Write short note on co-dominance.

Explanation

Co-dominance is the phenomenon in which two alleles express themselves independently when present together in an organism.
= In other words, it is the phenomenon in which offspring shows resemblance to both the parents e.g. ABO blood grouping in humans.
= ABO blood groups are controlled by the gene I.
= The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene.
= The gene I has three alleles `I^A ,I^B and i`.
= The alleles `I^A and I^B ` produce a slightly different form of a sugar while allele .i. does not produce any sugar.
= In humans, each person possesses any two of the three I gene alleles.
=`I^A` and i are completely dominant over i. When `I^B` and i are present only 1B express (because .i. does not have any sugar) same is the case with `I^A` and i.
= But when `I^A and I^B` are present together they both express their own types of sugars, this is due to co-dominance.
= Therefore, the red blood cells have both A and B types of sugars.
= Since, there are three different types of alleles there can be six different combinations.
= Hence a total of six different genotypes of the human ABO blood types are present as given below in the table.