GENETICS OF SOMATIC CELLS 415 



each twin transmitted to its progeny genes corresponding to the phenotypes of one of the 

 two cell populations. Owen concluded that an interchange of cells must have occurred 

 between bovine embryos as a result of vascular anastomoses. Stem cells appeared to 

 be capable of becoming established in the hematopoietic tissues of the co-twin hosts 

 and there continuing to provide a source of blood cells distinct from those of the host. 

 This finding was of the greatest importance for the understanding and experimental 

 study of immunologic tolerance. 



The initial discovery of Owen has been followed by analogous findings of blood- 

 group mosaicism, probably chimerical in origin, in sheep, 1288 man, 310 and fowl. 92 

 Sex-chromatin studies on neutrophilic granulocytes of human twins of different sex 

 suggested that chimerism may involve the stem cells of granulocytes and erythrocytes as 

 well. 117, 943 In two pairs of human chimeras with both partners examined, 228 the 

 ratio of the two populations of cells was distinctly different in the co-twins, the grafted 

 population being in a minority in three of the four persons. It was assumed that this 

 inequality resulted from a gradual selective overgrowth of the host's cells at the expense 

 of the co-twin's cells. 



It is not always easy to distinguish between mutation and chimerism. According 

 to Cotterman, 228 the latter is indicated by the existence of twinning, the presence of 

 mosaicism for two or more antigens simultaneously, and a not very unequal mixture of 

 two populations of cells. None of these is an infallible criterion. Highly unequal 

 mixtures may occur in cases of chimerism as mentioned above. Two or more antigens 

 could be expected to change simultaneously by some events belonging to the broad 

 category of mutations, such as deletion, somatic reduction, and crossing over. How- 

 ever, as long as the number of antigenic markers studied is relatively small and most of 

 them are unlinked, these mechanisms generally bring about a single difference only. 

 Most mutational mosaics are of the minute variety, with only a very small admixture 

 of cells of the mutant type. Cotterman lists three exceptions for which more nearly 

 equal mixtures could arise through mutation: (a) genie instability or variegation, 

 (b) neoplastic hemic diseases, and (c) very early genie mutation or chromosomal 

 aberration. The first of these is but little known in mammals and the second is dis- 

 cussed in the chapter on neoplasia. The third mechanism has been brought into the 

 foreground by the recent striking developments in the field of human cytogenetics. 

 Several cases of mosaicism have been discovered with two different types of cells; 

 each, with its distinctively different chromosomal equipment, could be shown to coexist 

 in the bone marrow and in other tissues as well. Most of these analyses concern the 

 behavior of the sex chromosomes, and they have not been shown to involve erythrocytic 

 antigens but are nevertheless relevant for the problem of mosaicism. Ford et al. 888 

 found that the bone marrow of a patient with Klinefelter's syndrome contained two 

 types of cells, one with 47 and one with 46 chromosomes. There were good reasons to 

 believe that the extra chromosome corresponded to the Y chromosome and that this 

 was a case of an XXYjXX mosaic. The genesis of this situation appeared to involve 

 two different events. It was assumed that the individual arose as a result of non- 



