Glossary
allele
An allele is one version of a gene; a person carries two for most genes, one on each chromosome of a homologous pair, and the pair is the genotype.
An allele is one version of a gene. A gene is a stretch of DNA carrying the instructions for a trait; the alleles are the variants of that instruction, and a person carries two of them for most genes, one on each chromosome of a homologous pair. The pair is the genotype — AA, Aa or aa — and the trait it produces is the phenotype. An allele is dominant when one copy is enough to show its trait, and recessive when the trait appears only if both copies are the same recessive allele. A Punnett square works out what a cross can produce: the alleles of one parent across the top, the alleles of the other down the side, and each box the combination a child could inherit. The ratios that come out of the square, such as 3:1 for a cross of two heterozygotes, are predictions about offspring — not a rule that a family of four will show three of one and one of the other.
Taught in
Related terms
- mitosis — Mitosis is the division that grows and repairs a body: one cell becomes two, and each daughter cell carries the same set of chromosomes as the parent.
- nucleotide — A nucleotide is the building block of DNA and RNA: a phosphate, a five-carbon sugar, and one of the nitrogenous bases.
- gene — A gene is a section of DNA that carries the instructions for a trait or a protein, and the alternative versions of a gene are its alleles.
- dominant — A dominant allele is one whose trait appears when at least one copy is present, masking the recessive allele paired with it.
- recessive — A recessive allele is one whose trait appears only when both copies are recessive, because a dominant partner masks it.
Source: The biology guide in this course: its genetics definitions follow the National Human Genome Research Institute's Talking Glossary of Genomic and Genetic Terms, and its process descriptions follow the US National Cancer Institute's SEER Training Modules. Both are US federal government material, not subject to copyright in the United States. The microbiology entries — the bacterial cell, the Gram stain and the chain of infection — were written for this course from the infection-control guidance the biology guide's own source list names (US Centers for Disease Control and Prevention), which the genetics glossary does not cover. The definitions above are written for this course in our own words. (NHGRI Talking Glossary accessed 2026-10-26; definitions written 2026-10-27; the CDC basis for the microbiology entries — Principles of Epidemiology in Public Health Practice, third edition — accessed 2026-10-28, the date the biology guide's own source list records)