At the time of conception, two living germ cell—the sperm from the father and the ovum, or egg, from the mother—unite to produce a new individual. The male and female germ cells are known technically as gametes, and the single cell they form at the moment of conception is called the zygote. Both the egg and the sperm contain tiny thread-like structures called chromosomes. These have been called, somewhat poetically, the “threads of life.”
In 1962, the Nobel Prize for Medicine and physiology was awarded to James Watson, Maurice Wilkins, and Francis Crick for the discoveries in the field of the chemistry and mechanics of heredity. Each chromosome (threads) is made up of long molecules of deoxyribonucleic acid, called DNA for short.
The atoms of a DNA molecule are arranged in two long strands twisted together in a complex double spiral, along each spiral are ultramicroscopic areas called genes, which are the bearers of heredity. The order of the atoms which make up a particular gene serves as a blueprint or master code which govern the formation of the parts of the body for which that gene responsible. The branch of biology concerned with the mechanism of heredity is called genetics.