Heredity, race, and society.

  • Dunn, L. C. (Leslie Clarence), 1893-1974.
Date:
1947
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Heredity, race, and society. Source: Wellcome Collection.

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    mountain top and the other left in the valley. The two halves of each plant have exactly similar heredities, and they develop into new whole plants being reared under different conditions. The differences in size, form, num ber of leaves, length of stem and roots which come to distinguish these plants may thus be traced directly to specified environmental conditions. Such knowledge is I very useful, and would be especially important for I human beings. » I Twins I In humans, obviously, it is not possible to make ex periments of this kind deliberately, but luckily, such experiments happen once in a while by accident. These accidents are called identical twins. In one birth out of about each 87, more than one baby is born. In a great majority of these cases two babies are born, rarely there are three, that is triplets (about one in 7,400 births), and still more rarely there are four (about one in 636,000 births). Quintuplets and sextuplets are ex tremely rare but not so rare as is popularly believed, since some sixty cases of quintuplets are recorded and even six cases of sextuplets. Of the twins born, about one quarter are identical twins, and it is easy to understand why these are so Ìinteresting and useful for answering questions about heredity and environment. Ordinary twins are merely a pair of babies who happen to be born at the same time. They come from two separate eggs and show the osarne hereditary differences as ordinary brothers and sis- \ ters. Identical twins on the other hand are like the two ; halves of the plant that was cut, for these twins arise Ì when a single fertilized egg, by some accident, gets split into two parts early in development, and each part grows into a separate and complete individual. Since I these individuals have the same heredity, any differences i between them must be due to environment, to nurture.