In biology, a germline is the line of cells that pass on genetic information from one generation to the next. The cells in which gametes (sperm and eggs) are produced are called germ cells. In sexually reproducing organisms, all of the cells descended from a single zygote contain the same genetic information as that zygote. However, during sexual reproduction, each parent contributes half of the genes (the gametes), so that the zygote and its descendants inherit a mixture of genes from both parents. Germline mutations occur when there is a change in the DNA sequence in the gametes or other reproductive cells; such changes can be passed on to future generations.
Mutations in somatic cells—the vast majority of cells in our bodies—are not passed on to offspring. So while you may have many mutated somatic cells, your children will not inherit those mutations unless they arise in your germline. That’s because only germline mutations can be transmitted to future generations through eggs or sperm.
Germline mutations are responsible for many hereditary diseases, including some forms of cancer. For example, BRCA1 and BRCA2 are two genes that normally help repair damaged DNA and prevent tumor growth; however, when these genes contain certain mutations, they lose their ability to perform these functions effectively, increasing an individual’s risk for developing breast or ovarian cancer. Other examples of conditions caused by germline mutations include Huntington’s disease and cystic fibrosis.
While most people think of germline mutations as being negative—and indeed, many are—there are also beneficial or “neutral” ones that can be passed down through families. For example, the CCR5-Δ32 mutation confers resistance to HIV infection; this mutation occurred naturally and over time has become more common in populations where HIV is endemic (such as sub-Saharan Africa). Similarly, sickle cell anemia confers resistance to malaria; this condition occurs when someone inherits two copies of a defective gene (one from each parent), which results in abnormal hemoglobin production. While sickle cell anemia can be debilitating—and even deadly—in regions where malaria is common, it confers a survival advantage because people with just one copy of the defective gene (and thus only partially impaired hemoglobin production) have increased resistance to malaria infections .