Ruling on Refraining from Marrying Relatives Due to Fear of Diseases

Question:
Some people do not marry their cousin’s daughter for fear that the children will be born with deformities. Does this contradict the perfection of tawhid (the oneness of Allah)?

Answer:
Many scholars have explained these matters, stating that a person should choose a suitable wife for his offspring. Some jurists even said that it is better to marry a woman who is not a relative, rather than one’s cousin’s daughter. This is an error; there is no basis for it. Whether she is his cousin’s daughter or not, or his maternal cousin’s daughter or not, the Prophet (peace and blessings be upon him) married his cousins’ daughters.
However, one should look into it. If the woman is from a family where the children suffer from mental deficiency, insanity, physical defects, or deformities, he should not marry from them. This is in the context of seeking the proper causes, because inheritance can transmit such conditions, and diseases can be hereditary—by the permission of Allah—appearing in the son, the son’s son, and the daughter’s son. If this family is known to have hereditary diseases that cause disfigurement in the children, he should avoid them. If nothing of the sort is known in the family, he may marry from them. These are matters known to occur through inheritance. For this reason, scholars and doctors advise paying attention to these matters.