Heredity, the transmission of characteristics from parents to their offspring, is a fundamental process in biology, shaping the diversity of life. This process explains why individuals resemble their parents and accounts for variations within a species. Biological instructions guide the development of these traits, passing from one generation to the next. Understanding the basic units of this inheritance clarifies how observable features are determined.
What a Gene Is
A gene is a fundamental unit of heredity, acting as a specific segment of DNA. These segments are organized at particular positions on structures called chromosomes, located within the nucleus of cells. Genes carry instructions for building specific proteins or controlling particular traits, such as hair or eye color. Proteins serve as the building blocks and perform numerous functions within an organism.
Genes vary in size, and the human body contains tens of thousands of them. While many genes code for proteins, some regulate other genes or produce functional RNA molecules. These instructions encoded within genes are crucial for an organism’s growth, proper functioning, and overall health.
What an Allele Is
While a gene represents a characteristic, an allele is a specific version or variant of that gene. For example, a gene might dictate eye color, but different alleles specify blue, brown, or green eye colors. These variations typically arise from genetic mutations.
Individuals inherit two alleles for most genes, receiving one from each biological parent. These alleles are situated at the same specific location, known as a locus, on homologous chromosomes. Although alleles represent different forms of the same gene, they still control the same trait. This inheritance pattern results in a unique combination of alleles within each individual, contributing to their distinct physical and biological features.
The Interplay Between Genes and Alleles
The relationship between genes and alleles can be understood by considering the gene as the general category of a trait and alleles as the specific options within that category. For instance, “hair color” is a gene, while “black hair,” “blonde hair,” or “red hair” are different alleles for that gene.
Having two copies of each gene, and thus two alleles, allows for various combinations that influence an individual’s traits. These combinations contribute to the genetic diversity observed within a population. The specific interaction between these two inherited alleles ultimately determines how a trait is expressed.
How Alleles Determine Traits
The combination of inherited alleles for a particular gene dictates an observable trait, known as the phenotype. The interaction between alleles is often described using terms like dominant and recessive. A dominant allele expresses its trait even when only one copy is present, masking the effect of a recessive allele. A recessive allele, conversely, only expresses its trait if an individual inherits two copies of it, one from each parent.
When an individual inherits two identical alleles for a gene, they are considered homozygous for that trait. This can be either homozygous dominant (two dominant alleles) or homozygous recessive (two recessive alleles). If an individual inherits two different alleles, they are heterozygous for that trait. In heterozygous individuals, the dominant allele typically determines the phenotype. For example, the allele for brown eyes is dominant over blue, so an individual with one brown and one blue eye allele will have brown eyes. Some alleles exhibit codominance or incomplete dominance, where both contribute to the phenotype or result in an intermediate expression.