272 CHROMOSOMES IN HEREDITY: MECHANICAL 



So much applies to simple inversions or external inverted trans- 

 locations (in which the two pairing segments are attached to two 

 centromeres), but we may equally have the pairing of dislocated 

 segments within one chromosome (inverted duplication), or as part 

 of a trivalent combination. Crossing-over within one chromosome 

 may be, so to speak, between a chromatid and itself or between two 

 sister chromatids. The first will give a ring chromatid, which will 

 perpetuate itself as a ring chromosome (as found in attached X's 

 with an inversion in Drosophila, Tiniakov, 1935). The second will 

 give a loop chromatid and a second division bridge. 



Where three chromosomes are associated in an interchange 

 hybrid or a polyploid, the two centromeres of the dicentric chromatid 

 sometimes pass to the same pole. Such a dicentric chromatid may 

 perpetuate itself normally, as in Ascaris, if the whole of each 

 daughter chromatid goes to the same pole. But sooner or later they 

 will go to opposite poles and a reciprocal bridge will be formed at 

 the mitotic anaphase (Tulipa, Upcott, unpub., cf. Ch. X (iii) ). 



The consequences of crossing-over in a dyscentric hybrid are 

 different in egg cells and male germ cells on account of the spatial 

 arrangements resulting. This is important, because genetical 

 observations on crossing-over in Drosophila are confined to the 

 female. Sturtevant (1926 h) found that crossing-over was sup- 

 pressed in inversion-heterozygotes between the dislocated segments 

 apart from double crossing-over. Yet later workers (Stone and 

 Thomas, 1935 ; Griineberg, 1935) have found that the viabifity of 

 eggs is not reduced by the proportion of the expected single cross- 

 overs. The reason for this is now clear. The four products of 

 meiosis lie in a row in the embryo-sac or egg mother-cell {egg 

 nucleus and polar bodies) and squarely or equidistant in the pollen 

 or sperm mother cells. Beadle and Sturtevant (1935) have found 

 that the cross-over chromatids taking part in the bridge never enter 

 into the functional female nucleus, which is an end nucleus in the 

 row. They conclude that the bridge holds together so that its parts 

 never .pass into this end nucleus. The deleterious effect of crossing- 

 over in inversion hybrids will therefore be less serious in female 

 than in male gamete formation. 



(d) Significance of Crossing-Over in Inversions. The consideration 



