When making a transgenic mouse, why can't you just mate 2 chimeric mice?

I read that when making a knockout mouse, you have to mate 1 chimeric mouse with a wildtype to generate a heterozygous mutant before mating that heterozygous mutant with another to generate the homozygous mutant. So my question is why can't you just breed 2 chimeric mice from the start, why is it necessary to go through such a long process? Thank you!When making a transgenic mouse, why can't you just mate 2 chimeric mice?
i think the basis of the long convoluted process is what is known as the ';test cross';. I think you want to make sure your chimeric mouse is a true breeding chimeric mouse. (Keep in mind, that I've actually never made knockout mouse...espeicially not with the process you're describing. We'd always use Cre recombinase and hope for a double cross over during meiosis)





a simple example of the test cross is using the pea plant:





1. purple flowers is dominant over white, so you can't tell whether you'd have a homozygous purple or a heterozygous purple.





2. you'd cross your mystery purple plant with a white plant. Since white is recessive, you'd know that this plant HAS to be homozygous white.





3. after the cross, you'd look at the offspring. if approx half of the offspring is purple, and the other half is white, you'd know you'd have a heterozygous purple plant. if you end up with all purple offspring, you know you'd have a homozygous purple plant. set up a basic punnett square and see for yourself if you don't really catch my drift. You'll see a big difference in the phenotypes of the offspring.