EVOLUTION OF SEX 



393 



the sex-differential system by interchange, secondly, reduce the 

 pairing segment to little or nothing, in which case a new method of 

 pairing becomes necessary ; and, thirdly, eliminate the Y , when 

 an adapted segregation mechanism becomes necessary. 



Table 58 

 Primitive and Advanced Types of Sex Determination 



Rana-Lebistes-Melandrium-Rumex Type. 



Sex determination : 

 Partly by difference between 

 XX and XY. 



Crossing-ovev : 

 Occurs between pairing seg- 

 ments of X and Y and hence 

 partial sex linkage occurs. 



Differential segments of X and Y : 

 They are a small fraction and 

 pairing is regular. 



Recessive factors in X : 

 Show in XY individuals in 

 reduced effect (Winge, 1931). 



Drosophila-Protenor-Lygctiis Type. 



Purely by difference in balance 

 between XfA and XX I A . 



Abolished between X and Y owing 

 to the pairing segment being 

 eliminated or inert. 



They are a large part, inert in Y , 

 active in X. 



Show in XY individuals in 

 unreduced effect, except those in 

 pairing segment. 



They have the same system of 

 sex-determination, as a rule. 



5. Related forms : 



They are hermaphrodite 

 {Melandriiim, Rana) or have 

 an opposite system of sex- 

 determination {Lebistes and 

 Platypcecilus) . 



6. Intersexual forms appear in both types either through (i) gene mutation 



or hybridisation in the diploid or (ii) different proportions of the active 

 chromosomes in polyploid and polysomic forms. 



7. Variation in X and Y : 



Still largely related on account 

 of the small differential seg- 

 ment. Hence size difference 

 not extreme. 

 Precocity of X and Y : 

 Absent. Pairing segments 

 associate by chiasmata in 

 prophase. 



Independent on account of the large 

 differential segment. Hence size 

 difference may be slight or 

 extreme. 



Frequent. Pairing may be post- 

 poned to metaphase of the first 

 or second division. 



Thus, if we assume that the differentiation of X and Y chromo- 

 somes has been accompHshed by a combination of structural and 



