Genetics holds the blueprints for all living things, dictating everything from eye color to how our bodies function. Genes are the fundamental units that pass traits from one generation to the next. Understanding the relationship between genes and their variations, known as alleles, provides insight into the diversity of life and how inherited characteristics manifest.
What Genes Are
Genes are specific segments of deoxyribonucleic acid (DNA) that serve as instructions for building proteins or controlling various traits. Each gene occupies a distinct, fixed position on a chromosome, called its locus. For instance, the human genome contains approximately 19,000 to 20,000 protein-coding genes spread across 23 pairs of chromosomes. These genes act as the foundational code, directing cellular processes and influencing physical and biological characteristics.
Genes provide the information necessary for cells to create specific proteins. Proteins are complex molecules that perform a vast array of functions, from forming structural components like hair and muscle to acting as enzymes that facilitate chemical reactions. While many genes code for proteins, some also produce functional RNA molecules or regulate the activity of other genes. This complex interplay ensures that an organism develops and operates correctly, with each gene contributing to the overall genetic makeup.
What Alleles Are
Alleles are different versions of a single gene. They arise from slight variations in the DNA sequence, meaning they code for the same trait but can result in different expressions. For example, a gene for eye color might have an allele for brown eyes and another for blue eyes. These variations contribute to the unique characteristics observed among individuals.
For most genes, an individual inherits two alleles, one from each parent. These can be identical or different. If both inherited alleles for a specific gene are the same, the individual is homozygous; if different, they are heterozygous. The interaction between these two alleles determines how a trait is expressed.
Some alleles are dominant, meaning their trait will be expressed even if only one copy is present, while recessive alleles only manifest their trait if two copies are inherited. For example, the allele for brown eyes is generally dominant over the allele for blue eyes.
How Genes and Alleles Work Together
A gene can be thought of as the category for a specific characteristic, such as “eye color” or “blood type.” Alleles represent the specific options within that category, like “brown eyes,” “blue eyes,” or “A blood type,” “B blood type,” and “O blood type.” This distinction highlights that while a gene provides the general blueprint, alleles provide the precise details of that blueprint.
When an individual inherits two alleles for a gene, one from each parent, these alleles interact to determine the observable trait. For example, the gene responsible for blood type has multiple alleles, including A, B, and O. An individual inheriting an A allele from one parent and an O allele from the other will have blood type A, because the A allele is dominant over O. The specific combination of alleles inherited, known as the genotype, dictates the physical manifestation of the trait, or the phenotype.