The instruction manual for every living organism is written in DNA, contained within structures called genes. While all members of a species share the same set of genes, the instructions within those genes are not static. This variation is the reason for the rich diversity seen across populations, from differences in human height and eye color to varied coat patterns in animals. Understanding this variety requires looking closely at the specific forms a single genetic instruction can take.
Alleles: The Different Versions of a Gene
The different versions of a gene for the same trait are known as alleles. A gene is a segment of DNA that provides the instructions for a specific characteristic, such as flower color or blood type. An allele represents a specific sequence variation within that gene, much like different options for a single ingredient in a recipe. For instance, the gene for eye color exists at a specific location on a chromosome, and the alleles determine if the eyes will be blue, brown, or green.
Each gene occupies a precise address on a chromosome, which geneticists call a locus. Organisms that reproduce sexually inherit two complete sets of chromosomes, receiving one chromosome from each parent. This results in every individual possessing two alleles for each gene, one on each paired chromosome. These two alleles may be identical or different, leading to the wide range of traits observed.
How Genetic Variation Arises
New alleles originate through random changes in the DNA sequence known as mutations. These alterations occur spontaneously, often during DNA replication when cells divide. A mutation in a reproductive cell can be passed down to an offspring, introducing a new version of the gene into the population.
One common type is a base substitution, or point mutation, where a single nucleotide base is swapped for another. This small change can alter the resulting protein structure or have no functional effect. More disruptive mutations include insertions or deletions, where one or more nucleotide bases are added or removed from the gene sequence.
An insertion or deletion can shift the gene’s “reading frame,” often leading to a non-functional protein. This significant change creates a novel allele that may result in a new trait or a medical condition. While some mutations are harmful, others can be neutral or provide a beneficial adaptation, driving the genetic diversity necessary for evolution.
Interaction and Expression
The two alleles an individual inherits interact to determine the physical characteristic that is expressed. The simplest form of interaction is dominance, where one allele completely masks the presence of the other. For example, a single dominant allele for brown eyes is sufficient to produce brown eyes, regardless of the second allele. The masked allele is termed recessive and is only expressed if an individual inherits two copies of it.
Not all alleles follow this simple pattern. In incomplete dominance, the resulting trait is a blend of the two alleles inherited. A classic example is the snapdragon flower, where crossing a red-flowered plant and a white-flowered plant produces offspring with pink flowers. Neither the red nor the white allele is fully dominant, resulting in an intermediate color.
A different pattern, called codominance, occurs when both inherited alleles are fully and separately expressed at the same time. The human ABO blood group system provides a clear illustration, where an individual inheriting both the A and B alleles has AB blood type. Both the A and B proteins are produced and displayed on the surface of the red blood cells.
Translating Alleles into Traits
The specific combination of alleles inherited by an individual constitutes their genotype. This is the internal genetic code for a specific trait, such as having two copies of the blue eye allele. The phenotype, conversely, is the observable, physical manifestation of that genotype, such as having blue or brown eyes.
The genotype serves as a blueprint, but the final phenotype can be modified by environmental factors. For instance, the alleles that determine skin color interact with the amount of sun exposure an individual receives, which darkens pigmentation. The observable trait is a result of the underlying genotype interacting with the environment.