Why do offspring resemble their parents? Such resemblances are passed on relatively unaltered from generation to generation through a process called heredity. The units of heredity are deoxyribonucleic acid (DNA) segments called genes. Encoded in every gene are biochemical instructions that determine the characteristics, or traits, of an organism. Genetics is the study of genes—how they operate and how they are transmitted from parents to offspring.
Even before the beginnings of written history people were aware that certain traits could be passed from parent to offspring. By selectively breeding animals or plants, humans produced livestock and crops that could provide food, pull plows, and supply companionship and protection. But while farmers and breeders learned to control the transmission of traits in agriculture, the actual process of heredity remained a mystery. Many theories were advanced. In ancient Greece, it was thought that traits were transmitted through the blood, and the word blood is still often used to denote ancestry. In the seventeenth century some biologists held that female eggs contained miniature offspring, with male sperm merely triggering embryonic development. Other biologists proposed the opposite—that tiny but fully formed offspring were present in the sperm.
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Back to Science key stage 3 Ages 11-14 years
