370 PERMANENT HYBRIDS 



unequal bivalents, viz., the differential segments divide equationally 

 at the first division and reductionally in the second. This is what 

 happens in Rattus in about one-twentieth of the spermatocytes. 

 Genetic evidence indicates that it also happens in Bomhyx (Gold- 

 schmidt and Katsuki, 193 1 ; cf. Mather, 1935) and the observation 

 of an equational first division of an " unpaired " X in Apodemtis is 

 most easily explained in this way. 



Where the differential segment is proximal, no special property 

 is required to permit of pairing segments at both ends of a sex 

 chromosome, as with a chromosome in a ring in (Enothera. Such a 

 system is actually developed in Humulus and elsewhere. But 

 where the pairing segment is proximal a very special adaptation 

 is required to permit of differential segments at both ends, for 

 these reasons : the pairing segment is intercalary ; it cannot, 

 therefore, pair by terminal affinity ; it can pair only by chiasmata ; 

 but effective crossing-over giving a single chiasma cannot occur in 

 segments lying between two pairs of differential segments without 

 removing one of them. Nevertheless in several species of Drosophila 

 the pairing segment includes or adjoins the centromere and both 

 arms are differential. Thus in D. melanogaster the X and Y chromo- 

 somes have one gene in common (" bobbed."). It lies near the 

 centromere, while both ends of the chromosomes are different. 



To account for pairing without visible crossing-over in these 

 species it was therefore necessary to suppose that crossing-over 

 betv/een them was regularly reciprocal and gave rise regularly to 

 reciprocal chiasmata in a part of the chromosomes near the centro- 

 mere where there were no differential genes (D., 1931 a). The Y 

 chromosome was known to be almost entirely inert and such an 

 inert region was afterwards found in the proximal part of the X 

 chromosome (MuUer and Painter, 1932). The view seemed to be 

 supported by the frequencies of pairing of X and Y in trisomic 

 forms, and also by various structural changes. For example, 

 X becomes " attached " to an arm of Y in a regular way such 

 as could result from exceptional single cross-overs near the centro- 

 mere and with a regular frequency of once in 2,000 times. Such 

 a change resembles the mutations which result from crossing- 

 over in (Enothera. These expectations have been borne out in 



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