Heredity is the biological process by which characteristics are passed from parents to offspring. Deoxyribonucleic acid (DNA) carries the instructions guiding the development and function of living organisms. Genes and alleles are foundational terms for understanding these instructions, though they are often used interchangeably. This article clarifies their distinct roles in shaping an organism’s traits.
What is a Gene
A gene is the basic physical and functional unit of heredity, a specific segment of DNA that holds coded information for various biological functions. Genes are arranged along chromosomes within the nucleus of cells. These DNA segments direct the synthesis of proteins or functional RNA molecules, which are crucial for an organism’s development and operation. For example, genes provide instructions for pigment production, such as eye color. The human genome contains approximately 19,900 protein-coding genes.
What is an Allele
An allele is a specific variant or form of a gene that determines the specific expression of a particular trait. For instance, a gene might dictate eye color, but different alleles would specify brown, blue, or green eyes. Organisms inherit two copies of each gene, one from each parent. These two alleles occupy the same specific location on homologous chromosomes. Alleles can be dominant, expressed even when only one copy is present, or recessive, expressed only if two copies of the recessive allele are inherited.
The Relationship Between Genes and Alleles
An allele is a version of a gene. A gene determines what trait will be expressed, such as hair color, while its alleles determine how that trait manifests, for instance, as brown, blonde, or red hair. Think of a gene as a specific recipe in a cookbook; it outlines how to make a particular dish, like “chocolate cake.” The alleles, in this analogy, would be the different variations of that recipe—for example, a recipe for “dark chocolate cake” or “milk chocolate cake”. Both recipes are for chocolate cake, but they yield distinct outcomes.
These two alleles then interact to determine the observable characteristic of the trait. If an individual inherits two identical alleles for a gene, they are considered homozygous for that trait. Conversely, if they inherit two different alleles, they are heterozygous. The interplay between these inherited alleles dictates the final observable trait, known as the phenotype.
Real-World Examples
The distinction between genes and alleles is evident in human traits like eye color and blood type. For eye color, the gene specifies the general trait, but various alleles dictate specific shades, like brown, blue, or green. For example, the allele for brown eyes is dominant over the allele for blue eyes. An individual with one brown-eye allele and one blue-eye allele will likely have brown eyes.
The ABO blood group system is another example. The ABO gene determines blood type, but it has three primary alleles: IA, IB, and i. The IA allele leads to type A blood, the IB allele to type B blood, and the i allele to type O blood. Both IA and IB alleles are dominant over the i allele, and they are codominant with each other. An individual inheriting an IA allele from one parent and an IB allele from the other will have type AB blood.