A plasmid bearing an active allele (gene X+) inserts into a recipient yeast strain bearing a defective gene X−) by homologous recombination. The result can be replacement of the defective gene X− by X+ (top) or its retention along with the new allele (bottom). The mutant site of gene X− is represented as a vertical black bar. Single crossovers at position 2 also are possible but are not shown.