78 THE CHROMOSOMES 



and consequently lead to very irregular distribution 

 of the chromosomes at the first meiotic division. 

 Such genes are known in Maize, Nicotiana and other 

 plants. In Drosophila melanogaster ' Go wen's Gene ' 

 produces similar effects on meiosis in the female. ^^ 



Behaviour of Sex-Chromosomes at Meiosis 



We have briefly considered (Chap. Ill) the different 

 types of sex -chromosomes as they appear at mitosis ; 

 it remains to be seen what happens to them at 

 meiosis. In the homogametic sex they behave, in 

 general, just like autosomes. Their behaviour in 

 the heterogametic sex depends on the following 

 general principles : 



(1) Non-homologous regions of chromosomes do 



not pau% and consequently no chiasmata can 

 be formed between them. 



(2) Homologous regions do pair at zygotene, even 



though other regions of the chromosomes in 

 question are not homologous. Chiasmata 

 will be formed in the homologous paired 

 regions, unless these are very short. 



(3) Pairing may be prevented, even between homo- 



logous regions, by extreme positive hetero- 

 pycnosis. 



(4) Univalent sex-chromosomes (i.e. sex-chromo- 



somes with no chiasmata) behave like other 

 univalent chromosomes and may show either 

 the first or the fourth type of behaviour 

 described above. 



Having stated these general principles we can 

 consider specific cases. In Locusts and Grasshoppers 

 we encounter one of the most highly evolved types of 

 sex-determining mechanism, which is, however, one 

 of the simplest to understand. The male is the 

 heterogametic sex, and there is no Y- chromosome ; 

 (that is to say, the female diploid set includes two 

 X-chromosomes, the male set only one, which is 



