Glossary
phenotype
A phenotype is the trait an organism actually shows, so two different genotypes can produce the same one.
A phenotype is the observable trait an organism has — a flower's color, a person's blood type, the shape of a seed. It is what the genotype produces, not the alleles themselves, and the two do not map one to one. A dominant allele shows itself in a heterozygote, so BB and Bb both give the dominant phenotype, which is why a ratio counted by appearance is 3:1 where the one counted by genotype is 1:2:1. The phenotype is also not written by the genes alone: the environment has a say, which is why identical genotypes can look different under different conditions. In incomplete dominance the heterozygote has its own intermediate phenotype, and in codominance both phenotypes appear at once. A question that asks what an organism looks like, or what proportion of offspring show a trait, is asking about the phenotype; a question about the letters in the boxes is asking about the genotype.
Taught in
Related terms
- genotype — A genotype is the pair of alleles an organism carries for a gene, written as letters such as BB, Bb or bb.
- 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)