In biology and genetics, the germline of a mature or developing individual is the line (sequence) of germ cells that have genetic material that may be passed to a child.
For example, gametes such as the sperm or the egg, are part of the germline. So are the cells that divide to produce the gametes, called gametocytes, the cells that produce those, called gametogonia, and all the way back to the zygote, the cell from which the individual developed.
Cells that are not in the germline are called somatic cells. This refers to all of the cells of body apart from the gametes. If there is a mutation or other genetic change in the germline, it can potentially be passed to offspring, but a change in a somatic cell will not be.
Germline cells are immortal, in the sense that they have the potential to reproduce indefinitely. This is largely due to the activity of the enzyme known as telomerase. This enzyme extends the telomeres of the chromosome, preventing chromosome fusions and other negative effects of shortened telomeres. Most somatic cells, by comparison, can only divide around 30-50 times according to the Hayflick limit. Certain somatic cells, known as stem cells, also express telomerase and are potentially immortal.
Not all multicellular organisms differentiate cells into somatic and germ lines. Notably, plants have no germline cells separate from stem cells.
Germline can refer to a lineage of cells spanning many generations of individuals—for example, the germline that links any living individual to the hypothetical first eukaryote of about 2 billion years ago, from which all plants and animals descend.