All are good responses but i try to give some information which is difficult believe in these days.We see some children behaves like their grandfathers or fore grandfathers because of their blood flows in that family simply we can say DNA in medicine. Some caste people belongs to a particular work, of course, may not now but in past. Those people have different mentalities and different inner powers in them. Simply we see brahmin students have bright mind as compare to others, Kshatriya people are more courageous than others, etc. Once we have to think how they get those powers without there is a great family seed. These days we see many strength less people, how it is possible? We have to think. This is my scientific DNA view.
Now coming to this world there is a sukshma prapancha (micro/nano world) which we can not see but vedic rishis can see like virus can not seen by man with naked eye but by using micro scope we can see. So we may not see the sukshma prapancha but there it is. Here there is system known as pitru devatha system which is corrupted by wrong deeds like our question. In a particular cast particular rights (dharmas) are there which are corrupted by this negligence. So entire positive energy in this atmosphere disturbed and negative energies increased which are caused to destruction of nature, what presently happening. You may asked about santana maharaja from Mahabharata, for this you have to study the Mahabharata clearly. Mathsya gandhi is not a own daughter of daasa raja. Coming to medieval India Kshatriya marriages were done but not inter cast. We have to know that Kshatriya is a ruler of a country. So there is no problem.
I proved here with the reference of DNA and also with the reference of our great rishis. I am neither a rishi nor a doctor but a man who believe that rishis are greater personalities. Just think ancient healthy satisfied people and today's unhealthy unsatisfied people. What is the difference and how it happens.