The Drosophila FM7-A Strain Contains a Fixed Translocation Involving the X and 4 Chromosomes

We discovered that a commonly used balancer chromosome - <i>FM7-A, Tb</i> ¹ - exists as an uncharacterized translocation between the progenitor <i>FM7a</i> chromosome and a normal chromosome <i>4</i> . Genetic and cytological data evidence show that <i>T</i> ( <i>1</i> ; <i>4</i> ) <i>FM7-A</i> translocates about 5% of chromosome <i>1</i> material to an entire chromosome <i>4</i> , thus the large "chromosome <i>1</i> " segregant is lethal without the small "chromosome <i>4</i> " segregant, though the latter is viable and fertile without the former. We characterize this balancer chromosome to inform <i>Drosophila</i> molecular-geneticists of the structure as it may affect their use of this balancer chromosome and the interpretations of their results.