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The Britannica guide to statistics and probability. The Britannica Guide to Sound and Light. A Babylonian tablet more than 6, years old, for example, shows pedigree s of horses and indicates possible inherited characteristics. Other old carvings show cross- pollination of date palm trees.
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Most of the mechanisms of heredity, however, remained a mystery until the 19th century, when genetics as a systematic science began. Genetics arose out of the identification of genes, the fundamental units responsible for heredity. Genetics may be defined as the study of gene s at all levels, including the ways in which they act in the cell and the ways in which they are transmitted from parents to offspring.
Modern genetics focuses on the chemical substance that genes are made of, called deoxyribonucleic acid, or DNA , and the ways in which it affects the chemical reactions that constitute the living processes within the cell. Gene action depends on interaction with the environment. Green plant s, for example, have genes containing the information necessary to synthesize the photosynthetic pigment chlorophyll that gives them their green colour.
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Chlorophyll is synthesized in an environment containing light because the gene for chlorophyll is expressed only when it interacts with light. If a plant is placed in a dark environment, chlorophyll synthesis stops because the gene is no longer expressed.
Genetics as a scientific discipline stemmed from the work of Gregor Mendel in the middle of the 19th century. Mendel suspected that traits were inherited as discrete units, and, although he knew nothing of the physical or chemical nature of genes at the time, his units became the basis for the development of the present understanding of heredity.
Curiosity must first have been based on human family resemblances, such as similarity in body structure, voice, gait, and gestures. Such notions were instrumental in the establishment of family and royal dynasties. Early nomadic tribes were interested in the qualities of the animals that they herded and domesticated and, undoubtedly, bred selectively.
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The first human settlements that practiced farming appear to have selected crop plants with favourable qualities. Ancient tomb paintings show racehorse breeding pedigrees containing clear depictions of the inheritance of several distinct physical traits in the horses. Despite this interest, the first recorded speculations on heredity did not exist until the time of the ancient Greeks; some aspects of their ideas are still considered relevant today.
Hippocrates c. Aristotle — bce emphasized the importance of blood in heredity. He thought that the blood supplied generative material for building all parts of the adult body, and he reasoned that blood was the basis for passing on this generative power to the next generation.
These male and female contributions united in the womb to produce a baby. The blood contained some type of hereditary essences, but he believed that the baby would develop under the influence of these essences, rather than being built from the essences themselves. These single hereditary factors were identified as genes.
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