330 CHROMOSOMES IN HEREDITY : PHYSIOLOGICAL 



Inert chromosomes arise, so far as we can judge, as small " frag- 

 ments " derived from the breakage of larger chromosomes. Such 

 are the supernumerary chromosomes of Zea, Tradescantia, Fritillaria, 

 Ranuncuhis, Tulipa and Paspaliim (Table i6). The large chromo- 

 somes from which these fragments have arisen must have been 

 inert in the neighbourhood of the centromere, otherwise the new 

 fragments will be active like those in Matthiola and Solanum. The 

 small chromosomes of Orthoptera which are so frequently hetero- 

 zygous for deficiencies are probably inert in part (D., 1936 h). How 

 chromosomes that are partially inert could have developed is shown 

 by the properties of Y chromosomes in certain animals with an 

 advanced degree of differentiation of sex chromosomes (Ch. IX). 

 This chromosome in Drosophila is almost entirely inert and has 

 concurrently lost the property of mutation. The property of 

 inertness is probably very general in Y chromosomes, since they 

 vary in size most frequently both within species and between species. 

 And again in many species they are lost altogether. The Y chromo- 

 some has become inert, according to Muller (1918) because crossing- 

 over is effectively abolished in it. Probably, as we shall see later, 

 the proximal parts of the other chromosomes in Drosophila are also 

 inert, as the}^ certainly are in the A^. This may also be due to 

 absence of crossing-over in them in the female (of. Mather, 1936). 

 Whatever the cause, their inertness is significant as a condition of 

 the origin of inert fragments. It is apparently as fragments of the 

 sex chromosomes that inert chromosomes have arisen in the 

 Hemiptera, since such inert chromosomes sometimes pair with the 

 A' or Y in Metapodius (Wilson, 1909) and Alydus (Renter, 1930). 



The study of inert chromosomes has been greatly advanced by 

 the hypothesis of Heitz (1929, et seq.) that the differential " hetero- 

 chromatin " is always inert. Muller and Painter (1932) found that 

 the proximal third of the A^ chromosome in Drosophila melanogaster 

 included the loci of none of the genes which have mutated. Heitz 

 (1933) was able to show that this region was differential in behaviour 

 at mitosis, thereby substantiating his principle. Similarly he was 

 able to show that the proximal parts of the autosomes were 

 differentially condensed, and to recognise the sex chromosomes of 

 D. funebris for the first time by their behaviour in this respect. 



