OPERATION OF THE GENETIC SYSTEM 59 



in producing the differences of sex? The way to determine 

 this is to alter the chromosome combinations experimentally, 

 and observe what differences this makes. This has been done 

 extensively, originally by C. B. Bridges in the fruit-fly, 

 Drosophila melanogaster.^ The results are of great inter- 

 est; they will therefore be examined. 



5. Alteration of the Chromosome Combinations in 

 the Fruit-Fly, and Its Effect on Sex 



It is important in following this work to have clearly in 

 mind the chromosomal conditions in the organism used as 

 an example. In this species of fruit-fly there are four pairs 

 of chromosomes, three of them (II, III and IV in figure 7) 

 being autosomes. In addition, the female has two large 

 straight X's, while the male has one straight X and a 

 hooked Y, of about the same size as the X (figure 7). 



The autosomes as well as X and Y are important for sex, 

 as it turns out. It will be convenient to indicate the three 

 autosomes simply by the letter A. Thus the male produces 

 two types of sperms, AX and AY, while the ova produced 

 by the female are all alike, having the constitution AX. 

 When the ovum AX is fertilized by a sperm AX, the result 

 is AAXX, a female. But when the ovum AX is fertilized by 

 the sperm AY, the result is AAXY, a male (see figure 5). 



The Effect of Changing the Numbers of X and Y 

 Chromosomes Present {"Non-disjunction"). — Bridges dis- 

 covered that sometimes in forming the ova, an accident 

 occurs of such a sort that the two X's do not separate into 

 different ova, but both go together into one ovum, leaving 

 another ovum without an X. Thus two kinds of ova are 

 produced, AXX and AO (using O to signify the absence of 

 X or Y) (see figure 20, at B). 



This failure of the two X's to separate is called non- 

 disjunction of the X-chromosomes. Later a race of fruit- 



