270 GENETICS 



That there are specific genes in the z/-chromosome 

 which, working in conjunction with autosomal genes 

 are capable of producing males, females or interme- 

 diates, in cases where the normal relationship is upset, 

 has been indicated very clearly, especially by Bridges in 

 recent experiments on DrosophUa. 



Sex, in other words, is now put upon a basis of 

 specific genes. We are, therefore, entirely rid of the 

 older ideas that the tf-chromosome is composed of a 

 different kind of chromatin from that found in the 

 autosomes and that the sex of the zygote depends upon 

 the amownt of a?-chromatin it receives. 



C. HETEROGAMETIC FEMALES 



The reverse of the foregoing mechanism, in which 

 two kinds of sex-determining sperm are present, is 

 found in the Lepidoptera and birds. In these groups 

 the presence of 2n -f- xx constitutes a male and 2n -\-x, 

 a female. The formulae in these cases are usually writ- 

 ten 2n -f- zz and 2n + 2, in order to distinguish them 

 from those of heterogametic males. 



The cytological proof for the s-chromosomes is not 

 as strong as for the o?-chromosomes, since both avian 

 and lepidopteran chromosomes are peculiarly difficult 

 to study. Nevertheless, the facts are well borne out by 

 breeding experiments in both groups. 



Definite results have been reached by Seiler and also 

 by Doncaster in experiments with moths, showing that 

 two types of ova are produced, namely, one which, 

 after extruding the ^-chromosome into the polar cell 



