In Vedic traditions the rishis saw that problems could occur with inbreeding or marriage between close relatives as it could lead to genetic problem for the offsprings. The entire purpose of the gotra system is to ensure humanity continues to evolve in the best possible way. This is why gotra is linked to some of the most divine rishis and there lineage. Manu is not the father of all humans. It all goes back to the seven main rishis and children of Manu and children of the prajapati etc….where humanity started. Vedic Gotra system had a scientific purpose and modern science is just now realizing this. The rishis were the true scientist of there time. Now you can see common problems occure when inbreeding happens as when 2 cousins get married. This leads to children having to suffer health issues. Some common problems are abnormal hair growth on face and other parts of the body, some have learning disability, some have extreme anger issues etc…