ORIGIN OF SEX 391 



of chiasmata and crossing-over, is a reaction of the chromosomes 

 to a genetic property of the organism. Locahsation, for example, 

 is found in Fritillaria Meleagris, but not in F. imperialis. 



(ii) Abnormal forms of meiosis are characteristic of one sex in 

 the Coccidae and in the Diptera, and affect equally like and unlike 

 chromosomes. They must therefore be determined by a genetic 

 property and not by hybridity as such. 



(iii) Crossing-over is generally reduced in sex heterozygotes 

 relative to the homozygotes in the autosomes as well as in the sex 

 chromosomes. This points to a unitary and therefore genotypic 

 control of the difference (Haldane, 1922 ; cf. Huxley, 1928 ; Crew 

 and KoUer, 1932). 



(iv) The mechanism of reciprocal crossing-over in Drosophila 

 must be genotypically determined, since it does not occur in the 

 female, and could not be due to a structural difference. 



These remarks suggest the means by which crossing-over may 

 be suppressed, viz., non-pairing and localised pairing due to non- 

 precocity or reduced precocity. The incidence of non-precocity 

 and abnormal meiosis will be discussed later (Ch. X). Localisation 

 is less easily detectable. It may be of two kinds. Most commonly 

 sex chromosomes that appear to have paired normally during 

 prophase are associated by terminal chiasmata at metaphase, as 

 in all plants. In these the localisation must be near the distal 

 ends of the chromosomes, and therefore terminalisation will occur 

 rapidly and the original position of chiasmata will not be ascertain- 

 able. The differential segments will then be proximal and the 

 pairing segments distal {v. Fig. 115). This condition is the same 

 as that in the complex-heterozygote. A second possibility is that 

 localisation occurs as in Mecostethus and Fritillaria in the neigh- 

 bourhood of the centromere. When this was median and both 

 arms were differentiated, chiasmata with reciprocal cross-overs 

 would need to be regularly formed to maintain the differentiation 

 of the X and Y chromosomes as in male Drosophila where visible 

 crossing-over is consequently suppressed. 



The best recognised change conditioned by the suppression of 

 crossing-over between the differential segments is the change to 

 inertness in the differential segment of the Y, a change evidently 



