Heredity, race, and society.

  • Leslie Clarence Dunn
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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    hybrid produces 1,024 distinct types of sex cells, and is capable of engendering that many different types of offspring. We see then, that the chance of two brothers or sisters receiving the same hereditary endowment from their father or from their mother is small. The chance that they receive the same endowment from both parents is quite negligible, unless, of course, the brothers are identical twins. As we now know, identical twins arise from a single egg, and have the same genes. How many genes there are in man is unknown, but it would seem reasonable to believe that man has at least as many as the vinegar-fly, Drosophila. To be sure, the number of genes is not precisely known even in that insect, which has been studied more than any other animal; but it is estimated that a Drosophila has several thousands, perhaps as many as ten thousand genes in each sex cell. To appreciate the difficulty of counting the genes in any animal or plant, one must have a clear idea about the method through which one learns about the exist ence of the genes in the first place. Mendel observed the segregation of the purple and white flower colors in the offspring of a hybrid between the purple and white varieties, and concluded that the pea plant must have a gene for flower color. Our knowledge of the genes for tasting PTC, for eye color, for albinism, and for brachydactyly in man comes from observations on the distribution of tasters and non-tasters, brown and blue eyes, albinos and non-albinos, and short and nor mal fingers in selected human pedigrees. But imagine that all the peas existing in the world or known to us are purple (or all white), or that PTC has never been prepared or tasted. The existence of the genes for the flower color in peas and for tasting PTC in man would not even be suspected. In other words, we find out about the existence of a gene for a given trait only if we observe individuals which differ with respect to that trait, and, strictly speaking, only if such individu als can be crossed and a Mendelian segregation in their offspring can be observed. By no means all traits in
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