Even at birth the whole individual is destined to die, and perhaps his organic disposition may already contain the indication of what he is to die from.

—Sigmund Freud, 1924

Genetics, the study of biologic inheritance, explains how and why certain traits, such as hair color and blood types, run in families. Each individual develops from a single fertilized egg, which contains all the information necessary for the development of innate mental and physical characteristics. This information is carried in twenty-three pairs of rod-shaped chromosomes (containing thousands of genes) that are responsible for determining and transmitting hereditary characteristics. Each pair of chromosomes includes one inherited from the mother and one from the father.

Genes determine specific physical features, such as height and the color of skin, hair, and eyes. Genes also direct the production of cell proteins needed for health and development. There are two types of...