Model Answer
0 min readIntroduction
In genetics, the concept of alleles describes the different forms of a gene that determine a particular trait. While most organisms are diploid, possessing two alleles for each gene, some genes exist in more than two allelic forms within a population. This phenomenon is known as multiple allelism. Multiple allelism expands upon Mendelian inheritance by demonstrating that genes can have more than two alternative forms, leading to a greater diversity of phenotypes. Understanding multiple allelism is fundamental to comprehending the genetic basis of various traits and diseases in diverse organisms, including humans.
What are Multiple Alleles?
Multiple alleles refer to the existence of more than two allelic forms of a gene within a population. While an individual organism can only possess a maximum of two alleles for a particular gene (one inherited from each parent), the gene pool of a population may contain several different alleles. This contrasts with simple Mendelian genetics, where a gene typically has only two alleles.
Understanding Multiple Allelism
Multiple allelism is the condition where a gene has more than two alleles. It’s important to note that an individual organism still only inherits two alleles for each gene, but the population as a whole exhibits a wider range of genetic variation at that locus. This leads to a greater number of possible genotypes and phenotypes compared to situations with only two alleles. The principles of segregation and independent assortment still apply, but the complexity of inheritance increases.
Examples of Multiple Allelism
1. ABO Blood Group System in Humans
The most classic example of multiple allelism is the ABO blood group system in humans. This system is determined by a single gene, I, which has three common alleles: IA, IB, and i.
- IA allele leads to the production of A antigens on red blood cells.
- IB allele leads to the production of B antigens on red blood cells.
- i allele leads to no antigen production.
The resulting genotypes and phenotypes are as follows:
| Genotype | Phenotype (Blood Group) |
|---|---|
| IAIA | A |
| IAi | A |
| IBIB | B |
| IBi | B |
| IAIB | AB |
| ii | O |
The IA and IB alleles are codominant, meaning that both antigens are expressed when both alleles are present (AB blood group). The i allele is recessive to both IA and IB.
2. Coat Color in Rabbits
Another example is coat color in rabbits, controlled by the C gene. There are at least four alleles: C (full color), cch (chinchilla), ch (Himalayan), and c (albino). These alleles exhibit a dominance hierarchy: C > cch > ch > c. This results in a range of coat colors depending on the genotype.
3. Self-Incompatibility in Plants
Self-incompatibility in plants is a genetic mechanism that prevents self-fertilization. It often involves multiple alleles at a single locus, where plants can only be fertilized by pollen with alleles they do not possess. This promotes outcrossing and genetic diversity.
Conclusion
Multiple allelism is a significant extension of Mendelian genetics, demonstrating that genes can exist in more than two allelic forms within a population. This phenomenon contributes to the vast genetic diversity observed in natural populations and plays a crucial role in determining phenotypic variation. Understanding multiple allelism is essential for comprehending the inheritance of complex traits and for applications in fields like medicine, agriculture, and conservation biology. Further research into the molecular mechanisms underlying multiple allelism will continue to refine our understanding of genetic inheritance.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.