358 RADIATION BIOLOGY 



sets of chromosomes. The later cell divisions were normal except that 

 all five sets of chromosomes were carried along instead of only two, and 

 the cells of the resultant embryo were thus all pentaploid. It is not the 

 rule in material in general, however, either in meiotic or in mitotic divi- 

 sions, for radiation to cause the union of daughter nuclei which would 

 normally separate. Thus the production of polyploid cells or offspring, 

 i.e., those having extra sets of chromosomes, is not a genetic effect of 

 radiation that has been found to be produced generally. 



3-2. Disarrangement of Cell Division and Induction of Aneuploidy 

 Involving Whole Chromosomes. An effect that has been much more 

 commonly observed, following ionizing radiation at any rate, is the 

 abnormal distribution of chromatids to the daughter nuclei, both at 

 mitotic and meiotic division. When the tAVo chromatids of a chromosome 

 that has doubled for mitosis are carried to the same pole, the process is 

 called "nondisjunction." The same term appKes when both pairs of 

 chromatids of a tetrad are carried to the same pole at the first meiotic 

 division, or both chromatids of a pair at the second meiotic division. In 

 any of these cases an equal number of cells is formed with extra and with 

 missing chromosomes, respectively. A related process is that in which a 

 given chromatid or pair of chromatids lags on the spindle in cell division 

 and fails to be carried to either pole. This event is in most material 

 followed by the eventual degeneration of the excluded chromatin. In 

 such cases, cells with missing chromosomes, but not cells with extra ones, 



are formed. 



Both nondisjunction and lagging occasionally occur without irradiation. 

 That they are far more frequent in irradiated material was first shown by 

 Mohr (1919) in a locust, Decticus, by cytological methods, and later by 

 Mavor (1921) in Drosophila by appHcation of the genetic methods by 

 which Bridges (1913, 1916) had previously demonstrated the spontaneous 

 occurrence of nondisjunction in that material. The induced displace- 

 ments of chromosomes at meiotic divisions were found to continue for 

 nearly a week after the irradiation had been applied. 



Both the addition and the subtraction of a chromosome, but to a 

 greater degree the latter, involve drastic departures from the normal gene 

 ratios and therefore imbalances in the concentrations, relative to one 

 another, of the different gene products. The imbalanced genetic com- 

 position, which is designated as aneuploid (a term applied generally when 

 any part of the chromatin, whether more or less than one chromosome in 

 extent, is present in the wrong amount relative to the rest of it), is there- 

 fore damaging to cell functioning. By proliferation of a cell damaged by 

 aneuploidy, a sector of tissue that is permanently abnormal will be 

 produced. If the affected cell is an embryonic one, it can give rise to an 

 abnormal portion of the body, the size of which will depend upon how 

 large a body region the affected cell is ancestral to. Sometimes the abnor- 



