I don’t know a simple answer.
If you believe in the divine origin of the Torah, then you believe that all mankind descended from Adam, and then from Noah after the flood, so it is not surprising to find various features among the Jews (or among any other people, for that matter).
There have been periods in which significant numbers of rapes of Jewish women occurred, such as the pogroms of the Chmielnicki uprising in 1648 and the pogroms of the 1880s, 1905 and -06 and in 1919.
It also is true that there have been marriages between Jews and non-Jews since the beginning of the Jewish people. In many of those cases, the non-Jewish partner converted to Judaism.
Conversion has been a major factor among the Jews at various times at well. At one point during the Roman empire, it is estimated that fully 10% of the population of the empire was Jewish. Many of those people were converts and many more were the children of converts. Doubtless, they brought many of their own ethnic features into the Jewish gene pool.
“There have been periods in which significant numbers of rapes of Jewish women occurred, such as the pogroms of the Chmielnicki uprising in 1648 and the pogroms of the 1880s, 1905 and -06 and in 1919.”
My opinion this is how Askenazi Jews got most of their fairer features, Slavic features when there are some. Rape during pogroms. Love affairs too happen when you have two peoples living in proximity. Tasting the forbidden fruit between young Slavs and Jews. With most of this being Slavic male and Jewish woman. Slavs being dominant the penalty for having an affair with their young females was much greater. Impregnating one forget about it!