Why did cutting off the tails of mice not produce tailless descendants?

The experiment of cutting off the tails of mice did not produce tailless descendants because physical mutilations or modifications acquired by an organism during its lifetime are not passed on to its offspring.123

This experiment, conducted by German biologist August Weismann, served as a critical challenge to the doctrine of Lamarckism, which erroneously assumed that acquired characteristics could be inherited. Weismann’s findings demonstrated that the hereditary material—which he termed the germ plasm—is distinct from the soma (the body cells).1234

Because the germ cells (eggs and sperm) are separate from the body cells, changes made to the body, such as the amputation of a tail, do not alter the genetic information contained within the germ cells. Consequently, the hereditary units for developing a tail remain intact in the germ plasm, ensuring that each generation continues to grow normal tails.1253