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It was shown by Lancefield (1929) that D. pseudoobscura A-B hybrid females, when back-crossed either to A or to B males, give sex-ratios among their offspring that may deviate widely (in either direction) from 1:1. My own experience confirms this result; I have, however, never found a significant excess of male offspring unless the father was descended from certain of the older mutant stocks. Back-crossing to males from wild stocks or to males whose ancestry could be traced wholly to recently collected wild stocks has never resulted in significantly more than 50 per cent male offspring.  It appears that a given combination of stocks usually gives approximately the same proportion of males in separate cultures, whether these are reared simultaneously or not. Some stocks appear to be heterogeneous, but the observations establish that the major causes of the variable sex ratio are genetic.  The analysis of the genetic basis involved is incomplete, but certain of its elements can now be stated. (1) The source of the A parent of the F1 female tested is very influential, that of the B parent less so-possibly of no significance. (2) The source of the male to which the F1 female is mated strongly influences the result; part but not all of this influence is due to the Y-chromosome carried by such a male.  Since the last-mentioned point involves a new type of effect of the Y-chromosome, it seems desirable to record it, even though the analysis is still unfinished.

An Effect of the Y-Chromosome on the Sex-Ratio of Interracial Hybrids of Drosophila Pseudoöbscura