500 MUTATION AND PLANT BREEDING 



To account for these losses he has developed an ingenious model 

 of double-loop pairing in the A region during meiosis. This allows 

 recombination either normally between homologous strands, or 

 internally between adjacent segments of the same strand (auto- 

 association). Although the proposed model is below the limits of 

 optical verification, it is based on good inferential evidence. (It is 

 known that the losses occur as a result of meiosis and, from inde- 

 pendent experiment, that the alpha and beta elements can engage 

 in oblique synapsis). Assuming the model to be correct, the evi- 

 dence shows that mutations formerly supposed to be "intra-genic" 

 on the basis of standard recombination tests, turn out to be "extra- 

 genic" when the tests are made more rigorous. The goat pen is 

 currently empty and the chances of further screening are limited 

 by the difficulty of obtaining new presumptive goats! 



The great technical difficulty of obtaining large enough popu- 

 lations to test rare mutants in this material probably sets a limit 

 to continued analysis along the same lines. However, it is amusing 

 to speculate where its continuation might lead. Because oblique 

 synapsis occurs between the alpha and beta elements it should be 

 possible to synthesize an extended series of repeats. The number 

 of possible recombinants would increase exponentially and models 

 based on triple, quadruple, etc., loop pairing would be permissible. 

 The possible exchanges would be difficult to portray in a 2 -dimen- 

 sional model; conversion to a 3-dimensional model would produce 

 a double helix, resembling" a simplified version of the Watson-Crick 

 formula. Diagrammatically, recombinations would involve exchanges 

 between neighboring gyres in the nucleotide sequence and their 

 consequences would not be distinguishable from those resulting 

 from Freeseian transitions. In short, the distinction between "extra- 

 genic" and "intra-genic" mutations would become solely a matter 

 of interpretation. 



In the higher organisms it would seem that a criterion other 

 than separability is needed to distinguish between extra- and intra- 

 genic events. An acceptable criterion should be based on function 

 and be capable of critical test. The cistron concept, developed in 

 microorganisms, fulfills these requirements. It is a functional unit 

 in the genetic sense and when the serial order of its base pairs is 

 interrupted, either by rearrangement or physical separation, that 



