Our bodies are made of countless cells, and within each cell’s nucleus lies our genetic instruction manual: DNA. This DNA is organized into structures called chromosomes, which contain segments known as genes. A gene is a specific sequence of DNA that carries the instructions for building a particular protein or influencing a specific characteristic, such as eye color or height. The precise location of a gene on a chromosome is referred to as its locus. Think of a chromosome as a long street, and a gene’s locus as a specific house number.
Alleles: The Building Blocks of Variation
Genes, while defining traits, can exist in different versions, and these variations are called alleles. An allele represents a distinct form of a gene found at the same locus on homologous chromosomes, which are the paired chromosomes inherited one from each parent. For instance, a gene for flower color might have one allele for red petals and another allele for white petals. These variations in alleles are the raw material for genetic diversity within a population.
Alleles arise through a process called mutation, which involves a change in the DNA sequence of a gene. While many mutations are neutral or detrimental, some can lead to beneficial traits, contributing to evolution and the diversity of life.
Genotypes: The Specific Pairings
The “combination of genes at a single locus” refers to an individual’s genotype, which is the specific pairing of the two alleles inherited for a particular gene. Since most organisms inherit one chromosome from each parent, they receive two alleles for every gene. These two alleles can be identical or different, leading to distinct genotypic classifications.
When an individual inherits two identical alleles for a specific gene, they are considered homozygous for that locus. This can be homozygous dominant, meaning both alleles are the stronger, expressed version (e.g., ‘AA’), or homozygous recessive, meaning both alleles are the weaker, masked version (e.g., ‘aa’). Conversely, if an individual inherits two different alleles for a gene, they are heterozygous, meaning they carry one of each version (e.g., ‘Aa’).
Phenotypes: How Combinations Express Traits
The genotype translates into an observable characteristic known as the phenotype. This can be a physical feature like eye color, a biochemical trait, or a behavioral pattern. The way alleles interact within the genotype determines which phenotype is expressed. The most common interaction involves dominant and recessive alleles.
In complete dominance, a dominant allele will completely mask the presence of a recessive allele when both are present in a heterozygous genotype. For example, if an allele for brown eyes (dominant) is paired with an allele for blue eyes (recessive), the individual will have brown eyes because the brown allele is fully expressed. The recessive blue eye allele is still present in their genotype and can be passed on to offspring, even though it is not outwardly visible.
Other patterns of allele interaction exist beyond complete dominance. In incomplete dominance, the heterozygous genotype results in a phenotype that is a blend or intermediate of the two homozygous phenotypes. For instance, if a red-flowered plant is crossed with a white-flowered plant, and the alleles exhibit incomplete dominance, their offspring might have pink flowers. Neither the red nor white allele is fully dominant, leading to an intermediate color.
Another pattern is codominance, where both alleles in a heterozygous genotype are simultaneously and fully expressed, without blending. A classic example in humans is the MN blood group system, where individuals with both M and N alleles will express both M and N markers on their red blood cells. Both parental traits appear together in the phenotype.
Impact on Individual Traits
The combination of genes at a single locus influences individual traits. Many human characteristics are determined by single-gene inheritance patterns. For example, the ABO blood type system in humans is governed by a single gene locus with multiple alleles (A, B, and O), demonstrating both codominance (A and B alleles) and complete dominance (A and B over O).
Other familiar human traits attributed to a single gene locus include earlobe attachment, where free earlobes are dominant over attached earlobes, and the ability to roll one’s tongue. While some traits like freckles or dimples are often cited as simple single-gene traits, their inheritance can sometimes be more complex, involving multiple genes or environmental factors. These examples illustrate how specific allele combinations at a single genetic location manifest as observable characteristics.