Glossary
genotype
A genotype is the pair of alleles an organism carries for a gene, written as letters such as BB, Bb or bb.
A genotype is the set of alleles an organism carries for a gene, written as the two letters it inherited, one from each parent: BB, Bb or bb. It is the genetic makeup, not the trait you can see — that is the phenotype. Two genotypes, BB and Bb, can produce the same phenotype when B is dominant, which is why a dominant-looking individual cannot be read off directly and a testcross may be needed to tell the two apart. When both alleles are the same the genotype is homozygous; when they differ it is heterozygous. A genotype ratio counts those classes, while a phenotype ratio groups by what is visible: Bb × Bb gives genotypes in the ratio 1 BB : 2 Bb : 1 bb, but phenotypes in the ratio 3 dominant to 1 recessive. A question that asks what an organism carries is asking for the genotype; a question that asks what it looks like is asking for the phenotype.
Taught in
Related terms
- phenotype — A phenotype is the trait an organism actually shows, so two different genotypes can produce the same one.
- 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)