372 PERMANENT HYBRIDS 



been connected near the centromere at metaphase. Their con- 

 nection may be on either side of the centromere, but is never on 

 both sides. It therefore shows both the variation and the inter- 

 ference characteristic of chiasma-formation. Furthermore, the 

 mechanical relationships of X and Y are entirely different from 

 those of the autosomes. The former show the characteristic tension 

 between the centromere and the nearest point of association, i.e., the 

 nearest chiasma. The autosome pairs, on the other hand, would 

 be incapable of showing chiasma-formation, even if they had 

 crossed over or not, because their four chromatids are equally 

 attracted to one another, lying equally parallel at diakinesis. They 

 are associated apparently at the centromere, but between diakinesis 

 and metaphase suddenly turn away from one another at this point, 

 very much as mitotic chromatids do at the beginning of anaphase. 

 Thus in the relationships both of the centromeres and of the bodies 

 of the chromosomes the autosomes are unique. The sex chromo- 

 somes, on the other hand, are normal in the formation of chiasmata 

 except in their being regularly reciprocal. 



These observations show that the properties of crossing-over 

 that can be deduced from general observations of structural hybrids 

 enable one to classify the types of sex-chromosome differentiation 

 that is found, according to the arrangements of differential and 

 pairing segments and to specify the kinds of crossing-over that will 

 be necessary to preserve it. Thus the system of reciprocal chiasmata 

 developed in Drosophila has been the condition of development of 

 unique sex-chromosome differences in this genus. Furthermore, 

 the pairing of sex chromosomes enables us to predict the occurrence 

 of partial sex-linkage in all organisms where pachytene association 

 and chiasma-formation occurs, in all, that is, except the most 

 extreme types. Such linkage has now been found in Drosophila 

 (Phihp, 1935) and in man (Haldane, 1936). 



(iii) Segregation of Sex Chromosomes. Paired sex chromosomes 

 separate reductionally at the first or second divisions, according to 

 the relative position of pairing and differential segments. Unpaired 

 sex chromosomes are a permanent property of the race or species 

 and behave regularly (unlike the unpaired chromosomes of 

 adventitious hybrids) in one of the following ways : — 



