Question

(a) Explain the phenomena of multiple alleles and codominance taking ABO blood group as an example.
(b) What is the phenotype of the following:
(i)$I^A$
(i) ii

Answer 1

Alleles refers to any of the several forms of a gene. These genetic variations arise usually through mutation and therefore are responsible for hereditary variations.

Complete answer:
To answer this question, we have to know about multiple alleles and codominance. Multiple allelism is the condition wherein three or more alleles are present in a gene.When two alleles of a gene are not dominant over each other and express themselves independent of the presence or absence of others, the phenomenon is codominance.

Multiple allelism is best illustrated by the ABO blood group structure in humans. In the inheritance of ABO blood group in humans, genes remain in three forms of alleles, such as- $I^A$, $I^B$, and $I^O$. Where $I^A$ and $I^B$ are codominant. They are in charge of type A and type B antigens, respectively, on the cell surface of erythrocytes. $I^o$ is a recessive allele and not able to produce antigen. It should be well known though that even if there are two or multiple alleles found in the population, the individual containing the population would possess only two such alleles. Therefore, in the case of ABO blood group structure, the hereditary of IA and IB alleles results in having a blood type AB.

Note: Alleles are mutated forms of a gene, and they are the reason for differences in phenotypic expression of a given trait. A gene for which at least two alleles remain is referred to as polymorphic. Examples in which a specific gene may remain in three or more forms of allele are called multiple allele conditions. Whilst multiple alleles structure and are arranged with inside a population