34 THE CHROMOSOMES 



aflFairs is called somatic pairing ; there is apparently 

 sufficient residual attraction to cause it lo develop 

 even in triploid nuclei (Fig. 206). Apart from this 

 and a few other similar cases chromosomes always 

 keep at a certain distance from one another through- 

 out the mitotic cycle as a result of the surface repul- 

 sion force already referred to. 



Genetically Active and Genetically Inert 

 Chromosomes 



No attempt will be made here to explain how the 

 idea that the genes are arranged in linear order along 

 the chromatids has been proved, since the matter is 

 dealt with fully in all text -books of genetics. Certain 

 special problems must, however, be gone into. It has 

 long been known that the Y- chromosome in Droso- 

 phila melanogaster is genetically almost inactive. 

 Males lacking it (called X-nought males) are viable 

 but sterile. It has been shown that the Y contains 

 two separate genes necessary to ensure fertility in 

 addition to the normal allelomorph of the gene 

 * bobbed ' i^s . apart from these it may contain a 

 few other genes. In Maize there is also a chromo- 

 some (the B-chromosome) which contains no known 

 genes and may be present any number of times in 

 the chromosome set or may be absent altogether 

 without in any way affecting the phenotypic appear- 

 ance of the plant. ^^* These, then, are examples of 

 inert chromosomes. It has recently been shown ^^^ 

 that the part of the X-chromosome in D. melanogaster 

 which is next to the spindle attachment (about one- 

 third of the total length) is almost completely inert 

 (up to the present this region only contains one known 

 gene, namely bobbed, which suggests that this region 

 is homologous with that part of the Y which likewise 

 contains bobbed). In addition there are almost 

 certainly small regions in the centre of chromosomes 

 II and III, on either side of the spindle attachment, 

 which are also inert. The possibility thus arises 



