Incomplete dominance, co-dominance and ABO blood groups
Not every allele pair shows full dominance. Snapdragon F1 is pink (incomplete dominance), while the AB blood group shows both alleles (co-dominance) of a gene with three alleles.
Why dominance is not always complete
A gene carries the information to make a product, often an enzyme. A changed allele may make the normal enzyme, a less efficient one, or no working enzyme at all. If it makes no working enzyme, it usually behaves as recessive, because one normal allele is enough. Dominance also depends on which trait you look at. In pea, BB plants make large starch grains and round seeds, while bb plants make small grains and wrinkled seeds. Bb seeds are round, so B looks dominant, but their starch grains are of intermediate size. So dominance is not a fixed property of a gene; it depends on the product and the phenotype we choose.
Incomplete dominance in snapdragon (dog flower)
- 1P: red-flowered (RR) × white-flowered (rr) snapdragon (Antirrhinum).
- 2F1: all Rr, with pink flowers, neither red nor white.
- 3F1 selfed: Rr × Rr.
- 4★F2: 1 red (RR) : 2 pink (Rr) : 1 white (rr).Red and white come back, so the alleles never blended
★ key step
Co-dominance and multiple alleles: the ABO blood groups
Red blood cells carry sugar polymers that stick out from their plasma membrane. Gene I decides which sugar is made. It has three alleles: Iᴬ and Iᴮ make slightly different sugars, and i makes none. Iᴬ and Iᴮ are each dominant over i. But when Iᴬ and Iᴮ are together, both sugars are made: this is co-dominance, and the person has group AB. A gene with more than two alleles shows multiple alleles. Since one person has only two alleles, multiple alleles can be seen only when a population is studied.
ABO blood groups: genotypes and sugars
| Genotype | Blood group | Sugar on red blood cells |
|---|---|---|
| Iᴬ Iᴬ | A | A-type sugar |
| Iᴬ i | A | A-type sugar |
| Iᴮ Iᴮ | B | B-type sugar |
| Iᴮ i | B | B-type sugar |
| Iᴬ Iᴮ | AB | Both A and B sugars (co-dominance) |
| i i | O | No sugar |
Incomplete dominance vs co-dominance
In both cases the F2 ratio is 1 : 2 : 1 for phenotype as well as genotype.
Solving a blood group puzzle
A man of blood group A and a woman of blood group B have a child of group O. (a) Give the parents' genotypes. (b) Which blood groups can their other children have, and in what ratio?
- Group O is ii, so the child got one i from each parent.
- The father is group A and carries i, so he is Iᴬ i.
- The mother is group B and carries i, so she is Iᴮ i.
- Father's gametes: Iᴬ and i. Mother's gametes: Iᴮ and i.
- Children: Iᴬ Iᴮ (AB), Iᴬ i (A), Iᴮ i (B) and ii (O), each 1/4.
(a) Father Iᴬ i, mother Iᴮ i. (b) AB, A, B and O in the ratio 1 : 1 : 1 : 1.
Before a blood transfusion, hospitals match blood groups so the patient's body does not attack the donor's red cells. Knowing ABO genetics also explains why siblings can have different blood groups.
2 marks: “Differentiate between incomplete dominance and co-dominance, with one example each.” Incomplete dominance: F1 is intermediate, e.g. pink snapdragon from red × white. Co-dominance: both alleles show fully in F1, e.g. blood group AB (Iᴬ Iᴮ).
3 marks: “Explain multiple allelism using the ABO blood groups. Why can multiple alleles be studied only in a population?” Gene I has three alleles, Iᴬ, Iᴮ and i, controlling the sugar on red blood cells; six genotypes, four groups; Iᴬ and Iᴮ co-dominant, both dominant over i. One person carries only two alleles, so all three are seen only across a population.
3 marks: “A red-flowered snapdragon is crossed with a white-flowered one. Work out the F1 and F2. How is this different from Mendel's monohybrid result?” RR × rr gives Rr, all pink. Selfing gives 1 red : 2 pink : 1 white. Mendel's F1 looked like one parent and F2 was 3 : 1; here F1 is intermediate and phenotype ratio equals genotype ratio.
Incomplete = in between (pink). Co-dominance = both co-exist (AB shows A and B).