Glossary
dominant
A dominant allele is one whose trait appears when at least one copy is present, masking the recessive allele paired with it.
A dominant allele is one whose trait is expressed when at least one copy is present, so an organism with one dominant and one recessive allele shows the dominant trait. The word describes the phenotype, not the allele's strength or its frequency: a dominant allele is not better, commoner or more useful, and a recessive allele is not weaker. In the cross Bb × Bb, three of the four boxes carry at least one B, so three quarters of the offspring show the dominant trait and one quarter show the recessive one, which is the 3:1 phenotype ratio. Dominance is written in capitals by convention and recessiveness in lower case, but that is bookkeeping, not biology. The simple version covers some traits and not others: in incomplete dominance the heterozygote shows a blend between the two homozygotes, and in codominance both alleles are expressed fully, as in blood type AB. A question that asks whether a dominant allele is the more common one is testing exactly this misconception.
Taught in
Related terms
- recessive — A recessive allele is one whose trait appears only when both copies are recessive, because a dominant partner masks it.
- 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.
- genotype — A genotype is the pair of alleles an organism carries for a gene, written as letters such as BB, Bb or bb.
- phenotype — A phenotype is the trait an organism actually shows, so two different genotypes can produce the same one.
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)