NATURE OF THE GENETIC EFFECTS 359 



mality of the affected cell will be so great as to result in its death or 

 failure to proliferate. The Ufe of an individual as a whole depends on a 

 more complicated and vulnerable organization than that of any part of it, 

 so long as that part has the aid of other, normal parts. It follows from 

 this that, in case a cell with a chromosome extra or missing happens to 

 be in the germ track, so as to give rise to an entire individual rather than 

 just a part of one, such an individual will be especially subject to abnor- 

 malities of development and functioning of varied kinds, so much so that 

 it will often be unable to live to maturity; i.e., a dominant lethal effect is 

 thereby produced. 



In case the extra or missing chromosome is a sex-determining chromo- 

 some, however, an individvial may be formed which does not show 

 abnormal effects of its gene imbalance, except for the usually minor 

 disturbances caused by an extra or missing Y or W chromosome. Never- 

 theless, the individual may be of the opposite sex from that which it 

 normally would have been, and so may constitute an exception to the 

 rules by which sex-linked characters are ordinarily inherited. If now 

 the aneuploidy had arisen in only one of the nuclei of an embryo of the 

 two- or four-cell stage, or some other very early stage, only a part (a half, 

 a quarter, or some other fraction) of the individual will have its genetic 

 sex altered. In that case, in organisms which, Hke insects, have auto- 

 nomic sex determination of their various parts, a gynandromorph will 

 result. In Drosophila irradiation of an egg before its fertilization can 

 so affect its cytoplasm as to cause lagging of chromosomes later, 

 during an early "cleavage" stage, and this is equally likely to affect 

 the chromosomes of either parent whether they had themselves been 

 irradiated or not, as shown by Patterson (1931b). Thus the male por- 

 tion of the resulting gynandromorph can, as the case may be, exhibit 

 the characters of either the maternal or the paternal sex-determining 

 chromosome. 



3-3. Effects on Crossing Over. Another way in which the transmission 

 of genes has been found to be altered by ionizing radiation is through its 

 effect on the frequency of crossing over. It was found by Mavor (1923) 

 that in Drosophila the frequency of crossing over was decreased by X-ray 

 irradiation in the region of the X chromosome that he was studying, and 

 by Mavor and Svenson (1924) that it was increased in the region that 

 was under their observation in the second chromosome. These influ- 

 ences persisted for more than a week, despite the occurrence of mitosis in 

 the affected germ cells between the time of treatment and the time of 

 crossing over. Some of this crossing over was probably produced in 

 oogonial cells, judging by Friesen's results on spermatogonia (see next 

 paragraph) ; this would explain a part at least of the apparent persistence 

 of the effect. Studies by the present writer (1925, 1926) showed that 

 the apparent contradiction in the effects on different chromosomes was 



