Allele
An allele is one of two or more alternative forms of a gene that occupy the same locus (position) on homologous chromosomes. In diploid organisms, each individual carries two alleles for each gene — one inherited from each parent — and these alleles may be identical (homozygous) or different (heterozygous). The interaction between alleles (dominance, co-dominance, incomplete dominance) determines the phenotype expressed by an individual.
Types of Allelic Relationships and Their Expression
| Relationship | Genotype Example | Phenotype Outcome | Example Trait |
|---|---|---|---|
| Complete dominance | Aa | Dominant allele masks recessive | Pea plant seed colour (Mendel) |
| Recessive | aa | Only expressed when homozygous | Cystic fibrosis (CFTR mutation) |
| Codominance | I^A I^B | Both alleles expressed simultaneously | Blood type AB |
| Incomplete dominance | Rr | Intermediate phenotype | Snapdragon flower colour (pink) |
| Multiple alleles | I^A, I^B, i | More than two alleles exist in population | ABO blood group system |
Interactive Tools
Wikimedia Commons, CC BY-SA
Related Terms
Gene
A gene is a specific sequence of DNA (or RNA in some viruses) that encodes the information required to produce a functional product, most commonly a protein or functional RNA molecule. Genes occupy defined loci on chromosomes and are the fundamental units of heredity, transmitted from parent to offspring. The modern molecular definition encompasses not only coding sequences (exons) but also regulatory elements such as promoters and enhancers that control when, where, and how much a gene is expressed.
Genotype
Genotype refers to the complete genetic constitution of an organism — specifically, the set of alleles present at a given locus or across all loci in the genome. Unlike phenotype (the observable characteristics), genotype is the underlying inherited sequence information that, in combination with environmental influences, determines the traits of an organism. Determining genotype is fundamental to genetic counselling, disease risk assessment, and molecular breeding programmes.
Chromosome
A chromosome is a long, thread-like structure composed of DNA and associated histone proteins (chromatin) that carries genes in a linear sequence. In humans, the diploid somatic cells contain 46 chromosomes (23 pairs), with each chromosome harbouring thousands of genes. Chromosomes are visible under a light microscope during cell division and are essential for the accurate segregation of genetic material to daughter cells.
Shortened from "allelomorph": Greek "allelos" (one another, of each other) + "morphe" (form, shape). The term was introduced by William Bateson in 1902 to describe alternative forms of Mendelian factors.