IRREGULAR RATIOS 20-7 



validating them. The search for precisely segregating genes com- 

 pels the selection of genetical material. In our own work on Neuro- 

 spora we were unable to classify the progeny of over two-thirds of 

 our matings. 



The economy that is achieved in describing living organisms 

 by genetical terms is enormous, for with morphological characters 

 that can only be designated descriptively, one cannot present a 

 pedigree involving many hundreds of individuals unless some short- 

 hand "all-or-none" designation can be discovered. If each culture 

 differs individually from every other one it is not feasible to de- 

 velop categories to fit them all. When we found crosses in Neuro- 

 spora whose progeny we could not analyze, we assumed that too 

 many factors were involved. Thus the only step that could be taken 

 was to inbreed for a few more generations until the stocks were 

 "cleaned up" and the characters were recognizable in their dif- 

 ferent combinations. The question is whether this process of 

 "cleaning up" stocks does not introduce some new factor or re- 

 m.ove some normal mechanism. 



Even in Drosophila when directly classifiable terminal char- 

 acters such as the number and position of bristles are studied, an 

 abundance of unclassifiable data accumulate which are presumed 

 to arise from multiple factors or other causes. In the genetical 

 analysis of the Smuts, Dickinson studied colonial characters and 

 color and found that segregation occurred in an irregular fashion 

 and he assumed irregularities of chromosomal behavior such as 

 a third division segregation to explain the effects. This is another 

 example of the pervading assurance that all genetical characters 

 segregate regularly. In our own collection of yeast pedigrees we 

 have hundreds of photographs of colonies but we have thus far been 

 unable to develop a publishable analysis. Descriptive characters 

 are simply too difficult to handle. 



However, biochemical characters such as ability to ferment 

 carbohydrates are easier to handle. In some fermentation pedi- 

 grees in yeast the inheritance was so precisely and regularly Men- 

 delian that it was clearly gene -controlled. In the pedigrees in 

 Chapter 19, the a/a, and G/g alleles segregated precisely accord- 

 ing to Mendelian expectations. This proved that the characters 

 were referable to two different loci which segregated regularly 

 in the reduction division. The irregular ratios involving the fer- 

 mentation of galactose and melibiose which were discovered later 

 (Chapters 20 and 26) raised the question of whether or not some 

 of the irregular ratios which occur in other organisms arise in a 

 similar manner. 



THE "GOOD GENE" CONCEPT 



Geneticists frequently speak of genes as if they fall into two 

 natural categories: "good" and "bad." A "good" gene is easy to 



