THE EFFECTS OF UNEQUAL CROSSING OVER AT THE BAR LOCUS IN DROSOPHILA

Abstract

1. Sixteen different kinds of changes at the bar locus are shown to occur exclusively, or nearly so, in eggs that undergo crossing over at or near the bar locus. 2. This result can be explained if it is supposed that such crossovers are unequal, SO that one daughter chromosome gets two representatives of the bar locus while the other receives none. 3. Only one mutation in this locus has been shown to have occurred in the germ track of a male. This one gave rise (from bar) to a new and less extreme allelomorph called infrabar. 4. Infrabar does not appear to represent a quantitative change in the bar gene. 5. When, by unequal crossing over, bar and infrabar come to lie in the same chromosome, they maintain their separate identities, and may be recovered again. 6. In such double forms the two elem,ents also maintain their sequence in the same linear series as the rest of the genes. It is thus possible to obtain bar-infrabar and also infrabar-bar. These two types look alike, but can be distinguished by their origin and by the usual testsf or determining the sequence of genes. 7. Facet counts are given for all the possible combinations of the following members of the bar series: round, infrabar, bar, double-infrabar, bar-infrabar, double-bar. 8. Analysis of these data shows that two genes lying in the same chromosome are more effective on development than are the same two genes when they lie in different chromosomes. 9. A general survey makes it seem improbable that many mutations in other loci are to be explained as due to unequal crossing over.