192 MUTATIONS 



of single component, nonheteroallelism, but not the other, which would 

 make one suspect that it were mitotic crossing over. 



Glass: May I take just two minutes to describe still another diffi- 

 culty in this problem that has not been mentioned and that I think is 

 rather instructive? 



In the attempted use of markers in tissue cultures, we tried to use 

 heterozygous albino rats and homozygous albino rats to provide two 

 tissues derived from the pigmented layer of the iris of the eye usable 

 for testing mutation in either direction, forward or reverse, at the 

 albino locus. 



The culture of pigmented cells proved unsuitable after a time be- 

 cause these cells, which were derived from the heterozygous animal, 

 spontaneously lost their capacity to produce pigment after being kept 

 in culture for a few weeks, so the percentage of nonpigmented cells 

 would exceed 15 per cent, and you could not detect any mutations 

 occurring at low frequencies in a culture like that. The albino strain — 



Magni: Excuse me, but were these cultivated in the presence of 

 mouse serum? 



Glass: No, I think they were cultivated in human serum. I would 

 have to check that to be quite sure. Mouse serum is a little difficult 

 to get in quantity. 



Stern: Just one question. Were homozygous pigmented tissues also 

 studied? Did they lose their pigmentation capacity, in tissue cultures? 



Glass: We started out with such cultures, but because it seemed 

 impractical to look for mutations there, for the very reason that Dr. 

 Lederberg suggested, we didn't press this; we didn't try to keep them 

 going very long, and so I'm not sure. 



Magni: I just want to mention here one experiment which has been 

 done by Dr. De Carli in our department. He has been able to keep the 

 antigenic property — that was group A, if I remember correctly — of a 

 human sample for eight months, cultivating the culture in the presence 

 of the homologous serum. If he shifted to another type of serum, the 

 antigenic property was lost in two or three generations. This is just 

 one example, but it could be taken into consideration. 



Glass: My comment on the other strain of cells is the more sig- 

 nificant one, I think. The albino cells were very constant in culture 

 and never showed any signs of pigment at all, although they were sub- 

 jected to a number of different treatments such as ultraviolet radiation. 

 X-ray, and so on. No cells that had regained the ability to produce 

 pigment were ever found. However, after studying these for a con- 

 siderable period of time, Kodani and I became convinced that these 



