THE MECHANICS OF THE CHROMOSOMES I29 



of chiasmata in the meiotic bivalent. The chromatid configurations 

 which occur can be classified as follows. If A, A' are two sister chroma- 

 tids and aa' the other two sisters, suppose A and a cross-over at the 

 first chiasma. Then at the second chiasma there are four possibiHties. 

 If A, a cross-over again the chiasmata are spoken of as reciprocal (re- 

 current compound cross-overs), if A' a' cross-over, they are comple- 

 mentary (independent compound cross-overs). In both cases the 

 original arrangement of the partners is restored and the two chiasmata 

 are said to be compensated or comparate. The other two possibiHties 

 are that Aa or A' a cross-over at the second chiasma and these give 

 non-compensating or disparate (diagonal and progressive) chiasmata. 



The types of gametes produced are different in the different cases. 

 They can be identified in diplo-haplonts such as Neurospora.^ It will 

 be noticed that if all types occur with equal frequency, it is only in 

 half the gametes that the original association of the genes at the ends 

 of the chromosome are still associated, i.e. that a double cross-over has 

 occurred. The variation in the intensity of interference along the 

 length of a chromosome might therefore be a variation, not in the 

 frequency with which successive chiasmata occur, but in the type 

 which occur. Actually Heame and Huskins^ have described a case in 

 which the cytological types do not apparentiy occur at random, there 

 being double as many non-compensating as compensating arrange- 

 ments. In Neurospora the genetic evidence indicates a considerable 

 excess of recurrents, but in other organisms the types seem to occur as 

 would be expected on a basis of chance, so that there is no "chromatid 

 interference."^ 



Note the distinction between these types of compound crossing-over 

 between the chromatids derived from one pair of chromosomes and 

 the recurrent and progressive types of double crossing-over between 

 different chromosomes in a polyploid (p. 107). 



7. Crossing-Over in Structural Hybrids 



Organisms which are heterozygous for some structural change, such 

 as a translocation or inversion, may be called structural hybrids.* 

 Chiasma formation is dependent on zygotene pairing and in such 

 organisms this is often interfered with by the hybridity. If, for instance, 

 part of a chromosome A is translocated on to the end of chromosome B, 



1 Lindegren and Lindegren 1937. - Hearne and Huskins 1935. 



3 Beadle and Emerson 1935, Mather 1933, cf. Darlington 1937. 

 * Darlington 1929. 



