466 GENETICS OF SOMATIC CELLS 



light on somatic changes in the cell. I would like to pick up a lead that he dropped 

 and, reversing the coin, discuss what his studies of somatic-cell change tell us about 

 the H-2 locus. 



Probably most of you know that there are reports of crossing over in the H-2 locus. 

 Crossovers have been observed by Dr. Sally Allen in Chicago, Dr. Gorer in London, 

 and Dr. Hoecker in Chile. The surprising thing in the light of what we know of com- 

 pound loci in lower organisms is the incidence of crossing over, which seems to run at 

 least 1 per cent. If we do think of this as a single locus, it must occupy, if this evidence 

 is reliable, a chromosomal segment of appreciable length. The antigenic components 

 which have been identified at the H-2 locus are now quite numerous. Dr. Stimpfiing 

 can tell you better than I can what the number is; it is over 20, I believe. These all 

 are potential markers by which crossing over could be studied. 



So far all the recombinants apparently do fall in linear order, which is, I believe, 

 evidence that they really are due to crossing over. We do not have visual markers on 

 both sides of H-2 on the ninth chromosome. The T or brachyury gene and the Fu or 

 fused gene are both on that chromosome; but they are on the same side of H-2 and not 

 on the opposite sides, and that has limited the efficiency of our test for actual crossing 

 over in the locus. 



The one point I would like to draw from Dr. Klein's interesting paper is this. It 

 does seem to me that the instance of apparent somatic crossing over, which he showed 

 on the board, has involved a separation of H-2 components. It seems to me that it is 

 going to be difficult to interpret this in any other way than crossing over, and perhaps 

 this does strengthen the evidence that we really do have a locus here in which crossing 

 over of something of the order of 1 per cent can occur. 



Dr. Lederberg: I would like to amplify the points that have just arisen and to 

 answer a particular question that Dr. Klein has raised. The latest map I have seen 

 (you probably have a better one) indicated at least five factors in the H-2 region, and 

 according to Hoecker they could be in the sequence D, C, V, K, and H. Is there any- 

 thing that would go beyond that or contradict it as far as the present data on germinal 

 transmission go? In many instances the decision as to order may depend on the 

 existence of one aberrant mouse. As things stand now, the data on the somatic 

 segregation do not particularly reinforce the sequence. That is, the statement that D 

 and K fall into some relationship to the centromere cannot be checked by any data 

 currently available in the field of murine genetics, unless there is some translocation 

 involving the //-2-bearing chromosomes that might be used for an independent check. 

 However, if one could put one more factor into the system, we could look for the mutual 

 consistency of the type of map which is available from the isolated apparent crossover 

 events in the germinal and somatic tissue. 



About the point that Dr. Herzenberg raised, which I think is quite an important 

 one, that of looking for more metabolic anomalies in mice, I wonder if it is necessarily 

 true that the present inbred strains furnish the best material for it, since they have been 

 necessarily selected for viability of homozygotes. Perhaps we should again start in- 



