PROBLEMS OF MEASUREMENT OF MUTATION RATES 41 



Atwood: No; 6 X 10^^ is a little less than the usual for AiB. 



Freese: What is the number of stem cells in the bone marrow? 



Atwood: About 10^^ I think, or maybe a little more than that. 



Goldstein: What is the average clone size from one stem cell? 



Atwood: If you assume the number of stem cells to be about 10^^, 

 then the clone size would be about 100, because of the order of 10^^ 

 erythrocytes in an individual. 



Dobzhansky: Could you make clear for me what you mean by 

 "mosaic"? Do you mean that this individual has two different geno- 

 types in different tissues, or do you simply mean that phenotypically 

 the blood cells have a different count? 



Atwood: Phenotypically, the cell population appears mixed. What 

 the origin of such mosaics may be, I don't know, but they are as- 

 sociated with the subgroup called A3. There is a disagreement about 

 whether A3 is really a mosaic of A2 and cells as Dr. Cotterman has 

 suggested. 



Neel: In the mosaics, what are the proportions of the two types of 

 cells? 



Atwood: About 70 per cent are A2B, apparently, and about 30 per 

 cent are apparently just B, although it has been claimed that these 

 inagglutinable cells adsorb anti-A. 



I infer from the isotope dilution experiments that the A3 allele in 

 the A1A3 individuals is also giving a mosaic phenotype when this is re- 

 vealed through loss of Ai. 



Cotterman: Would you put down the proportions of the other 

 phenotypes for the A1A3? 



Atwood: Where is that graph? This individual (AiB) has 6 X 10 -*. 

 Now, the AiAs's, those are the ones you want? 



Lederberg: This is the frequency of non-A cells, is that right? 



Atwood: Yes, in this A1A3 individual. It's 3 X 10"^. This one (A1A3) 

 is 1.6 X 10-^ 



Neel: Non-A, not non-Ai? 



Atwood: Yes, not just non-Ai. If you want to be strictly operational 

 about it, it is the frequency of cells that are not coprecipitated with Ai 

 carrier in the presence of unabsorbed human isoantibody that is 

 anti-A. These experiments, by the way, unlike the others, were done 

 with human anti-A rather than lectin. 



Auerbach: It's 30 per cent of 6. 



Atwood: Yes. 



Auerbach: If this is a very weak antigenic form, then it looks more 

 like a question of nonpenetrance, 30 per cent nonpenetrance. 



