How do you get a knockout mouse ?
So you knocked out a gene in embryonic stem cells you have taken from one white mouse. And then you put those cells into another blactocyst from a black mouse. Thus, you get a mix blactocyst. You put the blastocyst into a pseudo-pregnant mouse. The female gives birth to chimeric and normal mice. Then you take one chimeric mouse and mate it with a white wild type black. As a result you get mice of plain colour and not chimeras. One of these mice will be heterozygous for gene knockout. But why? Why is the offspring not chimeric? And also why the foster mother gives birth to both chimeras and plain colored ones? And also, when I knock out a gene, do I knock it out in both chromosomes? 
mouse.
So you knocked out a gene in embryonic stem cells you have taken from one white mouse. And then you put those cells into another blactocyst from a black mouse. Thus, you get a mix blactocyst. You put the blastocyst into a pseudo-pregnant mouse. The female gives birth to chimeric and normal mice. Then you take one chimeric mouse and mate it with a white wild type black. As a result you get mice of plain colour and not chimeras. One of these mice will be heterozygous for gene knockout. But why? Why is the offspring not chimeric? And also why the foster mother gives birth to both chimeras and plain colored ones? And also, when I knock out a gene, do I knock it out in both chromosomes? mouse.
1 Answer
White fur = dominant
Black fur = recessive (deduced by offspring)
Explanation:
Just because the offspring is heterozygous, doesn't mean it has to be chimeric. This means that the dominant gene expressed--white fur (dominant) over the recessive black.
Also, the offspring can be either chimeric or white furred due to the fact that in a punnet square, you have 3:1 ratio of dominant:recessive phenotypes.
