644 RADIATION BIOLOGY 



to such a degree that the sperm pronucleus cleaves and develops into a 

 normal, fertile, haploid male with paternal traits only. 



2-2b. Monosomic and Polysomic Types. If, in an otherwise diploid 

 organism, three chromosomes of one type are present, the individual is 

 called "trisomic." If one of the chromosomes is represented only once, 

 the individual is called "monosomic." Irregularities in the mitotic 

 mechanism account for such deviations from the normal diploid condi- 

 tion. Nondisjunction of chromatids in somatic mitosis, or of homologues 

 in meiosis, may result in the passage of both to one pole of the spindle. 

 Bridges (1916) found that eggs of normal, untreated females of D. 

 melanogaster may occasionally carry two X chromosomes or none, rather 

 than a single X. If such females are mated with males carrying sex- 

 linked dominant markers, the progeny arising from these exceptional 

 eggs can readily be detected. They will be either recessive females 

 (XXY, derived from XX eggs fertilized by a Y-transmitting sperma- 

 tozoon) or dominant males (XO, derived from no-X eggs fertilized by an 

 X-transmitting sperm), in contrast with the normal progeny, which are 

 dominant females and recessive males. Although the exceptional eggs 

 originate for the most part as a result of nondisjunction of the X, they 

 apparently can also arise from losses of that chromosome, since XX eggs 

 are less frequent (1 :2500) than no-X eggs (1 :600). 



The frequency of nondisjunction and chromosome elimination can be 

 increased by treatment of females with X rays, as was shown in the 

 studies of Mavor (1922, 1924a) and Anderson (1924, 1925a, b, 1931) on 

 D. melanogaster, and Demerec and Farrow (1930a, b) on D. virilis. Loss 

 of an X chromosome in a cleavage division of a normal female may give 

 rise to a gynandromorph composed of XX and XO tissues, and such 

 individuals have been obtained in X-ray experiments (Mavor, 1924b; 

 Patterson, 1930; Bonnier, Liming, and Perje, 1949). Losses of X and Y 

 chromosomes following irradiation of spermatozoa, which result in death 

 of the embryos, will be considered in the discussion of dominant lethals. 



Individuals that are monosomic for the fourth chromosome arise 

 spontaneously in Drosophila, and may also be induced experimentally. 

 Dobzhansky (1930) observed mosaics among the progeny of treated flies, 

 which were haplo-fourth in part of the body. The mosaic individuals 

 described by Mohr (1932) presumably belonged to this category, as did 

 some of the Minutes described by Muller (1928a, 1930) and others. 

 Flies that are trisomic for the fourth chromosome are viable (cf. Fig. 

 9-7e), although individuals tetrasomic for this chromosome are not. 



Nondisjunction or loss of the second or third chromosomes as a result of 

 irradiation of gametes would also lead to the formation of zygotes that 

 were either monosomic or trisomic for these longer autosomes; but indi- 

 viduals of these types have not been detected by genetic or cytologic 

 methods of analysis, and presumably are eliminated in embryonic stages. 



