Biology Forums - Study Force

As the first step in generating a ? -catenin knock-out mouse, you have used ES cells from a black mouse strain and selected for cells in which an appropriate transfected construct has undergone homologous recombination at the ?-catenin locus.  You then inject the transformed ES cells into the blastocysts of embryos from white mice and implant them into a pseudopregnant foster mother.   

(A) Are the ES cells you selected heterozygous or homozygous for the ? -catenin knockout allele?     
(B) Why do ES cells that are exposed to gancyclovir in your selection die if they have NOT undergone homologous recombination (HR) with the knockout construct, but live if they have undergone HR?  You can draw a picture if that helps to explain.
(C) Why, after you have generated black offspring from your chimeric mouse, do you have to test them with PCR to be sure they inherited the knockout construct?

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Seems right to me!  ;D ;D