I'm guessing that you heard about Tay-Sachs, which is most prevalent among Jewish people of Eastern European descent. This is an ethnic thing, not a religious thing. It's a disease of Eastern Europeans. The reason that Jewish people of Eastern European descent get it more than other Eastern Europeans is that they tend to marry each other, which perpetuates the gene causing the disease. It does not apply to Jews of other ethnic descent, or to people who convert to Judaism from another religion (what you meant when you said "take on thier beliefs").

Tay-Sachs isn't the only kind of heritable disease among ethnic groups. For example, Cooley's Anemia afflicts people of Mediterranian descent--especially Southern Italians for the same reason that Tay-Sachs afflicts Jews of Eastern European descent: they marry each other, and thus perpetuate the gene that causes it. It has nothing to do with religion or religious choice.