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THE AMEBIC AN NATURALIST 



[Vol. L 



methods of distribution — the segregation of Mendelian 

 allelomorphs exactly paralleling the pairing and separa- 

 tion of homologous chromosomes during the maturation 

 of the germ cells, and the random assortment of Men- 

 delian factors belonging to different, independently segre- 

 gating pairs, paralleling the random assortment of chro- 

 mosomes belonging to different pairs (Sutton). Still, 

 there was no indication of a connection between any par- 

 ticular chromosome and a particular character until the 

 work of McClung, Stevens and Wilson and others proved 

 that in many animals the "X-chromosome" contains, or 

 at least invariably accompanies, a factor for sex, inas- 

 much as all fertilized eggs which receive two X-chromo- 

 somes develop into females, while those with one X be- 

 come males. 



The next time that particular factors and chromosomes 

 were found to be correlated was in 1910, when Morgan 

 pointed out that the factor in Drosophila determining 

 whether an individual shall have red or white eyes, as well 

 as several other factors, must also be located in the X- 

 chromosome (or at least must accompany it in its segre- 

 gation). For, to put the whole argument briefly, the fact 

 that a red-eyed male bred to a white-eyed female produces 

 red-eyed daughters and white-eyed sons shows that the 

 female-producing spermatozoa— those that receive the X- 

 chromosome— also receive the factor for red, but the 

 male-producing sperm— which do not receive the X— also 

 fail to receive red. In other words, the factor for red was 

 judged to be in the X-chromosome, because in the male it 

 is always distributed to precisely the same spermatozoa as 

 those to which the X 's happen to be distributed. Bridges 

 has recently obtained evidence that in the female, too, such 

 "sex-linked" factors accompany the X-chromosome in 

 segregation. Ordinarily there is no opportunity for at- 

 tacking this question in the female, since the female con- 

 tains two X's, which are of course indistinguishable to the 

 eye, so that it would be impossible to tell whether or not a 

 particular one of the X's was always distributed to the 



