Fig. 2. Crossing scheme for isolating lethal X-chromosomal insertion lines. The sex chromosomes (left pair in each genotype) and the two large autosomes are shown schematically with females on the left. Relevant mutations are labelled accordingly. The P{lacW} insertion is indicated by the triangle and its white⁺ selectable marker is represented by the darker chromosome. The transposase source is indicated by the double-headed arrow. Balancer chromosomes are hatched. Absence of the crossed-out males in the F₃ generation indicates a potential lethal P{lacW} insertion. For further details, see Methods.