Dr Lawrence Schiffman gave a lecture about DNA. He said thst studies show that while approximately 90@ of Jewish males of European ancestry share the DNA source of YehudaTzvi but only 65% of females share it.
Among the reasons that led to this stem from the origin of the early Ashkenazi European communities. Many of the early settlers were traders following the old Roman routes, and they often traveled alone and took local wives. Even those who traveled with families had, early on, to take local wives for their children until the communities, which were never large, at least became big enough to begin marrying Jewish girls from other families.
Besides converts, thee was unfortunately another source of outside DNA, children that resulted from rapes that occured over the last two thousand years