Question

Define allele. Comment upon multiple allelism with examples.

Hint:

Alleles = alternative forms of a gene. Multiple allelism = more than two alleles in a population (e.g., ABO blood group).

Answer 1

Step 1: Definition of allele.
Alleles are the alternative forms of a gene that occupy the same locus on homologous chromosomes and govern the same trait but may produce different expressions. Example: The gene controlling plant height in pea has two alleles — Tall (T) and Dwarf (t).

Step 2: Multiple allelism.
When a gene has more than two alternative forms (alleles) present in a population, the condition is called multiple allelism. \[\begin{array}{rl} \bullet & \text{An individual can have only two alleles (one on each homologous chromosome).} \\ \bullet & \text{But in a population, there may be many alleles for the same gene.} \\ \end{array}\]

Step 3: Examples of multiple allelism. \begin{enumerate} \item ABO blood group system in humans: Controlled by a single gene 'I' with three alleles — \( I^A, I^B, i \). \[\begin{array}{rl} \bullet & \text{\( I^A \) = antigen A, \( I^B \) = antigen B, \( i \) = no antigen.} \\ \bullet & \text{Genotypes and phenotypes:} \\ \bullet & \text{\( I^A I^A \) or \( I^A i \) → Blood group A.} \\ \bullet & \text{\( I^B I^B \) or \( I^B i \) → Blood group B.} \\ \bullet & \text{\( I^A I^B \) → Blood group AB (codominance).} \\ \bullet & \text{\( ii \) → Blood group O.} \\ \end{array}\] \end{itemize} \item Coat colour in rabbits: Controlled by multiple alleles such as full colour (C), chinchilla (C\textsuperscript{ch}), Himalayan (C\textsuperscript{h}), and albino (c). \end{enumerate}

Step 4: Conclusion.
Multiple allelism increases genetic diversity within populations and gives rise to more phenotypic variations.