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HROMOSOME nondisjunction, the failure of the members of a chromosome 'pair to separate to the two daughter nuclej, was first described by BRIDGES (1913,1916). Theoretically, nondisjunction of the X chromosomes in XX females should result in the production of equal numbers of nullo-X and diplo-X gametes. Consequently, the two resulting classes of viable exceptional off spring, patroclinous sons and matroclinous daughters (presumably XO and XXY, respectively), should be recovered in statistically equal numbers. It was soon realized, however, that other events partially mimic the consequences of nondisjunction by giving rise to patroclinous sons but differ in yielding no matroclinous daughters. In the extensive data of SAPIR (1920) patroclinous sons were found to be about four times as frequent as matroclinous daughters. A much greater disparity observed in a particular strain of Drosophila mehogaster (SPIELER 1961) led to the investigations described in the present paper.An excess of patroclinous males would seem to be due to the recovery of more nullo-X eggs than diplo-X eggs. The unequal recovery could be caused by a variety of events which will be referred to generically as "chromosome loss.'' Chromosome loss will be said to occur whenever the ratio of patroclinous males to matroclinous females is significantly greater than one. This operational definition must be applied with caution, however, to those cases in which one might expect important viability differences to result from the relative mutational load of the maternal and paternal sex chromosomes.Although no attempt will be made to present an exhaustive list of the occurrences which may contribute to chromosome loss, a few will be mentioned briefly. One obvious possibility (which could take place at either meiotic division) would be separation of the X chromosomes followed by noninclusion of an X in one daughter nucleus. Or, after nondisjunction has occurred, there may be a directed orientation of the four meiotic products such that a nullo-X nucleus, rather than a diplo-X nucleus, more often becomes the egg pronucleus. Also, heterozygous X-chromosomal inversions produce nullo-X eggs by four-strand double exchange within the inversions ( STURTEVANT and BEADLE 1936) or by three-strand double exchange, one exchange within the inversion and one outside Statistical Methods. Iowa State College Press. Ames, Iowa. Nondisjunction involving the maroon-like and rosy mutants of Drosophila Accuracy of mutation rates. J. Genet. 43: 301-307.melanogaster.