You are a graduate student studying if the conserved Notch signaling pathway is essential in C. elegans. There are two Notch receptors called LIN-12 and GLP-1. "Null" non-functional alleles exist, but the genes are located 0.02 mu apart on chromosome 3. You want to create a double lin- 12 glp-1 null mutant to determine if the Notch signaling pathway is essential. Therefore, you cross a lin-12 (glp-1 +) mutant worm strain with a (lin-12 +) glp-1 mutant worm strain. The resulting F1s have the following genotype: lin-12 (glp-1 +)/(lin-12 +) glp-1 and are normal. The lin12 and glp-1 null mutants are in in the F1s. To get your double mutant lin-12 glp-1 worm strain, you backcross your F1s with the lin-12 (glp 1 + ) parental strain. To get at least one recombinant worm with the genotype lin-12 glp-1/lin-12 (glp-1+) you must screen offspring. These two genes are so close together that for practical purposes they are linked..