Actually, it makes sense from a genetic defect point of view. However, modern science tells us we should barbarically kill the mother as well as just the father and child. Other siblings should be wiped out, just in case the defects are recessive or only occasionally manifest. Furthermore, any paternal ancestors also living should be killed, as should, in rare cases there are any, any maternal ancestors still capable of reproduction.
Of course, a theological quandry pops up at this point. Given God has caused women in their 70's to bear children on several occasions, should infertile, post-menepausal women (gramma, great-gramma, etc.) be killed too, just in case? Or may we assume that any such pregnancies are the gift of God, and are thus something to be desired, even if the child should turn out defective?