GENETICAL CHARACTERS 11-2 



morphology among the progenies of hybrid yeasts; each haplophase 

 usually produces a relatively distinct colonial type; but in rather 

 extensive pedigrees we have not succeeded in making a genetical 

 analysis of genes controlling colonial characters, possibly because 

 interaction between these alleles occurs with high frequency. 



In Neurospora multiple factors interfere seriously with geneti- 

 cal analysis of morphological characters. The colonial morphology 

 of a clone carrying two different genes cannot be predicted on the 

 basis of the morphology of each haploid parent previous to the mat- 

 ing. Each gene affects the entire thallus; genes controlling the 

 shape of a colony or thallus do not express their effects in an iso- 

 lated locality. In Drosophila a character which affects the struc- 

 ture of the wing may have little perceptible effect on eye color; 

 when eye color and wing character are expressed independently 

 classification is easy. Much of the success achieved in Drosophila 

 genetics is due to cross-breeding stocks characterized by charac- 

 ters which make their appearance after differentiation is almost 

 completed on widely separated regions of the animal. The only 

 general principle concerning the effect of combining morphological 

 genes in Neurospora is that multiple mutants are generally weaker 

 than the single mutants. When more than two morphological char- 

 acters are involved the analysis often fails. Neurospora hybrids 

 heterozygous for three morphological characters, gap, crisp, and 

 pale, could be analyzed by inspecting the progeny, and an additional 

 physiological character, mating type, which could only be detected 

 by test matings, could be superimposed on the morphological char- 

 acters without complicating the analysis, but attempts to add one 

 additional character to this hybrid complicated the results beyond 

 the possibility of analysis. 



We have only used a few morphological characters in yeasts; 

 one of these is mucoid colonial growth. Variations in the substrate 

 affect some colonial characters considerably. We have photographed 

 hundreds of colonies of the haplophase cultures isolated in yeast 

 pedigrees. Some clear-cut distinctive, morphological patterns re- 

 appeared frequently among the progeny (fig. 11-1) but except for 

 mucoid, no simple ratios were obtained. There are two techniques 

 of studying colonial morphology; one is to plate out a culture and 

 examine the small, individual colonies, the other is to grow giant 

 colonies, preferably in gelatin at low temperatures. We have used 

 the former technique principally, for this also detects the other 

 variations that occur with high frequency in haplophase yeast cul- 

 tures. Winge and Laustsen (1937) have had some success using 

 giant colonies grown on gelatin, but for the analysis of extensive 

 pedigrees this might prove rather cumbersome. 



