一个学生的问题（about gene knockout）
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看到一本权威的E文教科书关于gene knockout的过程，插图如下
http://www.ncbi.nlm.nih.gov/books/bookres.fcgi/mcb/ch8f34.gif
Figure 8-34. General procedure for producing gene-targeted knockout mice. Embryonic stem (ES) cells heterozygous for a knockout mutation in a gene of interest (X) and homozygous for a marker gene (here, black coat color) are transplanted into the blastocoel cavity of 4.5-day embryos that are homozygous for an alternate marker (here, white coat color). The early embryos then are implanted into a pseudopregnant female. Some of the resulting progeny are chimeras, indicated by their black and white coats. Chimeric mice then are backcrossed to white mice; black progeny from this mating have ES-derived cells in their germ line. By isolating DNA from a small amount of tail tissue, it is possible to identify black mice heterozygous for the knockout allele. Intercrossing of these black mice produces individuals homozygous for the disrupted allele, that is, knockout mice. [Adapted from M. R. Capecchi, 1989, Trends Genet. 5:70.]
——不明白的是“Chimeric mice then are backcrossed to white mice; black progeny from this mating have ES-derived cells in their germ line.”这一句，不知道为什么这一步的结果会是black progeny，大概是遗传学没学好吧，不知道Chimeric mice的基因型究竟是怎样的
另外还有一个问题，是七年制教材《医学分子生物学》教科书上的，以前看书的时候想不通
第205页：
“5，嵌合体的杂交育种 对后代小鼠进行筛选，可以得到基因敲除的嵌合体小鼠。……然后经过杂交育种，按孟德尔遗传规律，其后代中有1／4的几率为纯合子，这样经过将嵌合体小鼠与正常小鼠进行交配，就可以得到基因敲除的纯系小鼠，即基因敲除小鼠”
问题来了:1/4的几率是如何算出来的？为什么“将嵌合体小鼠与正常小鼠进行交配，就可以得到基因敲除的纯系小鼠”？
考试以前问过分子生物学的老师，但是她也觉得这段话很奇怪。我想也许不是教材编写的问题，可能还是不太了解嵌合体的遗传学规律吧
希望有高人解惑：）
http://www.ncbi.nlm.nih.gov/books/bookres.fcgi/mcb/ch8f34.gif
Figure 8-34. General procedure for producing gene-targeted knockout mice. Embryonic stem (ES) cells heterozygous for a knockout mutation in a gene of interest (X) and homozygous for a marker gene (here, black coat color) are transplanted into the blastocoel cavity of 4.5-day embryos that are homozygous for an alternate marker (here, white coat color). The early embryos then are implanted into a pseudopregnant female. Some of the resulting progeny are chimeras, indicated by their black and white coats. Chimeric mice then are backcrossed to white mice; black progeny from this mating have ES-derived cells in their germ line. By isolating DNA from a small amount of tail tissue, it is possible to identify black mice heterozygous for the knockout allele. Intercrossing of these black mice produces individuals homozygous for the disrupted allele, that is, knockout mice. [Adapted from M. R. Capecchi, 1989, Trends Genet. 5:70.]
——不明白的是“Chimeric mice then are backcrossed to white mice; black progeny from this mating have ES-derived cells in their germ line.”这一句，不知道为什么这一步的结果会是black progeny，大概是遗传学没学好吧，不知道Chimeric mice的基因型究竟是怎样的
另外还有一个问题，是七年制教材《医学分子生物学》教科书上的，以前看书的时候想不通
第205页：
“5，嵌合体的杂交育种 对后代小鼠进行筛选，可以得到基因敲除的嵌合体小鼠。……然后经过杂交育种，按孟德尔遗传规律，其后代中有1／4的几率为纯合子，这样经过将嵌合体小鼠与正常小鼠进行交配，就可以得到基因敲除的纯系小鼠，即基因敲除小鼠”
问题来了:1/4的几率是如何算出来的？为什么“将嵌合体小鼠与正常小鼠进行交配，就可以得到基因敲除的纯系小鼠”？
考试以前问过分子生物学的老师，但是她也觉得这段话很奇怪。我想也许不是教材编写的问题，可能还是不太了解嵌合体的遗传学规律吧
希望有高人解惑：）
