A bit of simple genetics lesson may be needed here:
male is XY, the X comes from mother, Y from father. Female is XX, receiving one X from each parent. All other chromosomes undergo recombination and are a mixture/combination of the genes of each parent. Since the Y chromosome only comes from the father, it can be used to trace patrilineal lineage. The X cannot be used since it comes from both parents. Instead, mitochondrial DNA is used, since it is inherited from the mother.
What yichisdik is saying is that if one follows mitochondrial DNA, one sees non-semitic traits at a higher percentage than when one follows the Y chromosome. That can be explained by non-Jewish women joining the Jewish gene pool at a higher rate than non-Jewish men. Assuming that they converted k’din, there is no problem with matrilineal descent, mdd1. Academic papers do not care if they did, but I think we can be dan l’kaf zechus that the influx was due to conversion, if there really was such as significant influx.
The second point- if a non-Jew ravaged a Jewish woman, he would be introducing his non-semitic genes – for example eye and skin color- equally to daughters (if he also gives over his X chromosome) or his sons (if he gives over his Y chromosome). Following the Y chromosome would indicate the presence of “foreign” genes, but following the mitochondrial DNA would not show this, even among the daughters who would be inheriting non-semitic traits. For the argument presented above to hold about the purity of the lineages, one would have to conclude that this type of genetic “pollution” was much rarer than female conversion. Perhaps this was true, because the assaulted women were probably also murdered by their assailants, R”L. But it could still be a source of non-semitic traits within the Jewish gene pool, which I think is the point yichusdik was making.
Joseph, I agree with you that these sort of studies should be taken with a grain of salt, but not because of their academic quality (not being a geneticist, I can’t judge), but because they are often over-interpreted by eager laymen and Halacha is not determined by gene analysis. Anyway, most papers these days come out in digital form- theoretically there is a print version, but most people are not reading it. So you have to change your metaphor of “not worth the paper they’re printed on”. 🙂