Again such a nicely orchistrated technique. Used, for instance, when you’d like to look at a mutation which is lethal to an embryo: you’ll have to induce the mutation later in life in order to actually observe the effects. What to do? Insert a gene that induces mitotic crossing-over and place it under a heat-shock promotor. So when the time is ripe, the animal/plant is exposed to a moderately high temperature for maybe 10 minutes after which chromosomal segments will be exchanged during mitosis. Now, if you have an animal that was heterozygous for the mutation (one chromosome has it, the other not) the cell can survive, because there is still one functional gene. During mitosis, the chromosomes are duplicated.. it’ll have 2 genes with the mutation and 2 genes without the mutation. During normal mitosis the daughter cells will ALWAYS end up with the initial configuration of 1 mutated and 1 healthy gene. But since the gene is expressed that messes with the segregation of the chromosomes, some daughter cells will have 2 healthy genes and others 2 genes carrying the mutation -> the genotype you wanted to look at. How do you know which cells carry both mutations? By placing a construct next to that gene which causes a pigmentation when this process has occurred correctly. Very interesting technique, genes used are FLP/FRT or Cre/Lox.