An allele is a specific version of a gene, representing one of two or more forms that can exist at a particular location on a chromosome. These genetic variations are fundamental to heredity, influencing an organism’s observable characteristics. Every individual inherits a unique combination of alleles, which contributes to their distinct traits. Understanding alleles is central to how traits are passed down through generations and how biological diversity arises.
Understanding Alleles and Genes
Genes are segments of DNA that carry the instructions for building and maintaining an organism. Each gene occupies a specific position on a chromosome, known as a locus. Alleles are different versions of a particular gene. For instance, a gene might dictate eye color, while different alleles of that gene would specify blue, brown, or green eyes.
Humans inherit two copies of each gene, with one allele coming from each biological parent. These two alleles for a given gene are located at the same locus on a pair of homologous chromosomes. The combination of these inherited alleles determines the specific characteristics an individual will express.
How Alleles Determine Traits
The interaction between inherited alleles determines an individual’s observable traits, known as their phenotype. The specific combination of alleles an individual possesses for a given gene is called their genotype. Alleles often interact in a dominant-recessive manner. A dominant allele expresses its trait even if only one copy is present. A recessive allele expresses its trait only if two copies are present in the genotype.
For example, in Gregor Mendel’s pea plants, the allele for purple flowers is dominant over the allele for white flowers. A pea plant with one purple allele and one white allele (a heterozygous genotype) will display purple flowers. Only a plant with two white alleles (a homozygous recessive genotype) will have white flowers. For human earlobe attachment, the allele for unattached earlobes is dominant over the allele for attached earlobes.
Alleles and Human Diversity
The assortment of alleles within the human population drives human diversity. Different combinations of alleles contribute to a wide range of observable traits, from characteristics like eye and hair color to more intricate features such as metabolism and physical build. For instance, while brown eye color was once thought to be controlled by a single dominant gene, scientists now understand that at least eight genes influence the final eye color, with the OCA2 gene playing a major role in the brown/blue spectrum.
The ABO blood group system is another example of allelic variation contributing to human diversity. This system involves three common alleles: A, B, and O, located on chromosome 9. Alleles A and B are codominant, meaning both are expressed if present, resulting in AB blood type, while the O allele is recessive. This genetic variation allows for adaptation and evolution over time.
Alleles and Inherited Conditions
While most allelic variations contribute to the normal spectrum of human diversity, certain alleles can be associated with inherited genetic conditions. These conditions arise when a particular allele, often a mutated or non-functional version, disrupts normal biological processes. Such disorders can follow different patterns of inheritance, depending on the gene’s location and whether one or two altered copies are needed for the condition to manifest.
For instance, conditions like Huntington’s disease are inherited in an autosomal dominant manner, meaning only one copy of the altered allele is sufficient for a person to be affected. In contrast, conditions such as cystic fibrosis are autosomal recessive, requiring an individual to inherit two copies of the altered allele to develop the condition. In these recessive cases, individuals with only one copy of the altered allele are considered carriers and do not show symptoms.