Chapter 16. Mendelian Inheritance

Early theories of heredity predicted the transmission of acquired characteristics.

The first written speculations about mechanisms of heredity were made by the ancient Greeks. Hippocrates (460–377 BCE), considered the founder of Western medicine, proposed that each part of the body in a sexually mature adult produces a substance that collects in the reproductive organs and that determines the inherited characteristics of the offspring. An implication of this theory is that any trait, or characteristic, of an individual can be transmitted from parent to offspring. Even traits that are acquired during the lifetime of an individual, such as muscle strength or bodily injury, were thought to be heritable because of the substance supposedly passed from each body part to the reproductive organs.

The theory that acquired characteristics can be inherited was invoked to explain such traits as the webbed feet of ducks, which were thought to result from many successive generations in which adult ducks stretched the skin between their toes while swimming and passed this trait to offspring. A few decades later, however, Aristotle (384–322 BCE) emphasized several observations that the theory of inheritance of acquired characteristics cannot account for:

Traits such as hair color can be inherited, but it is difficult to see how hair—a nonliving tissue—could send substances to the reproductive organs.
Traits that are not yet present in an individual can be transmitted to the offspring. For example, a father and his adult son can both be bald, even though the son was born before the father became bald.
Parts of the body that are lost as a result of surgery or accident are not missing in the offspring.

From these and other observations, Aristotle concluded that the process of heredity transmits only the potential for producing traits present in the parents, and not the traits themselves. Nevertheless, Hippocrates’ theory influenced biology until well into the 1800s. It was incorporated into an early theory of evolution proposed by the French biologist Jean-Baptiste Lamarck around 1800. Charles Darwin, however, developed an alternative theory—the theory of evolution by natural selection (Chapter 21).

While traits acquired during the lifetime of a parent are not transmitted to the offspring, parental misfortune or misbehavior can nevertheless result in impaired fetal development and in some cases permanent damage. For example, maternal malnutrition, drug addiction, alcoholism, infectious disease, and other conditions can severely affect the fetus, but these effects are due to disruption of fetal development and not to changes in the genome.