292 CHROMOSOMES IN HEREDITY: MECHANICAL 



may be random (and apparently is in many organisms), although 

 their distribution in frequency and in relation to one another is 

 not so. 



A second m.ethod of observing interference has been attempted 

 by Belling in observing internode lengths. These distances, 

 however, are likely to be determined more by the conditions of 

 equilibrium in chiasma movement than by the original interference, 

 for it is probable that movement occurs as soon as chiasmata are 

 formed, in all organisms (Ch. XII). The frequency method is there- 

 fore the only one available for showing interference. 



It might be supposed that interference between successive cross- 

 overs might work either between chiasmata of any kind or between 

 chiasmata involving crossing-over between the same chromatids. 

 It might be chiasma interference (as shown cytologically) or chroma- 

 tid interference. Mather (1933, 1935 b) has shown that chiasma 

 interference, and also the formation of at least one chiasma by every 

 bivalent in the diploid, can be inferred from the breeding results in 

 Drosophila, while on the other hand (1935 b) there is no evidence 

 for chromatid interference. Whether chromatid interference also 

 exists can be found cytologically in ordinary circumstances only 

 from comparing the chromatid relationships at successive chiasmata 

 in inversions, which has not yet been done. Hearne and Huskins 

 (1935) conclude in Melanoplus that comparate chiasmata are more 

 frequent than disparate chiasmata, but we still do not know whether 

 the complementary or reciprocal types, or both, are responsible for 

 this excess over expectation. 



In the sex chromosomes of the male, unlike the autosomes of the 

 female Drosophila, pairing and crossing-over show evidence of 

 negative chromatid interference. This is absolute, giving pairs of 

 reciprocal chiasmata and no others in the X-Y pair (Ch. IX). In 

 female Bombyx there is a correlation in reductional as opposed to 

 equational division of different parts of the sex chromosomes which 

 themselves show no linkage (Mather, 1935 b). This seems to imply 

 a similar negative interference. 



(b) In Relation to Different Chromosomes. Schultz found, 

 from comparative studies of structurally homozygous and hetero- 

 zygous Drosophila, that high crossing-over in one chromosome goe§ 



