Heredity
Heredity is the transmission of characteristics--called hereditary traits--from parents to offspring. Genetics is the scientific and experimental study of heredity. The discovery of the principles of heredity in the 20th century was preceded by several thousand years of successful breeding of plants and animals.
Ancient Greeks proposed several theories about heredity, emphasizing the function of semen and sometimes postulating the existence of a female semen. Early European medical schools taught to their pupils that each human organ contributed to the formation of semen. With the introduction of the microscope in the 17th century, spermatozoa were found in semen. The anatomical study of ovaries of animals was also undertaken.
During the 17th and 18th centuries, it was generally assumed that embryos were totally formed after conception, hence development consisted only of growth. C. F. Wolff experimented with chicken embryos near the end of the 18th century, and definitely demonstrated the falseness of this idea. Most 18th-century scientists still believed that each body organ in the father and the mother contributed to the formation of either sperm or egg.
When Charles Darwin proposed his theory of evolution (1859), there was not an acceptable explanation of how species variations were produced and transmitted to descendants. Darwin himself believed in pangenesis, the theory that each organ produces factors called 'pangenes' that are incorporated to the sexual cells (gametes). With respect to variations, Darwin accepted the theory of Lamarck on the use and disuse or organs. Other biologist of the time, August Weismann (1834-1914), challenged lamarckism and said that modifications in other organs could not affect the organs that produced gametes.
In 1866, the monk Gregor Mendel published his work about transmission of seven hereditary traits in garden peas, where he enunciated what are now known as Mendel's laws. Despite the thorough foundation of his conclusions, the work was not recognized by other scientists. After experiments not so successful with other vegetal species, Mendel quit working on heredity. The importance of his results was only realized in the 20th century, when three biologists (including Hugo de Vries) independently rediscovered the work of Mendel.
Mendel discovered that what he called the factors or units of heredity operated in pairs. These pairs separate during sexual reproduction, and are united again when the male gamete fecundates the female gamete. Each factor corresponds to a hereditary trait, and is inherited independently of other factors. Factors do not mix but remain unalterable during transmission. These facts Mendel expressed in two laws: the law of segregation, and the law of independent assortment. Other important result of Mendel's research was that there exist two kinds of factors: dominant and recessive. When both types coexist in an organism, the dominant factor hides the recessive factor.
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