Estimation of relatedness by DNA fingerprinting.

The recent discovery of hypervariable VNTR (variable number of tandem repeat) loci has led to much excitement among population biologists regarding the feasibility of deriving individual estimates of relatedness in field populations by DNA fingerprinting. It is shown that unbiased estimates of relatedness cannot be obtained at the individual level without knowledge of the allelic distributions in both the individuals of interest and the base population unless the proportion of shared marker alleles between unrelated individuals is essentially zero. Since the latter is usually on the order of 0.1-0.5 and since there are enormous practical difficulties in obtaining the former, only an approximate estimator for the relatedness can be given. The bias of this estimator is individual specific and inversely related to the number of marker loci and frequencies of marker alleles. Substantial sampling variance in estimates of relatedness arises from variation in identity by descent within and between loci and, with finite numbers of alleles, from variation in identity in state between genes that are not identical by descent. In the extreme case of 25 assayed loci, each with an effectively infinite number of alleles, the standard error of a relatedness estimate is no less than 14%, 20%, 35%, and 53% of the expectation for full sibs and second-, third-, and fourth-order relationships, respectively. Attempts to ascertain relatedness by means of DNA fingerprinting should proceed with caution.

最近高变VNTR（可变数目串联重复）位点的发现，使得种群生物学家对通过DNA指纹技术在野外种群中得出个体亲缘关系估计值的可行性感到十分兴奋。研究表明，如果不了解相关个体以及基础种群中的等位基因分布情况，就无法在个体层面获得无偏的亲缘关系估计值，除非无亲缘关系个体之间共享标记等位基因的比例基本为零。由于后者通常在0.1 - 0.5的范围内，并且获取前者存在巨大的实际困难，所以只能给出一个亲缘关系的近似估计值。这个估计值的偏差因个体而异，并且与标记位点的数量以及标记等位基因的频率呈负相关。亲缘关系估计值中存在大量的抽样方差，这是由位点内和位点间的血缘同一性差异导致的，并且在等位基因数量有限的情况下，是由非血缘同一性的基因之间的状态同一性差异导致的。在检测25个位点的极端情况下，每个位点都有实际上无限数量的等位基因，亲缘关系估计值的标准误差对于全同胞以及二级、三级和四级亲缘关系分别不低于期望值的14%、20%、35%和53%。通过DNA指纹技术确定亲缘关系的尝试应该谨慎进行。