296 CHROMOSOMES IN HEREDITY : MECHANICAL 



All such conditions must affect the relation of length to map- 

 distance. There is, therefore, no special reason for expecting that 

 under any particular conditions the one should be a direct function 

 of the other. 



If the chiasma frequency in Drosophila melanogaster is such an 

 indirect function of chromosome length as in Stenobothrus, then the 

 fourth chromosome, although extremely short, should be of 50 units' 

 length, a condition which would not be easy to verify. But if one 

 of the fourth chromosomes were attached to a fragment of the 

 X chromosome the chiasma frequency should be reduced in the new 

 triple body, IV + X + IV. X, and the small unattached fourth 

 should frequently fail to pair, and therefore sometimes pass to the 

 same pole as its attached homologue. This is found by Stem 



(1931)- 



(vii) Crossing-over with Ameiosis. An important result of 



crossing-over between chromatids, pointed out by Janssens in 1909, 

 is that the separation of dissimilar chromatids occurs at both 

 divisions. Now one of these divisions may be suppressed, or the two 

 nuclei resulting from it may fuse again into one (Ch. X). In these 

 circumstances reduction of the chromosome number is suppressed ; 

 but segregation should not be ; it can still occur at the other division 

 if one of the pairs of chromatids that separate has crossed over with 

 a dissimilar partner — if, that is, any of the chromosomes have 

 paired by chiasmata. 



It is now possible to consider, therefore (contrary to the earlier 

 belief), that suppression of meiosis leading to failure of reduction 

 will not lead to failure of segregation when any two chromosomes 

 have paired. When the two chromosomes that have paired 

 separate at the first division, but are included in a single restitution 

 nucleus, their second division, which gives the two unreduced 

 germ-cells, will, on the chiasmatype hypothesis, not be equational. 

 Thus, if crossing-over must occur at the chiasma which con- 

 ditions their pairing, the second division will always be reductional 

 in regard to one of the cross-over segments. In either case the 

 result will be equivalent to mendelian segregation of the differences 

 in the pairs of chromosomes concerned. It follows, therefore, that 

 a hybrid in which a pair of chromosomes is occasionally found at 



