118 



in all possible combinations. Thus, a 

 common type of egg of a 3N female 

 has an extra set of autosomes. If we 

 represent a set of autosomes bv A, this 

 egg can be formulated as X+A+A 

 or X,2A. When such an egg is fertil- 

 ized by an ordinary sperm, that can be 

 formulated X + A, the zygote is 2X,- 

 3A. This type of zygote develops into 



BRIDGES 



an intersex, that is, into an individual 

 that is neither male or female but an 

 intermediate, or rather a mixture of 

 male and female parts, very similar in 

 type to the intersexes that Gold- 

 schmidt has worked with so exten- 

 sively in Lyma?itna dispar (Gold- 

 schmidt '20). 



The interpretation of these inter- 



Table I 

 Relation of Sex to Chromosomes in Drosophila nielanogaster 



* The haploid type has not been discovered. 



sexes in terms of genes carried in the 

 chromosomes was made possible as the 

 result of the very extensive studies of 

 the manner in which ordinary char- 

 acters are determined, and especially 

 from the study of the contrasted char- 

 acter changes brought about on the 

 one hand by the loss, and on the other 

 hand by the gain, of one of the small 

 round chromosomes, the fourth chro- 

 mosome. From such studies the view 

 had been reached that each character 

 of an individual is the index of the 

 point of balance in effectiveness of a 

 large but unknown number of genes, 

 some of which have a tendency to 

 change development in one direction 

 and others in the opposite (Bridges 

 '22). This conception of "genie bal- 

 ance" was applied to the sex characters 

 of the intersexes as follows: In chro- 

 mosome constitution the intersexes 

 differ from females only in that they 



have an extra set of autosomes. This 

 proves that the autosomes are con- 

 cerned with the determination of sex. 

 Moreover, they are male-determining 

 in their action, since the addition of a 

 set of autosomes causes the female to 

 assume male characteristics. That is 

 to say, in the autosomes there are genes 

 that tend to produce the characters 

 we call male, and these are more effec- 

 tive, either through greater numbers 

 or through greater potency, than the 

 total of autosomal genes tending to 

 produce the alternative characters that 

 we call female. On the other hand, the 

 X has a net female tendency, as shown 

 by the fact that the addition of an X 

 to a male group changes the individual 

 into a female. The net male tendency 

 of a set of autosomes is less than the 

 net female tendency of an X. This is 

 seen in the fact that in the individual 

 in which there are two of each. 



