`star` Codominance
`star` Phenotypes possible in Blood Groups
`star` Starch synthesis in pea


● We have seen the crosses where the `color{Violet}"F1 resembled"` either of the two parents (`color{Violet}"dominance"`) or was in-between (`color{Violet}"incomplete dominance"`).

● But, in the case of co-dominance the F1 generation resembles `color{Violet}"both parents"`.

● A good example is different types of red blood cells that determine `color{Violet}"ABO blood grouping"` in human beings.

● ABO blood groups are controlled by the `color{Violet}"gene I"`.

● The plasma membrane of the `color{Violet}"red blood cells"` has `color{Violet}"sugar polymers"` that protrude from its surface and the `color{Violet}"kind of sugar"` is controlled by the gene.

● The gene (I ) has three alleles `color{Violet}"IA, IB and i."` The alleles IA and IB produce a slightly `color{Violet}"different form"` of the sugar while allele i `color{Violet}"doesn’t produce"` any sugar.

● Because humans are diploid organisms, each person possesses any `color{Violet}"two of the three"` I gene alleles.

● IA and IB are `color{Violet}"completely dominant"` over i, in other words when IA and i are present only IA expresses (because i does not produce any sugar), and when IB and i are present IB expresses.

● But when IA and IB are present `color{Violet}"together"` they both express their `color{Violet}"own types"` of sugars: this is because of `color{Violet}"co-dominance"`.

● Hence red blood cells have both `color{Violet}"A and B types"` of sugars.

● Since there are three different alleles, there are `color{Violet}"six different combinations"` of these three alleles that are possible a total of `color{Violet}"six different genotypes"` of the human ABO blood types.


● The example of ABO blood grouping also provides a good example of `color{Violet}"multiple alleles"`.

● Here you can see that there are more than two, i.e., `color{Violet}"three alleles"`, governing the same character.

● Since in an individual only two alleles can be present, multiple alleles can be found only when `color{Violet}"population studies"` are made.

● Occasionally, a `color{Violet}"single gene product"` may produce more than one effect.


● For example, `color{Violet}"starch synthesis in pea"` seeds is controlled by one gene.

● It has two alleles (`color{Violet}"B and b"`).

● Starch is synthesised effectively by `color{Violet}"BB homozygotes"` and therefore, large starch grains are produced.

● In contrast, `color{Violet}"bb"` homozygotes have `color{Violet}"lesser efficiency"` in starch synthesis and produce smaller starch grains.

● After maturation of the seeds, BB seeds are `color{Violet}"round"` and the bb seeds are `color{Violet}"wrinkled"`.

● Heterozygotes produce `color{Violet}"round seeds"`, and so B seems to be the `color{Violet}"dominant allele"`.

● But, the starch grains produced are of `color{Violet}"intermediate size"` in `color{Violet}"Bb"` seeds.

● So if starch grain size is considered as the phenotype, then from this angle, the alleles show `color{Violet}"incomplete dominance"`.

● Therefore, dominance is not an `color{Violet}"autonomous feature of a gene"` or the product that it has information for.

● It depends as much on the `color{Violet}"gene product"` and the production of a `color{Violet}"particular phenotype"` from this product as it does on the particular phenotype that we choose to examine, in case more than one phenotype is influenced by the same gene.