A gene is a fundamental unit of heredity in living organisms, carrying the instructions that determine specific characteristics. These instructions are encoded within the long, complex molecule known as DNA. Every gene occupies a particular position on a chromosome, acting as a blueprint for building and maintaining an organism. Genes provide the information for creating proteins, which are the workhorses of the cell, performing a vast array of functions.
Alleles: The Different Versions
While every individual of a species has the same set of genes, these genes often come in slightly different forms, known as alleles. An allele is a specific variation of a gene, residing at the same location, or locus, on homologous chromosomes. Think of it like a recipe for a cake: while everyone might have a “chocolate cake recipe” gene, one person’s version might call for dark chocolate, while another’s specifies milk chocolate, representing different alleles.
These variations arise from differences in the DNA sequence of the gene. For instance, the gene responsible for eye color has alleles that can lead to brown, blue, or green eyes. Each individual inherits two copies of every gene, one from each parent. These two copies can be identical alleles or two different alleles, and their combination influences the observable trait.
How Alleles Arise
The main source of new alleles is mutation, which is a change in the DNA sequence of a gene. These changes can occur spontaneously, where errors occur during DNA replication. Environmental factors, such as exposure to certain chemicals or radiation, can also induce mutations. While some mutations can be harmful, others may have no noticeable effect, and some can even be beneficial, providing new variations that can help an organism adapt to its environment.
A single change in a DNA building block, called a nucleotide, can create a new allele. For example, if a gene’s sequence is “A-T-G-C” and a mutation changes it to “A-G-G-C,” this alteration could result in a slightly different protein or no protein at all, forming a new allele. These changes are random, but they are the main mechanism that generates the genetic diversity observed in populations.
How Alleles Influence Traits
The alleles an individual inherits determine their genotype, the genetic makeup for a particular trait. This genotype then influences the phenotype, the observable characteristic. For many traits, alleles exhibit a relationship where one allele can mask the presence of another, described using terms like “dominant” and “recessive” alleles.
A dominant allele will express its trait even if only one copy is present. For example, if a pea plant inherits one allele for tallness (dominant) and one for dwarfness (recessive), it will grow tall. The recessive allele will only express its trait if two copies of it are inherited, as its effect is masked by a dominant allele. This interaction explains why two parents with a dominant trait might still have offspring expressing a recessive trait if both parents carry a copy of the recessive allele.
Significance of Allelic Variation
The existence of different alleles is important for genetic diversity within a population. This diversity is important for a species’ ability to adapt and survive in changing environments. A population with more alleles is more likely to have individuals with traits that can withstand new diseases, climate shifts, or resource scarcity. This variation provides the raw material upon which natural selection acts, driving evolution.
Allelic variations also contribute to the unique characteristics of each individual. These differences can influence a person’s physical appearance, metabolic processes, and susceptibility or resistance to certain diseases. For instance, variations in specific alleles explain why some individuals respond differently to particular medications or are more predisposed to certain health conditions.