Germ Plasm Theory of Heredity

The concept that individuals inherited traits acquired through use and disuse by their parents dates back to the ancient Greeks and was formally stated by Jean-Baptiste Lamarck in the early 1800s. Throughout most of the nineteenth century, Lamarckism and the blending of the characteristics of the father and mother were the favored theories of heredity. Among the greatest of all biologists was the German August Weismann who, in a dramatic experiment, greatly weakened support for Lamarckism. He amputated the tails of 901 mice for five successive generations; no mouse was ever born without a tail and the tail lengths in the fifth generation were as long as those in the first. Rather than refuting Lamarckian inheritance by logic, Weismann did so by experimentation.

In his germ plasm theory, proposed in 1883 and detailed in his 1893 book, The Germ-Plasm: A Theory of Heredity, Weismann emphasized the stability of germ plasm (hereditary material, now called genes) that was transmitted, without change, from generation to generation. In his theory, the environment had little, if any, effect on the germ plasm, even if the environment altered external body characteristics. Weismann drew a clear distinction between body material (soma) and hereditary material (germ plasm). He postulated that in multicellular organisms, germ plasm is independent of other body cells; that somatic cells (non-sex cells) are involved in bodily activities but do not function in heredity; and, most importantly, that germ plasm is the essential element of germ cells or gametes (sperm cells and egg cells).

This illustration from “Thumbelina,” in a 1913 edition of Hans Andersen’s Fairy Tales, shows an old field mouse giving shelter to a tiny girl. An experiment in which five successive generations of mice had their tails amputated challenged the Lamarkian view of inheritance, as the tail lengths of the fifth generation matched those of the first generation.

Weismann’s germ plasm theory had a great influence on biological thinking—namely, the rediscovery of Mendel’s rules and the role of chromosomes in inheritance—but a number of recent findings have severely undermined its validity. Weismann’s view that there is only a set quantity of germ plasm, which is reduced with successive generations after successive somatic cell divisions, was undermined when the clone ewe, Dolly, produced by somatic cell nuclear transfer, was found to possess a full complement of germ plasm. In addition, Lamarckism has been recently resurrected as epigenetics.