78 AN INTRODUCTION TO MODERN GENETICS 



between the centromere and the ends of the chromosomes.^ It has Httle 

 observable consequence, as the homologous parts of the X and Y are 

 nearly empty of active genes. This part of the chromosomes is therefore 

 called the inert part of the X; it is made of heterochromatin (p. 40) 

 and shows the characteristic differential condensation. Such condensa- 

 tion or heteropycnosis is in fact a fairly common property of sex chromo- 

 somes, and, as was mentioned above, may be found in XX-XO or 

 derived mechanisms, where there is no apparent need for an inert 



/I 

 abed e 



Fig. 33. Homologous and Differential Parts of Sex Chromosomes. — The 



homologous parts of the sex chromosomes are represented by simple lines, the 

 differential, sex-determining parts by dotted lines (for X) and wavy lines (for Y). 

 Centromeres black. The upper row shows the zygotene pairing, the lower the 

 metaphase configurations. A a short differential segment in one arm (e.g. Lebistes). 

 B pairing segments at one end (e.g. Melandrium). C pairing segments at both ends 

 (e.g. Phragmatobia). D differential segments at one end, two alternate metaphase 

 configurations (e.g. Mammalia). £ differential segments at both ends (e.g. Droso- 

 Ph'la). (After Darlington.) 



pairing part of the chromosome differentiated from the sex determining 

 part. 



The necessity to prevent the mingling of the sex-determining regions 

 of the X and Y chromosomes, which is expressed in the development 

 of inert pairing regions in forms such as Drosophila, is probably also 

 responsible for the generally observed reduction in frequency of 

 crossing-over in the heterogametic sex.^ In Drosophila and other 

 Diptera this goes so far as a complete suppression of crossing-over of 

 all chromosomes in the male. This is correlated with the development 

 of a completely abnormal mechanism of pairing and segregation 

 (p. 123), a development which has brought about the peculiar paradox 

 that in the male Drosophila the only chromosomes associated by cross- 

 ing-over (chiasmata) in the ordinary way are the XY pair, although it 

 1 Phillip 1935. 2 Haldane 1922, Huxley 1929, Eloff 1932. 



