Falling prey to temptation of the Judaistic attitude - one of the Abrahamic belief-systems - there are some who consider Sanatana Bharat also to be comprising of one PEOPLE based on DNA. Followers of Judaism pride themselves as One People (racially), using One Language (Hebrew), believing in One God, following One Book (sort of), having One History, etc.
Firstly, we mention that we neither deny nor are opposed to the possibility that most Indians (most people in the subcontinent) share similarity in DNA.
However, the attitude mentioned above is the very anti-thesis of Sanatana Dharma. The Truths purported to be Understood and Lived in Sanatana Dharma are language-transcending, history transcending, scripture transcending, etc.
Sanatana Dharma (and any civilization worth its name) sustains itself over generations, not by genetics rather by memetics (educations). Sanatana Dharma is carried over by Dwijas (known as twice-borns). If we ponder as to what "twice born" means, it becomes clear at once that the second (and more important) birth comes as a result of education.
Thus, even a white European or a black African can become a dwija if a competent preceptor initiates him into the knowledge and practice of dharma; like wise even the self proclaimed "brahamanas" (for example having titles like "Sharma", "Mishra" in north india, and "Ramanujan", "Thiruvachagam" in south india) cease to be dwijas if they are ignorant and are bereft of the practice of the Dharma.
In a somewhat oversimplified manner we could understand it by the following example. Can someone be a non-practicing vegetarian? Can someone be called a vegetarian, even though you consume meat regularly, merely because his parents were vegetarians?
Thus, in our opinion, it is a grave mistake to conflate commonality of DNA with absence of significant differences. A practitioner of Islam, inasmuch as he takes Koran as word of God and Muhammad as a role model, is a mortal danger to non-Muslims, no matter how much their DNA's may match.
Similarly, the commonality of DNA means that we should not call someone a brahmana merely because he claims that his parents are brahmanas, nor should we deny the possibility of becoming a dwija to someone who may be born to parents belonging to scheduled caste, for example. A good birth might imply some advantage but it can no more be like the advantage one has by being born to "educated" parents, say for example, being born to doctor parents. His early childhood might accord him a head start, that's about it. But to become "educated" himself, the ward has to study and learn for himself and then also practice it. What we need is not a caste-less society that some DF's (including the otherwise bright Subramanian Swamy) profess, rather we need to rediscover the True Varna System and the Associated Certification Process.
Sanatana Dharma is not a genetic civilization, it is a memetic civilization!