Genotype

Typically, one refers to an individual's genotype with regard to a particular gene of interest and, in polyploid individuals, it refers to what combination of alleles the individual carries (more at homozygous, heterozygous). Any given gene will usually cause an observable change in an organism, known as the phenotype (otherwise we wouldn't be interested in it). The word genotype and phenotype are distinct for at least two reasons:

1. To distinguish the source of an observer's knowledge (I know about genotype because I looked at the DNA; I know about phenotype because I looked at an organism.)
2. Genotype and phenotype are not always directly correlated. Some genes only express a given phenotype in certain environmental conditions. Conversely, some phenotypes could be the result of various genotypes.

See also : genetics, Genotype-phenotype distinction