Understanding the Variations of Genes: A Study of the HBB Gene

An allele is a variant form of a gene. When the copies of a gene differ from each other, they are identified as alleles. A given gene may have multiple different alleles, but typically only two alleles are present at the gene's locus in any individual. This article explores the reasons behind the variations of alleles, using the HBB gene as a practical example.

Why Alleles Differ

The differences between alleles can result from various factors. These may include:

A mistake in DNA replication or DNA repair that doesn't severely impact fertility. The alleles providing an advantage, such as the ability to metabolize lactose as an adult or possessing an attractive characteristic like unusually colored hair or eyes. For instance, the HBB gene, located on the 11th chromosome, instructs the body on how to produce beta globin, a subcomponent of hemoglobin crucial for red blood cells' ability to bind and transport oxygen.

Changes in nucleotides can lead to different codons, which code for different amino acids. While some changes may have no phenotypic effect, others like the HBB allele 'HbS' can have profound physiological impacts.

The HBB Gene and Its Alleles

The human HBB gene is an excellent example of gene variation. It has more than 1,600 nucleotides, and the sequence can vary from one individual to another. In a typical scenario, the sequence 'GAG' is found, which codes for glutamic acid. However, a single nucleotide change can turn this into 'GTG', which codes for valine, leading to significant physiological effects.

This change in a single nucleotide can have significant implications, as seen in the sickle-cell trait. The normal red blood cell (left) versus the red blood cell with sickle-cell trait (right) illustrates the difference.

Each possible variant of the HBB gene is an allele. For human HBB, there are over 1,000 known alleles. The one responsible for the sickle-cell trait is called HbS. If an individual inherits two alleles of HbS, they will develop sickle-cell disease, a condition that shortens life expectancy and is a leading cause of death worldwide. However, having just one allele of HbS and one normal allele provides natural resistance to malaria, making it a beneficial allele in tropical regions.

Summary:

Alleles are variants of a gene. Depending on the gene and the significance of the variation, the impact can range from having no phenotypic effects to causing life-changing conditions.

Conclusion

Alleles play a crucial role in genetic variability, shaping human traits and health. Understanding the HBB gene provides insights into the complex interplay between genetic variation and physiological outcomes.

Reference: HBB Gene and Sickle Cell Anemia