Glossary
recessive
A recessive allele is one whose trait appears only when both copies are recessive, because a dominant partner masks it.
A recessive allele is one whose trait appears only when both copies of the gene are recessive, because a single dominant partner masks it. That is why a recessive trait can disappear for a generation and then return: two parents who each carry one hidden copy can produce a child with two, and the child shows the trait while neither parent does. In the cross Bb × Bb, one box in four is bb, which is the 1 in the 3:1 phenotype ratio. A recessive allele is written in lower case by convention, and it is neither rarer nor weaker than a dominant one — it is simply an allele whose product is not enough to show when a working copy is also present. A testcross reveals whether a dominant-looking individual carries a hidden recessive copy: cross it with a homozygous recessive and look at the offspring. X-linked recessive conditions are commoner in males because a male has one X, so a single recessive allele on it has no matching allele to mask it.
Taught in
Related terms
- dominant — A dominant allele is one whose trait appears when at least one copy is present, masking the recessive allele paired with 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)