Biology CO DOMINANCE

### KEY TOPICS

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

### CO-DOMINANCE

● 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.

### PHENOTYPES POSSIBLE IN BLOOD GROUPS

● 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.

### STARCH SYNTHESIS IN PEA

● 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.