GENERAL ZOOLOGY 



males, and Drosophila, which is an example of forms with XT males, the males 

 are the digametic sex, or the sex that gives rise to two kinds of gametes with 

 respect to the X-chromosome. 



The diagrammatic simplicity of such a method of sex determination became 

 somewhat confused when the observation was made that in some species of 

 moths and in birds the female was the digametic sex; that is, a female moth 

 produces two kinds of ova, one with an X-chromosome and one without, 

 whereas all the spermatozoa are alike in carrying an X-chromosome. Many 

 facts indicate an undeniably close correlation between number of X-chromo- 

 somes and sex, however. One of the most interesting is the occurrence of 

 bilateral gynanders, which are male on one side and female on the other. 

 It is clearly established that gynanders in Drosophila, for example, arise 

 from female-producing zygotes (2/1 + 2X). At the time of the first division 

 one of the X-chromosomes, either the paternal or the maternal, is lost on the 

 mitotic spindle, so that one nucleus has 2A + 2X chromosomes and the other 

 has 2A + X chromosomes. The former gives rise to the female half, the 

 latter to the male half of the gynander. If the X-chromosome is lost in some 

 later division, only a limited region of the female will exhibit male characteristics. 



In spite of such confirmation of the sex-chromosome theory of sex determi- 

 nation, it should be obvious that the study of inheritance has shown char- 

 acters to depend on combinations of the genes located in the chromosomes, 

 not on the chromosomes as such. Yet with sex, the presence of two particular 

 chromosomes was assumed to condition the differentiation of one sex, and one 

 of these chromosomes was assumed to condition the differentiation of the other 

 sex. Either one of the X-chromosomes of a female could enter a male- 

 producing zygote, and the X-chromosome of a male could pass into a female- 

 producing zygote. No gene for maleness or femaleness has been located on 

 the chromosome maps of Drosophila or with certainty been identified in any 

 species, although reported by some investigators. The discovery of inter- 

 sexes among the off^spring of triploid females by C. B. Bridges in 1921 led to 

 his formulation of the Theory of Genie Balance to explain the determination 

 of sex. When disjunction occurs in the germ cells of triploid (3.4 -|- 3X) 

 females, some eggs with 2A + A' chromosomes and some with .4 + 2A' are 

 formed, among others. If a 2.4 + A' ovum is fertilized by an .4 + .V sperm, 

 a 3.4 + 2X zygote results and develops into an intersex, or an individual 

 that is male in some parts and female in others. These male and female 

 parts are not clearly segregated into halves or quarters as in gynanders but 

 are completely blended, and intersexes range from almost total females to 

 almost total males. Bridges' theory is that sex is the result of the inter- 

 action of many different genes, some of which are to be thought of as female 

 determiners and others as male determiners. Both types of genes are located 

 on all the chromosomes. However, there are more female than male deter- 

 miners in the X-chromosomes, whereas the reverse situation is true in the 

 autosomes. The distribution and effect of these genes are such that, when a 

 zygote has a 2.4 + 2A' constitution, the female determiners on the X- 



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