Question

(i) What are multiple alleles? How do they differ from simple dominant-recessive allele pairs?

(ii) Explain the concept of multiple alleles with the help of a human trait studied by you.

(iii) Mention all possible genotypes with their corresponding phenotypes of the human trait as explained by you.

(iv) Comment on the type of inheritance pattern involved in the example explained by you.

Hint:

Use the ABO blood group: alleles IA, IB and i, six genotypes, four phenotypes.

Answer 1

Step 1: Part (i): Multiple alleles:
When a gene has more than two allelic forms in a population, these forms are called multiple alleles.
An individual still carries only two of them, one on each homologous chromosome.
In a simple dominant-recessive pair, the gene has just two alleles in the population, such as T and t.

Step 2: Part (ii): ABO blood groups:
The human ABO blood group is controlled by the gene I on chromosome 9.
It has three alleles: \( I^{A} \), \( I^{B} \) and \( i \).
\( I^{A} \) makes sugar A on the red cell surface, \( I^{B} \) makes sugar B, and \( i \) makes no sugar.

Step 3: Part (iii): Genotypes and phenotypes:
\( I^{A}I^{A} \) and \( I^{A}i \) give blood group A.
\( I^{B}I^{B} \) and \( I^{B}i \) give blood group B.
\( I^{A}I^{B} \) gives blood group AB.
\( ii \) gives blood group O.

Step 4: Part (iv): Pattern of inheritance:
\( I^{A} \) and \( I^{B} \) are both dominant over \( i \), so A and B show dominance over O.
\( I^{A} \) and \( I^{B} \) are codominant: in \( I^{A}I^{B} \) both sugars are made. So the pattern is multiple allelism with codominance.

Final Answer:
Multiple alleles are three or more alleles of one gene. In ABO blood groups, \( I^{A} \), \( I^{B} \), \( i \) give six genotypes and four phenotypes, with codominance between \( I^{A} \) and \( I^{B} \).