>> @akuperma are you trolling?
He is, unfortunately, right in certain circumstances. In some corners of Jewish society, more than 50% of people are intermarried. Thus, having a non-Jewish last name increases a chance that the mother is Jewish (unless you are talking about several generations of intermarriage).
I presume this joke was posted here 4 years ago: what is the difference between Donald Trump and a Reform Jew? His grandchildren are Jewish.