Genetics was the field of science concerned with the study of genes, the method common to many species by which traits and characteristics were passed from one generation to the next in reproduction. The term was also used to refer to anything pertaining to genes. One specializing in genetics was known as a geneticist.
Each individual has a unique genetic structure. Likewise, every alien species has a different genetic pattern. For example, Bajoran genes are quite different from those of Cardassians. This fact proved troublesome for the like of Seska, who was exposed as a Cardassian spy disguised as a Bajoran Starfleet officer when it was found that the genetic markers in her blood were those of a Cardassian. (VOY: "State of Flux")
Khan Noonien Singh and the other augments of the Eugenics Wars were products of eugenics. Their genetic makeup was chosen through selective breeding and their DNA altered through the process of genetic engineering. (TOS: "Space Seed")