INTRODUCTION 



Sex, as a genetic phenomenon, has frequently been discussed in this 

 book in relation to heterochromatin (see I 2 C d ee), genie balance 

 (III 5 C a aa), kinetics of genie action (III 5 C a aa), and gene- 

 controlled variegation (1 S C c dd ddd). I have no intention of re- 

 viewing and analyzing the various details of the problem of sex deter- 

 mination in their entirety, which would require a volume. This has 

 been done by me (Goldschmidt, 1920Z7, 1931, 1932Z?), by Hartmann 

 (1943), by Witschi (1929), and for plants by Correns (1928) and by 

 Allen (1940), not to mention more popular treatments by Crew (1927, 

 2d ed. 1954), and still more popular books like Caullery's (1941). 

 What we are interested in here is the basic genetic situation as far as 

 it can be generalized without going into the details of all established 

 or assumed variants and, further, the relation of the facts and general- 

 izations of the genetics of sex to those of general genetics. Thus we 

 shall select specific aspects of the problem of sex determination which 

 we consider to be of importance for the general theories of genetics. 



The study of the genetics of sex begins with the discovery of the 

 mechanism of the sex chromosomes (McClung, Stevens, Wilson) which 

 established that one sex is heterogametic, the other, homogametic in 

 regard to the X-chromosome and that, as a consequence, two types of 

 fertilization occur, leading to 50 per cent X and 50 per cent XX 

 individuals, the two sexes. The same fact was established on a purely 

 formal genetic basis when Bateson and Doncaster showed that sex- 

 linked heredity is based upon the coupling ( or repulsion ) of sex deter- 

 miners and sex-linked genes, making sex determination comparable 

 to a Mendelian backcross. This comparison was further confirmed 

 when Correns found that sex behaved as a Mendelian backcross in 

 crosses between monoecious and dioecious plants, though the introduc- 

 tion of monoecism complicated this example and, therefore, prevented 

 it from exercising a major influence upon the development of the 

 subject. The basic notions were finally established when Morgan and 

 later Bridges proved that sex-linked loci are located in the X-chromo- 

 some, thus establishing the cytological and the Mendelian descriptions 

 of sex determination by way of the homogamety-heterogamety and 

 the homozygosity-heterozygosity scheme as one and the same thing. 



The modern study of the genetics of sex begins with the discovery 

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