Codominance MCQ - Practice Questions & Answers

Quick Facts

Codominance

A variation on incomplete dominance is codominance, in which both alleles for the same characteristic are simultaneously expressed in the heterozygotes.
Hence, in the case of co-dominance, the F1 generation resembles both parents.
A good example is the different types of red blood cells that determine ABO blood grouping in human beings.
ABO blood groups are controlled by the gene I, located on chromosome number 9.
The gene (I) has three alleles $I^{A}$ , $I^{B}$ and $i$ .
The alleles IA and IB produce a slightly different form of the sugar while allele $i$ doesn’t produce any sugar.
Because humans are diploid organisms, each person possesses any two of the three I gene alleles.
IA and IB are completely dominant over $i$ , in other words when $I^{A}$ and $i$ are present only $I^{A}$ expresses (because $i$ does not produce any sugar), and when $I^{B}$ and $i$ are present $I^{B}$ expresses.
But when IA and IB are present together they both express their own types of sugars: this is because of codominance.
Hence red blood cells have both A and B types of sugars.
Since there are three different alleles, there are six different combinations of these three alleles that are possible a total of six different genotypes of the human ABO blood types.
In a self-cross between heterozygotes expressing a codominant trait, the three possible offspring genotypes are phenotypically distinct.
However, the 1:2:1 genotypic ratio characteristic of a Mendelian monohybrid cross still applies.