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Prof. E. B. Wilson. 



number of specially trained co-operating observers, it has been more and 

 more conclusively demonstrated that the units of each group are more or less 

 firmly linked together in heredity, while those belonging to different groups 

 are quite independent. This at once suggests that the units of each group 

 (or corresponding things on which they depend) are borne by a particular 

 chromosome which constitutes their common vehicle of transmission, and 

 that to this fact is due their cohesion or linkage in heredity. Conversely, the 

 several groups are independent of one another, because of the independence 

 of the chromosomes which bear them. This hypothesis would have been a 

 plausible one even were the number of chromosomes in Drosophila unknown. 

 In point of fact, however, the gametic number of chromosomes in this species 

 {or of chromosome-pairs in the diploid groups) is actually the same as that of 

 the linkage -groups, namely, four (fig. 1, d, e). It is at least an odd coinci- 

 dence that one of these chromosomes, like one of the linkage-groups, is 

 extremely small. One is tempted to guess that this may explain why for a 

 long time but three linkage-groups could be identified, and that the fourth 

 thus far contains but a single character, recently discovered by Muller. 



Thus far, admittedly, the hypothesis presents a somewhat speculative 

 aspect ; but fortunately there is a means of testing it specifically, for the 

 •cytological evidence demonstrates that one of the four chromosomes is 

 definitely connected with the determination of sex. One of the four groups 

 of units, therefore, should likewise exhibit some special relation to sex. And 

 this is in accordance with the facts, for every one of the 31 characters of the 

 first group exhibits sex-linked heredity, of the same type as that which 

 appears in colour-blindness or haemophilia in man. It was pointed out, in 

 1910-11, by Morgan, Gulick, and myself that the heredity of sex-linked 

 characters of this type exactly follows the course of the X-chromosome ; that 

 is to say, that the history of such characters is precisely such as it should be 

 if they were dependent upon factors borne by this chromosome. Like the 

 latter, the sex-linked units are always simplex or haploid (hence heterozygous) 

 in the male ; and they zigzag between the sexes in exactly the same way. 

 No other group shows this relation. 



Without entering far into the detail, let me illustrate these phenomena by 

 a single example, that of the so-called " criss-cross " heredity. The normal 

 Drosophila possesses red eyes ; a common mutant has white eyes, recessive to 

 red. If a pure-bred white-eyed female be paired with a normal red-eyed 

 male, all the resulting sons are white-eyed, like their mother ; all the 

 daughters red-eyed, like their father. Exactly analogous results appear when, 

 instead of white eyes, any other units of the first group, such as yellow body 

 colour or miniature wings, are similarly tested. The results at once lose their 



