Allele

A recent study conducted in South Africa has found that certain protective HLA-B alleles, which previously helped individuals survive HIV/AIDS and reduced the transmission of the virus before the widespread availability of Antiretroviral Therapy (ART), are no longer undergoing the same degree of natural selection. The study suggests that the extensive use of ART has significantly slowed this genetic shift, as medical treatment has reduced the survival advantage once conferred by these protective alleles.

What is an Allele?

An allele is a variant or alternative form of a gene. Genes occupy specific positions, known as loci (singular: locus), on chromosomes. At a particular genetic locus, different versions of the same gene may exist, and these different versions are called alleles.

Alleles are responsible for the variation in inherited characteristics among individuals. They influence how specific traits are expressed and contribute to the genetic diversity observed within a population.

Alleles in Humans

Humans are diploid organisms, meaning that they possess two copies of each chromosome, one inherited from the mother and the other from the father. Consequently, every individual generally carries two alleles for each gene, with one allele inherited from each parent.

The combination of these two alleles constitutes the genotype of a particular gene and determines how that trait may be expressed.

Homozygous and Heterozygous Conditions

When an individual possesses two identical alleles for a gene, the condition is known as homozygous. In contrast, when the two alleles are different, the individual is described as heterozygous.

The interaction between these alleles determines how a trait is manifested in the organism.

Role of Alleles in Phenotype Expression

Alleles contribute to an organism’s phenotype, which refers to its observable characteristics or outward appearance. These characteristics may include traits such as eye colour, hair colour, blood group, or susceptibility to certain diseases.

In many cases, one allele may be dominant, while the other is recessive. When an individual carries one dominant and one recessive allele, the dominant allele generally determines the phenotype.

For example, in the case of hair colour, the allele for dark hair is often dominant over the allele for light hair. Therefore, a child will typically have dark hair if at least one dominant allele is inherited. Light hair may be expressed only when both inherited alleles are recessive.

Importance of Alleles in Genetic Diversity

Alleles play a crucial role in maintaining genetic diversity, which is essential for the survival and adaptation of species. The presence of multiple alleles within a population ensures that offspring inherit a unique combination of traits from their parents.

This diversity enhances a population’s ability to adapt to changing environmental conditions and resist diseases, thereby contributing to long-term evolutionary success.

Alleles and Evolution

The frequency of alleles within a population is not constant and may change over time through evolutionary processes. Factors such as mutation, genetic recombination (crossing over), natural selection, genetic drift, and environmental pressures influence allele frequencies.

When a particular allele provides a survival or reproductive advantage, natural selection tends to increase its frequency in the population. Conversely, alleles that reduce fitness may become less common over generations.

The recent study on HLA-B alleles and HIV resistance is an example of how medical interventions can alter evolutionary pressures by reducing the selective advantage associated with certain genetic traits.

Conclusion

An allele is an alternative form of a gene that plays a fundamental role in determining inherited traits and generating genetic diversity. Through interactions between dominant and recessive alleles, organisms express a wide range of characteristics. Changes in allele frequencies over time form the basis of evolution and natural selection, making alleles a central concept in genetics, heredity, and evolutionary biology.