THE CONTROL OF RECOMBINATION 

 TABLE 25 



CHIASMA FORMATION AND CHROMOSOME PAIRING IN 

 NORMAL AND ASYNAPTIC MAIZE (BEADLE, 1933) 



When the asynaptic plants are bred, one might expect their 

 progeny to show a great reduction in the frequency of crossing-over. 

 The reduction, however, is not uniform. On the female side, in 

 fact, there is no reduction at all. The explanation is instructive in 

 several ways. Either the action of the gene in upsetting meiosis is 

 different on the female side. Or the selected sample of the products of 

 meiosis is different. The germ cells which are effective in breeding are 

 those with a complete set of chromosomes. Complete sets are pro- 

 duced only when the mother cell, at least in the large female cell 

 where unpaired chromosomes are always lost, has had complete 

 pairing. And complete pairing has been attained in those cells only in 

 which the chromosomes had a normal, or nearly normal, frequency 

 of chiasmata. In the male cells incomplete pairing will sometimes give 

 complete sets by chance. And on the male side some reduction of 

 crossing-over is found. The results are : normal, 24 per cent recom- 

 bination between the genes shrunken and waxy; female asynaptic, 

 24 per cent; male asynaptic, 13 per cent. 



Apart from low-frequency adjustment there are two other 

 methods of reducing the effective frequency of recombination. 

 One is by locaHzation. In a greater or less degree aU organisms have 

 some localization of chiasmata. This localization seems to be 

 produced by a delay in pairing, so that the later pairing produces 

 no coiling strain or is even totally inliibited. Crossing-over is then 

 confined to the region where pairing began. In some species, such 

 as the grasshopper Afecoifef//;/-? grossus or the lily, Fritillaria meleagris, 

 the crossing-over is localized near the centromere. In others, a more 

 numerous congregation, it is localized near the ends, for example in 



Elements 0} Genetics 289 T 



