Here's why:
* Germline vs. Somatic Cells: Genetic variation comes from changes in the germline cells, which are the cells that give rise to sperm and egg cells. Body cells, or somatic cells, make up the rest of the body.
* Inheritance: Mutations in somatic cells only affect the individual organism in which they occur. They are not present in the germline cells and therefore are not passed on to the next generation.
* Example: If a person develops skin cancer due to a mutation in a skin cell, their offspring will not inherit that specific mutation.
While somatic mutations don't contribute to genetic variation in a population, they are important for several reasons:
* Cancer: Somatic mutations are often the cause of cancer.
* Evolution: Somatic mutations can sometimes contribute to the evolution of a species if they occur in a cell that will contribute to the reproductive system. This is rare, but it can happen.
In summary, while mutations in body cells can affect an individual's health, they do not impact the genetic variation that gets passed down from one generation to the next.
