Gene Versus Allele Concepts (Pseudoalleles) | Botany Optional for UPSC PDF Download

Gene

Genes are the fundamental units of heredity, carrying the genetic information that guides the development of an organism's traits. They are comprised of segments of DNA that code for specific characteristics, such as eye color, immune system function, or hormone production. Each individual inherits two copies of each gene, one from each parent, which are collectively known as alleles. Genes play a pivotal role in passing features from one generation to the next and are responsible for the vast array of variations in human appearance and biology.

Allele

• When genes undergo mutations, they can take on different forms. These alternative forms, known as alleles, vary slightly in the sequence of their base DNA but still encode for the same trait. For instance, there may be alleles for brown hair and blonde hair. The presence of different alleles is what leads to diversity in heritable traits. Humans, being diploid organisms, carry two alleles for each gene, contributing to the uniqueness of their characteristics.
• In human genetics, the presence of multiple alleles for a given trait is the reason behind the immense variability among individuals. Take eye color as an example: it is influenced by multiple genes, with each gene having unique alleles that determine whether the eyes will be brown, blue, green, or hazel. The specific combination of alleles on an individual's chromosomes determines their eye color.

Difference Between Gene and Allele

Pseudoalleles

• Pseudoallelism is a genetic condition in which two genes with similar functions are located very closely to each other on the same chromosome, causing them to be genetically linked. This term was introduced by scientists Morgan in 1928 and Lewis in 1948. Essentially, it means that these two genes, often referred to as pseudoalleles, tend to be inherited together, and because they have related functions, they may functionally appear as a single gene.
• In some rare cases, these closely linked pseudoalleles can undergo recombination, which separates them. One theory suggests that pseudoalleles may originate from gene duplication events, and the duplicated genes can then evolve to develop new functions.

Characteristic of pseudoalleles

• They are closely linked alleles that can undergo crossing over.
• They affect the same trait or character.
• While they are often inherited together, they can occasionally be separated by recombination.

Example:

• Red eye colour of Drosophila has different mutants like white and apricot. 
• They affect pigmentation i.e., affect the same character. So, they are allelic. 
• They can undergo recombination, i.e., they are nonallelic.

Multiple alleles

•  Multiple alleles refer to a genetic concept where the term "allele" represents different forms or variants of a gene or a pair of genes that dictate a particular trait. When gene mutations occur, they can lead to the creation of numerous alternative forms of a gene, and when the number of these altered forms exceeds two, they are collectively termed "multiple alleles."
• In the genome of a diploid organism, which possesses two sets of chromosomes, any two of these multiple alleles can be present. However, in a haploid state or within a gamete, only one of these alleles can be found.
• Multiple alleles consistently occupy the same specific location or locus on a chromosome, and they invariably impact the same characteristic or trait within an organism. Importantly, there is no occurrence of crossing over between the various member alleles of the same series of multiple alleles. This ensures that these alleles remain distinct and do not merge through genetic recombination.

Important Examples of Multiple Alleles:

• Wings of Drosophila 
• Coat Colour in Rabbit
• Self-Incompetebiliyt in Plants
• Blood Groups in Man
• The ‘Rhesus’ Blood Group in Man