Allele: variant forms of a gene at a chromosomal locus
An allele is a specific variant of a gene at a particular location on a chromosome. Alleles determine genetic variation, influence traits, and are central to inheritance, population genetics, and medical genetics.
Overview
An allele is a specific version of a gene found at a particular position, or locus, on a chromosome. In molecular terms an allele corresponds to a particular DNA sequence variant at that location — ranging from a single nucleotide change to larger insertions or deletions — that can affect how a gene functions. The concept links the molecular sequence on the chromosome with observable inheritance patterns and trait variation.
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Basic characteristics and types
Most multicellular animals and plants are diploid: they carry two sets of chromosomes, one inherited from each parent. For each genetic locus they therefore possess two alleles (except on the sex chromosomes in many species). When both alleles are the same the individual is homozygous at that locus; when they differ the individual is heterozygous. Alleles may be described as dominant, recessive, co‑dominant, or part of an allelic series depending on how they influence the trait in heterozygotes.
How alleles arise and persist
New alleles originate by mutation — random changes in the coding DNA or regulatory regions of a gene — and are shaped by natural selection, genetic drift, migration, and mating patterns. In populations the frequency of an allele can rise, fall, or be maintained (for example, by balancing selection). Population genetics uses allele frequencies to describe genetic variation and predict evolutionary change.
Examples and applications
Some well‑known examples illustrate allele concepts: the ABO blood group system is controlled by multiple alleles that produce different sugar‑adding enzymes; the sickle‑cell allele of the hemoglobin gene causes a disease in homozygotes but can convey malaria resistance in heterozygotes. Understanding alleles is essential in medicine (diagnosing hereditary conditions), agriculture (selective breeding), conservation biology, and forensic genetics.
Distinctions and important terms
- Gene vs allele: a gene is a unit of heredity; an allele is one of the variant forms that gene can take at its locus. See also gene.
- Locus: the specific chromosomal position where alleles occur; see chromosome for context.
- Wild‑type and mutant: 'wild‑type' often denotes the common or reference allele, while 'mutant' denotes a variant that arose by mutation.
Alleles provide the raw material for genetic diversity. Their study links molecular biology, classical Mendelian inheritance, and population‑level processes to explain why organisms vary and how traits are transmitted across generations.
Questions and answers
Q: What is an allele?
A: An allele is a form of a gene at a particular position on a chromosome.
Q: How many sets of chromosomes do typical plants and animals have?
A: Typical plants and animals have two sets of chromosomes, one set inherited from each parent.
Q: Are organisms with two sets of chromosomes called diploid or haploid?
A: Organisms with two sets of chromosomes are called diploid.
Q: How many alleles do diploid organisms have at each gene locus (except on the sex chromosomes)?
A: Diploid organisms have two alleles at each gene locus (except on the sex chromosomes).
Q: What is a homozygote?
A: A homozygote is an individual with two identical alleles at a particular gene locus and is said to be homozygous.
Q: What is a heterozygote?
A: A heterozygote is an individual with two different alleles at a particular gene locus and is said to be heterozygous.
Q: Where is the bit of coding DNA located in an allele?
A: The bit of coding DNA in an allele is located at a particular position on a chromosome at a gene locus.
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Author
AlegsaOnline.com Allele: variant forms of a gene at a chromosomal locus Leandro Alegsa
URL: https://en.alegsaonline.com/art/2774