I was reading this little 'article' about homologous recombination and knockout mouse. According to this article you first remove embryonic stem cells from a gray-fur blastocyst, then insert the construct and place them back into the blastocyst. Further the text states:
A knockout mouse has had both alleles of a particular gene replaced with an inactive allele. This is usually accomplished by using homologous recombination to replace one allele followed by two or more generations of selective breeding until an breeding pair are isolated that have both alleles of the targeted gene inactivated or knocked out.
However what I don't get here is that we have cells from an embryo, and thus have cells with both alleles. Hence, the regions surrounding these alleles will be homologous, so why would this step result in heterozygous (and thus require selective breeding afterwards) instead of homozygous mouse direcly?