198 PHYSICAL BASIS OF HEREDITY 



in which sex is determined by a chromosome mechanism, 

 it were possible to change the sex by other agencies in 

 spite of the chromosome arrangement, the latter relation 

 would be entirely thrown out of gear and males would 

 transmit sex-linked characters and sex itself like females, 

 and females like males. As no such cases have been 

 found, it is futile to discuss such a possibility. 



It has been shown that only the female-producing 

 sperm in phylloxerans and aphids becomes functional, 

 hence it is obvious why all the fertilized eggs develop 

 into females. In daphnians and other Crustacea it is not 

 known whether one class of spermatozoa degenerates, but 

 the results are explicable on such a view. In rotifers 

 the production of males only by certain females is due to 

 the eggs developing by parthenogenesis with the haploid 

 number of chromosomes and this explains also the case of 

 the bees, wasps and other hymenoptera. If a queen bee 

 is unfertilized or if her supply of sperm gives out she 

 produces only males. If she contains sperms, then any 

 egg that is fertilized produces a female, and as Petrunke- 

 witch showed several years ago, spermatozoa are to be 

 found in eggs laid in worker cells — ^such eggs being known 

 to produce workers ( 9 $ ). In rotifers, too, the presence 

 of a large and a small class of sperm suggests that only 

 the former is functional. 



Certain females of Drosophila give a sex ratio of two 

 females to one male. By making such a female hetero- 

 zygous as to her X-chromosomes (each carrying different 

 factors) it can be determined that the half of the expected 

 sons that die are the ones containing one of these two 

 chromosomes. It is easily possible by means of linked 

 genes to locate a factor in the sex-chromosome (Fig. 91) 

 and to show that whenever it goes to a male the fly dies. 

 All the daughters survive because the lethal factor being 

 recessive does not harm a female whose other^ chromo- 

 some comes from a normal father. The scheme is shown 

 on the next page. 



