THE BEHAVIOUR OF INDIVIDUAL CHROMOSOMES 77 



usually either before, or after, but not synchronously with, the auto- 

 somes, and divides equationally in two at the second division, or (2) 

 divides equationally at the first division and passes to one or other pole 

 at the second. 



In Xy organisms pairing between the X and Y is very variable in 

 intensity, probably correlated with the length of chromosome through 

 which the sex differential is spread, and which must therefore be 

 guarded from crossing-over. At one end of the series, in organisms with 

 diffuse sex differentials, e.g. Lygaeus, the chromosomes divide equa- 

 tionally in the first division, and pair transitorily before segregation in 



A B C D E* F 



Fig. 32. Segregation of Sex Chromosomes. — The sex chromosomes are drawn 

 in black; the diagrams represent anaphases of the first division of meiosis seen in 

 side view. A, unpaired X passing to one pole before the rest of the set (e.g. 

 Stenobothrus). 6, unpaired X passing to one pole after the rest of the set (e.g. 

 Aphis). C, unpaired X split into two chromatids, one of which goes to each pole 

 at the first division (equational separation) (e.g. Plotinus). D, unequal XY segre- 

 gating with the other chromosomes (many animals). £, unequal XY segregating 

 before the other chromosomes (e.g. man). F, unequal XY" separating equationally 

 at first division (e.g. Lygaeus). 



(Modified from Darlington.) 



the second division. At the other end, are organisms in which the sex 

 differential is locaUzed at one point and pairing of the X and Y at the 

 prophase of the first division is apparently normal, and crossing-over 

 occurs between them (Lebistes). Drosophila occupies an intermediate 

 position, since pairing occurs, but there is less affinity between an X 

 and a Y than between two Xs in XX Y flies. This differential affinity 

 is due to the fact that only part of the X possesses a homologue in the 

 Y; the limits of the homologous section can be determined by studying 

 the pairing of XF flies containing small redupHcated sections of the X. 

 Within the homologous segment of the X and Y pairing occurs and 

 crossing-over takes place, as is necessary if the chromosomes are to 

 remain associated at metaphase (p. 121). This causes no difficulty unless, 

 as is the case in D. melanogaster, the pairing segment lies between two 

 sex-determining segments which might get separated by crossing-over 

 and thus lead to a disturbance of the sex-determining mechanism. In 

 such a case the crossing-over is probably by two reciprocal chiasmata, 

 both on the same side of the centromere, so as to preserve the relation 



