Recessive gene

In genetics, the term recessive gene refers to the an allele that causes a phenotype that is only seen in a homozygous genotype. Strictly, it is the phenotype (or trait) which is recessive, not the gene.

That is dominance/recessiveness refers to phenotype, not genotype. Consider sickle cell anemia as an example. The sickle cell genotype is caused by a single base pair change in the beta-globin gene: normal=GAG (glu), sickle=GTG (val). There are several phenotypes associated with the sickle genotype: 1) anemia (a recessive trait), 2) blood cell sickling (partially dominant), 3) altered beta-globin electrophoretic mobility (codominant), and 4) resistance to malaria (dominant). This example demonstrates that one can only refer to dominance/recessiveness with respect to individual phenotypes.