108 



SCIENCE 



[N. S. Vol. XL. No. 1020 



11. Non-disjunctional females are diploid 

 in constitution, for (see 10) tBey can be 

 heterozygous in various sex-linked genes, and 

 they then give all the expected classes in the 

 normal proportions. 



12. Half of the expected class of sons (from 

 any non-disjunctional female by any male) 

 transmit the power of producing exceptions, al- 

 though they themselves do not possess that 

 power. Thus, half the white sons of a white 

 non-disjunctional female outcrossed to wild fe- 

 males give in Fj only the expected classes of 

 wild-type females and wild-type males. If 

 these wild-type daughters are tested by out- 

 crossing to barred males exceptions are found 

 among their offspring. 



13. Those sons (half the expected class) 

 which do transmit the iwwer of producing ex- 

 ceptions transmit it to only some of their 

 daughters and not to all. Thus of the wild- 

 type daughters tested (in 12) by barred males 

 only approximately half showed exceptions, the 

 others giving only expected classes. 



14. The patroclinous sons of a non-disjunc- 

 tional female do not transmit non-disjunction 

 to any of their offspring. 



15. Attempts to localize a gene for non-dis- 

 junction in the series of sex-linked genes 

 showed that no such definite locus in the series 

 could be assigned to non-disjunction as has 

 been assigned in the case of all sex-linked 

 genes. Non-disjunction was found to assort 

 freely from all sex-linked genes tried. 



16. Attempts to obtain pure stock of non- 

 disjunction failed. Had there been an X- 

 chromosome gene involved, the rigorous 

 method used would not have failed to yield a 

 stock pure for that gene. 



17. Eecent cytological investigation has 

 clearly shown that normally in Drosophila 

 ampelophila the female bears two X-chromo- 

 somes, and the male an unequal X-Y pair 

 (see 9). 



This last fact requires us to substitute the 

 term F-bearing sperm for "no-X " sperm and 

 F-egg for no-X egg in our explanation. 



The breeding work (especially 13, 15 and 16) 

 showed that an -Y-chromosome gene could not 



be the cause of the phenomenon, and the cyto- 

 logical work (see lY) supplies us with an ade- 

 quate cause in the supposed presence in the 

 exceptional females of a F-chromosome in ad- 

 dition to the normal two X-chromosomes. 



The prediction was accordingly made that 

 half the daughters of a non-disjunctional fe- 

 male would be found to contain in addition to 

 the two X-chromosomes a supernumerary 

 chromosome which is a Y. 



18. Cytological investigation has shown that 

 approximately one half of the daughters of a 

 non-disjunctional female do in fact contain a 

 supernumerary Y-chromosome, while the re- 

 maining half contain only the normal two X- 

 chromosomes. 



The sex-chromosomal constitution of a non- 

 disjunctional female is XXF. In 90 per cent, 

 of the maturations the sex-chromosomes must 

 be placed in opposition to each other in such 

 a way that an X and Y both pass to one pole, 

 and a single X to the other. In ten per cent, of 

 the maturations, however, the chromosomes 

 must oppose each other in such a way that the 

 Y passes to one pole by itself and the two X- 

 chromosomes pass to the other pole together. 



The fertilization of these types of eggs by 

 the spermatozoa of an unrelated male of the 

 constitution XY explains the genetical results 

 as follows: 



The XX eggs (5 per cent.) by the F-sperm 

 give females (XX = female) having each a 

 supernumerary F-chromosome. These females 

 will be exact duplicates of their mother with 

 respect to their chromosomes, and the hreed- 

 ing work showed that they are exact duplicates 

 with respect to ail sex-linJced genes (see 1, 2 

 and 5). This parallelism can only he explained 

 hy assuming that the X-chromosomes do in 

 reality hear the genes for the sex-linJced char- 

 acters. 



Furthermore, since these females received 

 no X-chromosome from the father they can 

 neither show nor transmit sex-linked charac- 

 ters from the male (see 1 and 2). 



Since the composition of these females is 

 XXY, they will themselves have the power of 

 producing exceptions as did their mother (see 



