CYTOGENETIC ANALYSIS 509 



chromosomes. These marker chromosomes are very useful in identifying cells in 

 mixed populations. The Z)-chromosome of strain L-P59 604 and the E and F chromo- 

 somes of strain L-M 606 have been used to advantage in population analysis. A novel 

 attempt made by Harris and Ruddle 522 was especially interesting. A translocation 

 between two chromosomes resulted in a very long chromosome. This element is so 

 long that during telophase their ends are still near the equator. Thus when the 

 daughter nuclei restitute, the long chromosome forms a finger-like projection at one 

 side of the nulceus. In mixed populations, this type of cell can be identified with 

 interphase nuclei. 



The work by Hsu 601 suggests that different doses of a marker chromosome (the D 

 chromosome of strain L-P59) will cause the cells to prefer different cultural environ- 

 ments. Variation in chromosomal dose (and karyotype in general) undoubtedly 

 means variation in genetic makeup of the cells. Studies on the relationship between 

 enzymatic activities and certain chromosomes should yield some interesting information 

 in cellular physiology and somatic-cell genetics. 



Dr. Bender: As was brought out by Dr. Snell after the last paper, there are two 

 points of view about what we have come to call mammalian somatic-cell genetics. 

 One is to regard it as a useful new tool for the study of classical mammalian genetics. 

 The other is simply the converse: to consider classical mammalian genetics as a useful 

 source of material for the study of somatic-cell genetics. There has already been a good 

 deal of discussion of the latter point of view. Perhaps something should be said about 

 the former. 



There are already several ways in which somatic-cell techniques can help in the 

 study of mammalian genetics. An important way is the use of techniques in vitro for 

 the measurement of mammalian, and particularly human, mutation rates. We have 

 made a beginning in this work by measuring somatic chromosome mutation rates in 

 man, in the spider monkey, and in the Chinese hamster. 70 ' 71 - 72, 73 We have been 

 able to measure spontaneous and X-ray-induced aberration rates in diploid cells both 

 in vivo and in vitro. Also, we are now making parallel measurements of human genie 

 mutation rates at the ABO blood-group locus in diploid somatic cells in vitro, using a 

 fluorescent antibody assay technique. It is already clear that such studies will con- 

 tribute heavily to mammalian genetics in the future. It is also clear, however, that 

 many problems of specific interest to the discipline of somatic-cell genetics will emerge 

 from such studies. 



As an example of such a problem, I will cite the difference that we have found 

 between the rates of spontaneous chromosomal aberration found in vivo and in vitro. 

 The epithelioid cells that have been investigated so far have a spontaneous aberration 

 rate of about 1 per cent, while the rates for fibroblasts range up to almost 30 per cent. 

 The mammalian somatic cells that have been studied in vivo have extremely low 

 spontaneous aberration rates. To add to the puzzle, all of the somatic cells that have 

 been investigated, in vivo or in vitro, have about the same X-ray-induced chromosomal 

 aberration rates. 



