Meiosis in the Sex Chromosomes 71 



called heterochroinatin to distinguish it from the active or euchro- 

 matic regions. These two types of chromatin stain somewhat 

 differently during mitosis. Although most of the Y chromosome is 

 inert, heterochromatic material is not confined to the Y chromo- 

 some, for about one-third of the X chromosome nearest the cen- 

 tromere and small regions of the autosomes on either side of 

 their centromeres appear to be heterochromatic. 



As in Drosophila, there is an homologous segment in both the 

 X and Y chromosomes of human beings, but this segment is 

 small in comparison with the nonhomologous regions and only 

 a few genes have been found in it. A number of loci are found 

 in the X chromosome which are not represented in the Y chromo- 

 some, and several genes are known in the part of the Y chromo- 

 some not represented in the X chromosome. Some of the char- 

 acters produced by genes on. the X and Y chromosomes in man 

 have been mentioned in Chapter 3. 



Meiosis in the Sex Chromosomes 



The meiotic behavior of the chromosomes in individuals with 

 two X chromosomes, such as the females of many species, or 

 with two Z chromosomes, as the males of birds and a few other 

 animals, is exactly like the meiotic behavior of the autosomes. 

 This is to be expected since the two X chromosomes are homol- 

 ogous throughout their length. Thus they pair at zygotene, 

 exchange segments, and form chiasmata in exactly the same 

 way as autosomes. In the heterogametic sex, however, the be- 

 havior depends upon the presence or absence of the Y chromo- 

 some and, if a Y chromosome is present, upon the extent of 

 homology between it and the X chromosome. In organisms of 

 the XO type naturally there is no chromosome with which the 

 X chromosome can pair (Fig. 23). It will usually pass intact 

 to one or the other pole at the first meiotic anaphase and divide 

 equationally at the second. If it does so, it goes to the pole 

 either before or after but never at the same time as the auto- 

 somes. Sometimes the univalent X chromosome divides equa- 

 tionally at the first division and reductionally at the second. In 

 either case only two of the four resulting cells contains an 

 X chromosome. In the XY type in most organisms, pairing may 

 occur between the X and Y chromosomes provided they have a 

 segment in common, but pairing is always between homologous 



