296 



SCIENCE 



[Vol. LV, No. 1420 



two female exceptions were the daughters of 

 different X-rayed females. The total number 

 of daughters produced by the X-rayed females 

 was 512 and the two exceptions formed .39 per 

 cent, of them. The total number of sons 

 produced by the same X-rayed females was 

 467 and the twelve exceptions form 2.5 per 

 cent, of them. It should further be noted that 

 four of the thirteen fertile X-rayed females 

 produced less than ten offspring and that each, 

 i. e., 100 per cent of the nine X-rayed females 

 which produced more than ten offspring pro- 

 duced one or more exceptional daughters or 

 sons. 



The exceptional son of the control female 

 was mated and found to be sterile. Eight of 

 the twelve exceptional sons of the X-rayed fe- 

 males were mated and proved sterile. The 

 other four exceptional sons of the X-rayed fe- 

 males were either dead when found or died 

 soon afterwards. Bridges^ and Safir^ both 

 found that exceptional males arising from pri- 

 mary non-disjunction were sterile. One of 

 the exceptional daughters was fertile, the other 

 sterile. 



It is, of course, not certain that both of the 

 exceptional daughters obtained from the X- 

 rayed females were produced as a result of the 

 X-rays. Safir'', working on primary non-dis- 

 junction in white-eyed stock, found two excep- 

 tional females and thirteen exceptional males 

 in a total of 21,773 offspring. A consideration 

 of this and the fact that no exceptional daugh- 

 ters were produced by the control females and 

 that the exceptional daughters came from dif- 

 ferent X-rayed parents makes it seem extreme- 

 ly unlikely that both the exceptional daughters 

 were the result of natural non-disjunction. It 

 is interesting to note that the unequal ratio of 

 exceptional males to exceptional females, six 

 to one, found in the offspring of the X-rayed 

 females, is approximately the same as that 

 found for natural non-disjunction in white- 

 eyed females by Saflr. 



Bridges in his work on non-disjunction^ has 

 clearly demonstrated that the exceptional males 

 arising from primary non-disjunction come 



e Genetics, Vol. 5, No. 5, 1920. 



from eggs which are without an X chromosome 

 and are fertilized by an X chromosome bearing 

 sj^erm. Such males have the chromosome 

 formula XO. The exceptional females caused 

 by primary non-disjunction, he shows to be due 

 to the fertilization of an egg having two X- 

 chromosomes by a Y chromosome bearing 

 sperm. Such females have the chromosome 

 formula XXY. This evidence seems to justify 

 the assumption that the exceptional flies pro- 

 duced as a result of the action of X-rays have 

 the chromosome formulas found by Bridges to 

 occur as the result of natural primary non- 

 disjunction; and the conclusion that an effect 

 of X-rays when applied to eggs during matura- 

 tion is to cause non-disjunction. This makes 

 it probable that the X-rays have a direct effect 

 on the germ cell, causing an increase in the 

 tendency, already seen in the occurrence of 

 uatvu'al non-disjunction, for the two X chromo- 

 somes to fail to separate in the maturation of 

 the egg. The question as to whether this effect 

 is due to a modification of the physical prop- 

 erties of the X chromosomes or of the proto- 

 plasm surrounding them will not he diseui-sed 

 here. 



When primary exceptional females are 

 crossed to males like their fathers they produce 

 further exceptions. As shown by Bridges^ 

 secondary exceptional females occur when the 

 two X chromosomes are included in the mature 

 egg and exceptional males when the two X 

 chromosomes pass into a polar body and leave 

 only the Y chromosome in the mature egg. 

 When the egg containing two X chromosomes 

 is fertilized by a sperm containing a Y chromo- 

 some the resulting female lives and is lite its 

 exceptional mother. When the egg containing 

 only the Y chromosome is fertilized by a sperm 

 containing an X chromosome the resulting male 

 lives and is like its father. The two exceptional 

 daughters of the X-rayed females were mated 

 to eosin-eyed piiniature-winged males. The 

 daughter which proved to be sterile was found 

 on dissection to have only one undeveloped 

 ovary. The other which was fertile produced 

 174 regular daughters, 198 regular sons, 1 ex- 

 ceptional daughter, and 5 exceptional sons. The 



