Figure 1. A genetic map. (A) The X chromosome. The right end of the X contains several let (lethal) genes and gas-1. The let genes were tested to determine whether they were allelic to the gas-1 gene (see materials and methods). All complemented gas-1, so gas-1 represented an unidentified gene from this region. The portion of the chromosome duplicated in mnDp1 (X,V) is shown by the solid line below the X chromosome; the portion deleted by mnDf1 is shown by a dashed line. (B) An expanded view of the right end of the X chromosome. The position of gas-1 was determined from three-factor crosses with unc-7 and lin-15. By scoring recombinants between unc-7 and lin-15 for the gas-1(fc21) mutation, we determined that gas-1 was positioned approximately 17% of the way from lin-15 toward unc-7. This positioning was used to direct the screen of cosmids for mutant rescue (see Figure 2). (mu = map unit.)