Two children that have the same father but different mothers are forbidden to have relations on grounds of incest. If their father is a gentile (and they have different Jewish mothers), it is not really their father, therefore are they still forbidden to have relations?
This question was raised by the Assia journal conjunction with artificial fertilization.
It is a problem to take sperm from another Jew because the child could marry his brother. It may be permitted to take sperm of a non-Jew in this case because they are not considered to be brothers.
, שהטעם "עיקרי לאיסור הזרעה מיהודי זר לאשת-איש - "שמא ישא אחותו מאביו" - לא קיים במקרה שהזרע הוא של גוי. זאת מכיון שאין הולד מתייחס אחר אב-גוי גם כשהוא נולד כדרך כל הארץ, לא כל שכן בצורת הזרעה מלאכותית.
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