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By Theodosius Dobzhansky

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By Theodosius Dobzhansky

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The quarter of the century immediately preceding the rediscovery of Mendel's laws, roughly from 1875 till 1900, saw a rapid develop- ment of cytology, the study of the cell. The following quarter of a century, approximately 1900-1925, brought an even more rapid prog- ress of cytogenetics, a synthesis of the findings of cytology and genetics concerning the mechanisms of the transmission of heredity. Nuclei and Chromosomes. Cell nuclei were described by Brown in 1831, even before the promulgation of the cell theory by Schleiden and Schwann in 1839.

He pub- lished the results of his work in 1865 in a single paper. Although he stated very clearly the laws of heredity now regarded of great im- portance, his work failed to attract the attention of contemporaries. In 1900 these laws were rediscovered independently by Correns in Germany, De Vries in Holland, and Tschermak in Austria. Their importance was recognized, and the development of a new science, genetics or the study of heredity, proceeded apace. Mendel crossed varieties of peas differing in single contrasting traits, or characters, such as a variety with purple and one with white flowers, with yellow and with green seeds, with round and with wrinkled seeds, tall and dwarf varieties, etc.

The effects of different genes are additive; that is, each gene adds a certain amount of pigment regardless of presence or absence of other pigment genes. A Negro, then, would carry six pig- ment genes, and a white no pigment genes (this is admittedly inexact, since whites who are not albinos always have some skin pigment). A Negro X white marriage produces Fi hybrids (mulattoes), who should be PipiPzp^PaPa- With three pigment genes the mulattoes have, on the assumptions made, half as much skin pigment as the Inheritance of the Skin Color in Man 41 Negro parent.

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Evolution, Genetics, and Man by Theodosius Dobzhansky


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