While both are fundamental to understanding heredity, genes and alleles describe distinct aspects of our genetic makeup. Genes represent the basic units of inherited information, while alleles refer to the different versions of these genes. This distinction is central to comprehending how traits are passed down and expressed across generations.
What is a Gene?
A gene is a fundamental unit of heredity, a specific segment of DNA that carries instructions for building one or more particular proteins or functional RNA molecules. These instructions act as a blueprint, guiding the development and functioning of an organism. Genes are arranged linearly along chromosomes within the nucleus of cells. Genes vary in length, ranging from a few hundred to over two million base pairs. Humans have approximately 20,000 protein-coding genes, which collectively provide the information necessary for various biological traits and functions.
What is an Allele?
An allele is a specific variant or form of a gene. While a gene dictates a particular trait, such as eye color, different alleles of that gene account for the variations observed in that trait, like blue eyes or brown eyes. Alleles occupy the same specific position, known as a locus, on homologous chromosomes. Individuals typically inherit two alleles for each gene, receiving one from each biological parent. These alleles can be identical or differ slightly in their DNA sequence, influencing how a particular trait is expressed.
The Relationship Between Genes and Alleles
Alleles are distinct versions of the same gene, meaning they all relate to the same trait but present different expressions of it. For example, a gene might determine hair color, while different alleles of that gene could specify brown, blonde, or red hair. This pairing of alleles at a specific gene locus determines an individual’s genetic makeup for that trait.
How Alleles Influence Traits
The combination of alleles an individual inherits for a particular gene dictates their observable traits, known as their phenotype. Alleles interact in various ways to produce these traits. This interaction is often described as dominant and recessive.
A dominant allele expresses its associated trait even if only one copy is present. In contrast, a recessive allele will only express its trait if an individual inherits two copies of it, one from each parent. For example, the allele for brown eyes is dominant over the allele for blue eyes, meaning a person with one brown and one blue allele will likely have brown eyes. Similarly, in human ABO blood types, the A and B alleles are dominant over the O allele, and A and B can also be co-dominant, resulting in AB blood type.