26 



SEX-DETERMINATION 



matter how this may be estabhshed. Since in these non- 

 disjunctional types the autosomes are the same in both 

 sexes, maleness and femaleness would seem to be characters 

 that are based upon the number of X-chromosomes present 

 in the zygote. It would appear from this work of Bridges 

 that the X-chromosome itself is neither male-determining 

 nor female-determining but is of such a kind that when one 

 is present in association with a diploid set of autosomes 

 development is swung in the direction of maleness, whereas 

 when two are present it is swung in the direction of female- 

 ness. The egg possesses the capacity to develop in either 

 direction, the direction taken being determined by the rela- 

 tive amount of X-chromosome-borne chromatin. 



Then in 1922 came the work of L. V. Morgan on attached 

 X-chromosomes which finally showed that two X's in the 

 zygote, irrespective of their origin, resulted in the produc- 

 tion of a female. The culture of Drosophila used by L. V. 

 Morgan exhibited a sex-linked recessive character yellow 

 body-color. Homozygous yellow females mated to wild- 

 type (grey body-colored) males gave only yellow daughters 

 and grey sons, a constant and complete reversal of the ex- 

 pected criss-cross mode of inheritance. Cytological examina- 

 tion showed that in these females instead of the usual two 

 X's there was a single V-shaped chromosome and, in 

 addition, usually a supernumerary Y. Others possessed a 

 single X in addition to this V. The V was shown to be two 

 attached X's which did not disjoin during meiosis. 



G=the gene for grey body-colour; g=yellow; ^=attached. 

 (i) rarely survives. (4) is non- viable and does not appear. 



