The characteristics of all living organisms are determined by a fundamental biological process: heredity. This process explains how traits are passed from parents to their offspring. Understanding how these characteristics are inherited requires exploring the basic units of genetic information that dictate an organism’s features and functions.
What is a Gene?
A gene is a specific segment of deoxyribonucleic acid (DNA) that serves as a blueprint for building and maintaining an organism. These segments are organized along thread-like structures called chromosomes, which are found within the nucleus of cells. The primary function of most genes is to provide instructions for creating specific proteins or functional RNA molecules. These proteins and RNA then carry out the various tasks that determine an organism’s traits, such as eye color, hormone production, or immune system function.
What is an Allele?
While a gene defines a particular trait, an allele represents a specific variant form of that gene. For instance, a gene might determine eye color, but different alleles of that gene could lead to brown, blue, or green eyes. Alleles occupy the same specific position, known as a locus, on homologous chromosomes. Homologous chromosomes are pairs of chromosomes, one inherited from each parent, that contain the same set of genes. The existence of these different versions for the same gene is what contributes to the variation observed in traits among individuals.
How Genes and Alleles Work Together
For sexually reproducing organisms, such as humans, an individual inherits two alleles for each gene, receiving one from each biological parent. The combination of these two inherited alleles is called the genotype. The genotype then dictates the observable trait, known as the phenotype, which is how the characteristic physically appears.
The interaction between these two alleles determines the expressed trait. For example, the allele for brown eyes (often represented as ‘B’) is dominant over the allele for blue eyes (represented as ‘b’). If an individual inherits at least one dominant brown eye allele (genotype BB or Bb), their eyes will appear brown. However, blue eyes will only appear if an individual inherits two copies of the recessive blue eye allele (genotype bb).
The Impact on Traits and Diversity
The interplay between genes and their various alleles is central to the unique combination of traits observed in every individual. Each person possesses a distinct set of alleles, which contributes to their individual characteristics, from hair color to susceptibility to certain conditions. This vast array of genetic variations within a population is known as genetic diversity.
Genetic diversity is important for the survival and adaptation of species. It provides the raw material for natural selection, allowing populations to respond to changing environments. When there is more variation in alleles, there is a higher probability that some individuals will possess traits better suited to new conditions, enabling the species to continue. This constant interaction between genes and their alleles is a driving force behind the evolution of life on Earth.