HEREDITY AND VARIATION 



chromosomes overbalance the male determiners on the autosomes. When the 

 zygote has a 2A + XO or 2A + XT constitution, the male determiners on 

 the autosomes overbalance the female determiners on a single X-chromosome; 

 the Y-chromosome appears to carry no genes related to sex. In the 3,4 + 2X 

 individuals neither set of genes overbalances the other, but both are some- 

 what effective in molding the appearance of the intersex. This theory is 

 strengthened by the occurrence of what are known as superfemales and super- 

 males in which the chromosomal make-up is 2/1 + 3.Y [ovum (.4 -|- 2X) 

 + spermatozoon (.4 + X)] and 3.4 + XT [ovum {2 A + X) + spermatozoon 

 (.4 + T)], respectively. Although triploid (3.4 + 3.1") and tetraploid 

 (4.4 + 4X) individuals are females, as would be expected on the basis of an 

 explanation in terms of genie balance, haploid individuals {A + X) are males 

 in bees and other forms that normally produce males parthenogenetically. 

 Haploid drosophilas have not been found, so that the theory remains untested 

 in a crucial case. More facts are necessary before the final word can be said 

 on the mechanism of sex determination at fertilization. We have seen an 

 apparentlv satisfactory theory in terms of 2X versus XO or XT zygotes fail 

 to explain accumulated observed facts and hence undergo modification. Such 

 is the method of science — observation, explanation, further observation or 

 experimentation, and modification of explanation when necessary — a con- 

 tinued seeking for the whole truth. 



As in all development, the environment influences the differentiation of 

 so-called sex characters. A zygote in which the genie balance is female- 

 determining may develop into a male in an atypical environment. In 1915 

 Emil Witschi found that if frogs were forced to develop at a high tempera- 

 ture, they were all males because the female-producing genotype had been 

 overridden by environmental conditions. Sometimes reversal of sex occurs. 

 A hen, for example, may change into a cock and produce spermatozoa. Al- 

 though the mechanism of the overriding of the inherited constitution, the 

 genotype, is not clear, this is only a special case of the effect of the en- 

 vironment, as noted previously (pp. 167 and 195). 



The problem of sex determination is not really a special one, although it 

 has long been so treated. It is well known by geneticists that single pairs 

 of genes do not by themselves produce an effect in the organism. They al- 

 ways depend on the presence of other genes, on a given pattern of distribution 

 of the cytoplasm of the zygote during cleavage and the cell movements during 

 development, on interactions between differentiating cells, and on the chemi- 

 cal and physical conditions of the external environment. In the ordinary 

 course of events the genes are the part of this complex most often varied; that 

 is, new combinations of genes occur with each fertilization, but development 

 proceeds under practically uniform conditions in the great majority of cases. 

 Genes initiate chains of reactions which may be environmentally modified 

 during cell differentiation. When the environment is altered, its importance 

 in the complex of factors determining what the individual will be, not only 

 with respect to its sex but also its other characteristics, is appreciated. 



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