Problem
Assume you're trying to make a mutant mouse by homologous recombination: Why is it important to use ES cells from a mouse strain which have a different coat color than the strain from which you harvest the early-stage embryo? What must be true about the ES-strain coat collor allele, in Mendelian terms, for this strategy to work? Could you successfully make mutant mice using this approach without using ES cells from a strain with a different coat color? Why/why not, and if yes, is there anything you'd have to do differently to make sure you really have mutant mice?