BALANCE OF SEX-DETERMINING FACTORS Since the difference in sex-chromosome content is the only apparent difference in the constitution of the sexes it follows that the X or something lodged in it is female-determining (in cases similar to man or Drosophila), whilst male-determination is an affair of the rest of the chromosomes (exclusive of the Y) or autosomes. If the female-determining factor in the X be sym bolised as F and the sum of the male-determining factors in one set of autosomes as M, then whilst both male and female possess 2M, the females possess 2F, the males IF. Thus 2F must be >2M and IF must be <2M.
Triploid Forms.—Bridges found a strain of Drosophila which was triploid, i.e., which possessed some or all of its chromosomes in triplicate instead of the normal duplicate. In the individuals produced by the triploid strain, three sets of autosomes (3A con taining 3M) were in some individuals associated with IX, in others with 2Xs, in others with 3Xs. Investigation revealed the fol lowing sex types. By representing the efficiency of the female determining factors (X-borne) as roo and that of the male determining factor complex as 8o, a series of sex-indices can be made.
Thus sex-determination cannot be the function of sex-chromo somes alone ; on the contrary, sex must be determined by the inter action of factors borne upon sex-chromosomes and autosomes. The addition of more autosomal factors to the usual 2X : 2A balance so disturbs this that femaleness is transformed into intersexuality. The difference in the sexual characters of individuals possessing 2X chromosomes associated with 3 of each autosome (2X :3A) and of those which have but two IV chromosomes instead of three (2,7( :3A—IV) is to be regarded as an indication that the IV chromosome carries, like the X-chromosome, a net balance of female-determining factors. Sex-determination is thus a matter of the correct quantitative balance between the amount of male and of female determining factors of the fertilised egg and if one does not exceed the other by a certain amount intersexual forms result.
The monogametic female is a female because she has the con stitution (FX)(FX)MM and because 2F> 2M, whilst a male is a male because he has the constitution (FX)Y MM and because IF< 2M. In forms with digametic female the male-determining factors must be borne upon the X-chromosomes. Then a male is a male because his constitution is (MX)(MX)FF and because 2M> 2F, a female is a female because her constitution is (MX)YFF and because r M< 2F. In either case when M =F, intersexuality will result. Of course, these formulae must not be taken too literally: they are but convenient symbolism. But it is clear that in Drosophila the effective factor in the establishment of maleness, femaleness and intersexuality is the numerical ratio of X-chromosomes to autosomes.
The end-result of the interaction of male and female differentiat ing substances is the establishment of a particular physiological state, maleness or else femaleness, within the developing fertilised egg and in this or that internal environment the development of the individual proceeds. In Drosophila an individual is a male and develops the attributes of the male because in its beginning it had a chromosome constitution symbolised as XY; because in this XY individual the relation of male and female determining factors was such that an internal environment of maleness be came established within the developing egg ; because in this environment the structures pertaining to the XY sex equipment developed ; and because the impress of the external environment did not or could not affect the developing individual so as to override the action of the sex-determining mechanism. For similar reasons the XX individual becomes possessed of typical female characterisation.