370 RADIATION BIOLOGY 



early embryonic stage. Thus the individual with a translocation, though 

 appearing normal, is usually capable of producing only about half as 

 many viable (surviving) offspring as a really normal individual can. In 

 consequence, in most species, if a number of individuals with transloca- 

 tions have arisen in a given generation, this number will be reduced to 

 approximately a half, an eighth, a sixteenth, etc., in successive genera- 

 tions, until at last the translocations (with the possible exception of 

 whole-arm translocations) have completely died out. 



In primates, however, where there is but one foetus per pregnancy and 

 its early death in utero is rather promptly followed by another pregnancy, 

 this process tends to compensate for the deaths and thus to allow the 

 so-called " semisterile " individual with the translocation (or, if this 

 individual be a male, his mate) to bear nearly as many offspring as an 

 entirely normal individual does. This will greatly delay the dying out 

 of the translocation. In man, especially civilized man, the additional 

 compensatory factor enters in that a couple subject to involuntary abor- 

 tions or miscarriages consciously tries to bring their total number of off- 

 spring up to or even beyond the average number. In this way the trans- 

 location, along with the "semisterility" occasioned by it, must become 

 actively perpetuated. 



When two breaks occur in homologous chromosomes of a diploid cell, 

 they will usually be at nonidentical points. If all broken ends succeed in 

 uniting, but in such a way that the newly constituted chromosomes are 

 made of one portion from one homologue and the other portion from the 

 other, we again have the possibility of the configuration being either 

 aneucentric (with acentric and dicentric chromosomes) and so becoming 

 lost, or eucentric (with monocentric chromosomes) and so allowing the 

 daughter chromosomes ("chromatids") to be transmitted regularly at 

 succeeding mitoses. In the latter case one of the newly constituted 

 chromosomes will be deficient for some genetic material, while this mate- 

 rial will be twice represented (duplicated) in the other new chromosome. 

 The cells derived by mitosis from this cell have the normal number and 

 kinds of genes, however, since the lack in one chromosome is exactly 

 complemented by the duplication in the other. But if this unequal 

 exchange between homologues has occurred in a germ cell, then, after 

 its cycle of mitoses has been completed and it undergoes the meiotic 

 divisions, a gamete is finally produced which has just the deficiency or 

 just the excess of genes. The zygote resulting from such a gamete is 

 therefore aneuploid: it has an "imbalanced" gene content, since its genes 

 are in abnormal ratios to one another, and it is correspondingly abnormal. 

 Only very small deficiencies are compatible with the life of an individual 

 as a whole, even when the homologous chromosome (that which was 

 received from the other parent) is a normal one. Considerably larger 

 duplications than deficiencies can usually be tolerated but, depending on 



