CHAPTER 10 



Sex Determination^ 



It was realized in the very early days of Mendelism that there is 

 some connection between sex determination and Mendelian heredity. 

 Even before the rediscovery of Mendel's^ work, Bateson in 1894 pointed 

 out that sex is an example of discontinuous variation, and included it in 

 the body of facts which, presented as Materials for the Study of Varia- 

 tion^ nearly brought him independently to the idea of discontinuous 

 hereditary units. Correns^ was the first to give an actual demonstration 

 that the connection was a real one. He showed that in Bryonia the male 

 is heterozygous for sex factors, and produces two classes of pollen, 

 while the female corresponds to the homozygous recessive. This was 

 soon followed by the discovery of unequal sex chromosomes by 

 McClung. 



Since that time, the subject of sex determination has developed into 

 a detailed and fairly comprehensive part of genetics. A wide view of 

 sexuality in the biological reakn raises considerable difficulties of a kind 

 which previously could only be considered from a philosophical point 

 of view. What in fact do we mean by sexuality? In its common use the 

 word sex is applied to diploid animals and plants which elaborate 

 haploid gametes which unite in fertilization. If we consider that the 

 essential characteristic of sex is its subservience to a particular kind of 

 reproduction, it would seem logical to define it to apply to the actual 

 gametes whose union is the fundamental reproductive act. This, how- 

 ever, runs counter to the commonsense usage. The difficulty of choosing 

 a suitable meaning is increased when we attempt to deal with sexuality 

 in lower plants which have an equal alternation of generations, since 

 here we frequently find that the diploid phase is asexual, while the 

 haploid phase is sexual in the sense of bearing sexual organs and elabo- 

 rating gametes, which latter are sexual in the sense of actually imiting 

 in fertilization. 



In this account, a discussion of these possibilities will be postponed 



^ General references: Bridges 1925, 1932, Crew 1927, Goldschmidt 1923, 

 19316, Hammerling 1937, Mainx 1933, Wettstein 1936, Witshic 1929, Plants, 

 Allen 1932, Correns 1928a, Lr Plants, Hartmann 19296, Kniep 1928. 



^ Mendel himself considered the question. See his letters to Nageli, published 

 in Correns 1924. » Correns 1907. 



