78o 



NATURE 



[August i8, 192 i 



The Determination of Sex. 



By Prof. R. Goldschmidt, Kaiser Wilhelm-Institut fiir Biologic, Berlin-Dahlcm. 



IN this communication it is proposed to give an 

 exposition of the subject of the determination 

 of sex presenting chiefly the line of argument 

 which the writer has been able to develop from 

 recent work on the question. In doing so it will 

 be convenient to confine our attention to one line 

 of thought, though this will compel us to omit 

 mention of much important work upon the prob- 

 lem. Further, it is proposed to limit the account 

 to the writer's own field of work- — namely, the 

 animal kingdom. For a more complete account 

 we refer the reader to the author's book, " Me- 

 chanismus und Physiologic der Geschlechtsbestim- 

 ung " (Borntraeger, Berlin, 1920). 



The situation in regard to sex which is typical 

 in nature is that out of a number of fertilised 

 eggs of a given species about equal numbers of 

 male and female individuals are developed. The 

 problem of the determination of sex, then, pre- 

 sents itself in the form of two principal questions : 

 first, what is the mechanism which, at a certain 

 moment, separates the flow of development into 

 two different streams— those of female and male 

 differentiation ; and secondly, what is the material 

 difference in the two sets of individuals thus 

 separated, and how does the supposed difference 

 act physiologically in order to direct individual 

 development along female or male lines? .We 

 may call the first of these questions the problem 

 of the mechanism of distribution of the two sexes ; 

 while the siecond is the problem of the physiology 

 of sexual determination. 



It will be clear to every student of biology that 

 the first problem in question is part of the general 

 problem of the mechanism of heredity — i.e. it is 

 concerned with the transmission of genetic pro- 

 perties from parent to offspring and their distribu- 

 tion among offspring. Therefore the study of the 

 mechanism of distribution of the sexes has formed 

 an integral part of modern work in genetics, and 

 partaken of its triumphal progress. We may 

 safely say that to-day, in the light of Mendelism 

 and the work accomplished in the realms of cyto- 

 logy, the problem is solved as completely as the 

 methods of biology permit. 



The first successful attack upon the problem 

 was made when Doncaster and Raynor discovered 

 and studied the famous case of sex-linked inherit- 

 ance in the currant-moth,- and Bateson and 

 Punnett furnished the Mendelian analysis of the 

 case. By following the hereditary distribution of 

 a somatic character closely linked with the dis- 

 tribution of sex, the inference could be drawn that 

 one sex must be heterozygous for a Mendelian 

 factor connected with sexual differentiation, and 

 the other sex homozygous. Thus one sex produces 

 two kmds of gametes in respect to the factor in 

 question, the other sex only one kind. The re- 

 sulting situation is, therefore, the same as in a 

 back-cross between a hybrid Aa and the pure 

 recessive form aa\ both types reappear again in 

 NO. 2703, VOL. 107] 



the offspring in equal numbers. Since then an 

 immense number of cases of sex-linked inherit- 

 ance have been analysed, all with the same general 

 result ; one sex is homozygous in regard to a sex- 

 differentiator, and produces one type of gametes 

 — i.e. it is homogametic; the other sex is hetero- 

 zygous, and produces two types of gametes — i.e. 

 it is heterogametic. There is one complication 

 so far as certain groups of animals are con- 

 cerned : in mammals and in most of the insects 

 the male is the heterozygous sex, whereas in moths 

 and birds it is the female which produces the two 

 kinds of gametes. The possible meaning of these 

 two types is, however, a question of detail which 

 does not concern us here. 



Almost simultaneously with the solution of the 

 problem of the mechanism of distribution of sex 

 in terms of Mendelian symbolism, McClung an- 

 nounced that the odd chromosome found in the 

 sperm-cells of certain Orthoptera and Hemiptera 

 might act as a differentiator of sex. Since then 

 the study of the sex-chromosomes has progressed 

 with a rapidity and success which have rivalled 

 Mendelian discoveries regarding sex. The simple 

 result which stands out to-day as one of the basic 

 facts of cytology is this : all the cells of the 

 body of many animals contain in one sex either 

 an odd chromosome, called an X-chromosome, or 

 an unequal pair of chromosomes, called an X-Y 

 group. The cells of the other sex contain, instead, 

 two X -chromosomes. As is well known, all sex- 

 cells undergo a reduction division which reduces 

 the somatic number of chromosomes to one-half ; 

 this reduction is brought about by a pairing of 

 each maternal with a corresponding paternal 

 chromosome and subsequent disjunction of whole 

 chromosomes during the meiotic division. An 

 odd X-chromosome, whether it has a Y-partner 

 or not, must, therefore, pass undivided to one of 

 the daughter-cells during the meiotic division. 

 The result is the production of two mature sex- 

 cells, one with X, the other without X. In other 

 words, the sex containing the odd X (or the 

 X— Y group) forms two kinds of gametes, which 

 are with and without X respectively — i.e. it is 

 heterogametic. The other sex, however, with its 

 two X's, produces only gametes with X, and is 

 therefore homogametic. In fertilisation, then, an 

 X-gamete of the latter sex may unite either with 

 a Y-gamete, or with an X-gamete of the 

 heterogametic sex. The result is XX- and XY- 

 zygotes- — i.e. the two sexes. 



The close parallelism between the genetic and 

 cytological facts led Gulick, Morgan, and the 

 writer to venture the opinion that the genetic facts 

 of sex-linked inheritance could be completely ex- 

 plained, if it were assumed that Mendelian factors 

 which are inherited in -that peculiar way are 

 carried within the X-chromosomes. Such an 

 assumption would lead to the view that the Alen- 

 delian explanation of- sex-linked inheritance and 



