340 



SCIENCE 



[N. S. Vol. LIU. No. 1371 



cliromosome may be tHe vehicle of trans- 

 mission, a matter wliicli his colleague, Winge, 

 has under investigation. 



In all previously known eases of sex-linked 

 inheritance, the egg as well as the sperm may 

 serve as a vehicle of transmission. In Droso- 

 phila and man it is supposed that the X chro- 

 mosome bears sex-linked characters, the female 

 being in formula XX, so that every egg after 

 maturation contains an X, a bearer of sex- 

 linked characters; but in the sarae species the 

 male is XY in formula, a T replacing one of 

 the X's found in the female. This X con- 

 sequently will occur in only half of the sperm 

 produced by the XT male, namely those which 

 pass into female offspring, but the other 

 sperms will contain T instead of X and they 

 will pass into male offspring. However, up to 

 the appearance of Schmidt's paper, no char- 

 acters had been observed to follow the path of 

 a T chromosome in transmission, so that 

 Morgan characterizes the T chromosome of 

 Drosophila as " empty." 



Before the mechanism of transmission of 

 sex-linked characters had been worked out, I 

 suggested in 1909^ that the T chromosome 

 afforded a suitable vehicle for transmitting the 

 secondary sex characters of males. But until 

 Schmidt's publication was made this sug- 

 gestion had found no support in known facts, 

 and the demonstration by Morgan and others 

 that characters which make their first appear- 

 ance or are most often found in males, may 

 nevertheless have their genetic basis in an 

 X chromosome, seemed to discredit the T 

 chromosome as a possible organ of inheritance. 

 The discovery of Schmidt leads me to call 

 attention to my earlier suggestion, not for the 

 mere satisfaction of saying " I told you so," 

 but to renew the further suggestion which I 

 then made and which still lacks verification, 

 that the T chromosome may contain the clew 

 to the explanation of that other and very 

 different type of sex-linked inheritance found 

 in Abraxas and subsequently found to occur 

 also in poultry. 



2 " A Mendelian View of Sex-heredity, ' ' Soienoi! 

 Vol. 29, pp. 395-400, March 5, 1909. 



Studies of sex-determination made in the 

 last twenty years show unmistakably that in 

 dioecious species the chromatin composition of 

 the nucleus of the egg determines the sex of 

 the individual into which the egg develops. 

 Further, in many cases, if not in all, the 

 quantity of chromatin is clearly decisive be- 

 tween maleness or femaleness in the individ- 

 ual which develops from the egg. Thus in 

 parthenogenesis an egg which develops with- 

 out chromatin reduction (in maturation) 

 regularly develops into a female; but an egg 

 which first undergoes chromatin reduction 

 (usually by exactly half the total number of 

 chromosomes), before it begins development 

 into an embryo, if it remains unfertilized, 

 now develops into a male. Yet if the egg, 

 after undergoing chromatin reduction in 

 maturation, receives a new complement of 

 chromatin by being fertilized, it is restored 

 again to the female status. Femaleness thus 

 goes with the full chromatin equipment of the 

 species, maleness with a less complete chro- 

 matin equipment. 



It has further become clear through studies 

 of sex-linked inheritance that not all kinds 

 of chromatin are equally influential in deter- 

 mining sex, but that a particular chromosome 

 called X is of preeminent, if not exclusive 

 importance in determining sex. In the case 

 first clearly worked out by Wilson, that of the 

 squash-bug, the female bears in each cell- 

 nucleus a pair of X's, whereas the male eon- 

 tains but one. As the remaining ten chro- 

 mosomes of this species are paired in both 

 sexes, the total chromosome count for the 

 female is 20 -|- 2X = 22 ; for the male it is 

 20H-X = 21. The only discoverable differ- 

 ence between the two sexes is in the number 

 of the X chromosomes. The number is two in 

 the female, one in the male. The metabolic 

 grade of maleness is attained when, in addi- 

 tion to the other ten pairs of chromosomes, a 

 single X chromosome is present in the cell, 

 but the grade of femaleness is attained only 

 when the further addition of a second X is 

 made. Every kind of chromatin of the 

 species is present in the male, but a partic- 

 ular kind of chromatin is present in less 



