Improvement of litter size in a strain of mice at a selection limit.
Improvement of litter size in a strain of mice at a selection limit.
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在选择极限下提高小鼠品系的产仔数。
DOI：
10.1017/s0016672300012246
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发表时间：
1971
期刊：
影响因子：
--
通讯作者：
D. Falconer
中科院分区：
文献类型：
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作者：
D. Falconer
一种对高产仔数的选择不再有反应的小鼠，通过继续选择进行近亲繁殖。均值的下降证明了残留遗传方差的存在。有4个系在近亲交配中幸存下来，其中一个达到了20代，平均与原始菌株相等，从而反驳了显性是残余变异的主要原因。将所选的4个自交系进行杂交，得到一个与原自交系平行的新品系。新菌株比原菌株每窝提高1.5只小鼠。因此，近亲繁殖的选择能够达到超越原始选择所达到的极限的进步。残余变异是由具有简单优势的基因引起的这一假设经过检验，看它是否可以用相关参数的合理值来解释观察结果。近交和杂交改良需要消除约30个隐性基因，效应（纯合子差异）为0.5表型标准差，基因频率为0.2。将选择的自交系的平均水平与不选择近交的下降率结合起来考虑，表明有近交的选择消除了大约75%的分离性隐性基因。因此，导致剩余方差的基因数量约为40个。这些基因产生的加性方差刚好与实现遗传力的零估计值一致。考虑到最初的选择表明，当反应停止时，大约一半的基因可能仍在分离。因此，该假设要求基础种群中的基因数量约为80个。所需的基因数量虽然很大，但似乎并非不可能，基因具有简单优势的假设可以解释所有的观察结果。
A strain of mice that had ceased to respond to selection for high litter size was inbred with continued selection. Depression of the mean proved the existence of residual genetic variance. Four lines survived the inbreeding, and one reached 20 generations with a mean equal to the original strain, thus disproving overdominance as a major cause of the residual variation. The four selected inbred lines were crossed and a new strain derived from the cross was maintained in parallel with the original strain. The new strain showed an improvement of 1·5 mice per litter over the original strain. Thus selection with inbreeding was able to achieve an advance beyond the limit attained by the original selection.The hypothesis that the residual variation was due to genes with simple dominance was tested by seeing if it could account for the observations with reasonable values of the relevant parameters. The improvement made by the inbreeding and crossing required the elimination of about 30 recessive genes with effects (homozygote difference) of 0·5 phenotypic standard deviations and gene frequencies of 0·2. Consideration of the mean levels of the selected inbred lines in conjunction with the rate of depression found on inbreeding without selection showed that the selection with inbreeding had eliminated about 75% of the segregating reces-sives. The number of genes contributing to the residual variance was therefore about 40. The additive variance generated by these genes was just consistent with the estimate of zero from the realized heritability. Consideration of the original selection showed that about half the genes could have been still segregating when the response ceased. The hypothesis therefore requires the number of genes in the base population to have been about 80. The number of genes required, though large, does not seem impossible, and the hypothesis of genes with simple dominance can account for all the observations.