carries sugar polymers that stick out from the plasma membrane. The kind of sugar present is determined by the I gene.
The I gene has three alleles: IA, IB, and i.
- IA and IB each produce a slightly different form of sugar.
- i does not produce any sugar at all.
Since humans are diploid, every person carries any two of these three alleles.
- IA and IB are both completely dominant over i. So if IA and i are together, only IA expresses (because i makes no sugar). Similarly, IB and i together give only IB expression.
- But when IA and IB are present together, both express their own types of sugars. This is co-dominance. As a result, red blood cells have both A-type and B-type sugars on their surface.
Because there are three alleles, six different combinations are possible, giving six genotypes. The table below shows these genotypes and the corresponding blood types (phenotypes).
| Allele from Parent 1 | Allele from Parent 2 | Genotype of offspring | Blood type of offspring |
|---|---|---|---|
| IA | IA | IA IA | A |
| IA | IB | IA IB | AB |
| IA | i | IA i | A |
| IB | IA | IA IB | AB |
| IB | IB | IB IB | B |
| IB | i | IB i | B |
| i | i | i i | O |
So, how many phenotypes are possible? There are four blood types: A, B, AB, and O.
The ABO blood group example also illustrates multiple alleles — more than two alleles (here, three) governing the same character. Since an individual can have only two alleles, multiple alleles are observed only when studying populations.
Dominance is not an autonomous feature of a gene
Occasionally, a single gene product can produce more than one effect. Consider starch synthesis in pea seeds, controlled by one gene with two alleles: B and b.
- BB homozygotes synthesise starch efficiently, producing large starch grains. Their seeds are round.
- bb homozygotes have lower efficiency, producing smaller starch grains. Their seeds are wrinkled. …