486 The Determination of Sex 



are not present, such individuals are converted into males which 

 are sterile because the two ix™ genes are more potent than the 

 two F genes. A number of other modifying genes which delay 

 the transformation of FF ix^ix"^ females into males is also pres- 

 ent. This delay causes the individuals to be intersexes possessing 

 both ovaries and testes instead of males. 



Drosophila virilis is not the only species of that genus in which 

 diploid intersexes have been found that have resulted from the 

 action of a single gene. As in D. virilis, a recessive gene is the 

 causative agent for certain intersexes in D. simulans, but in D. 

 pseudoobscura intersexes have been produced by a dominant 

 gene. In the last species, Dobzhansky and Spassky have found 

 that most of the diploid intersexes have two sets of genital ducts 

 and external genitalia, one of which is almost always more 

 female-like whereas the other is usually more male-like. Be- 

 cause of these two sets of reproductive organs, these intersexes 

 could also be termed hermaphrodites. There is, however, only 

 one pair of gonads so that these aberrant flies have generally 

 been regarded as intersexes rather than hermaphrodites. The 

 intersexes were genetically females, as was revealed by a cyto- 

 logical examination which showed that they possessed two X 

 chromosomes and two sets of autosomes, and they arose as a 

 result of a dominant gene whose normal allele is believed not 

 to be involved in any way in the mechanism of sex determination. 



Heteropycnosis 



One physical feature of the X chromosome in many animals 

 is of considerable interest. In the heterogametic sex of a number 

 of animals the sex chromosomes are much more condensed dur- 

 ing certain stages of division than the autosomes. Such chromo- 

 somes, therefore, are much more compact and stain more deeply 

 than the other chromosomes. They are said to be heteropycnotic 

 and the phenomenon is heteropycnosis (Fig. 139). The con- 

 densed condition is often noticeable during the period of growth 

 of the spermatocytes. Because of the compact, deeply staining 

 nature of the X chromosome during this stage, it has the appear- 

 ance of a karyosome or chromosome-nucleolus. Because of the 

 difference in appearance between these heteropycnotic sex chro- 

 mosomes and the autosomes, the sex chromosomes were once 



