just google
With all due respect, rumor is easy to create (even unintentionally) and very hard to kill.
I'm sure I can find links for "evidence" that the letter "e" is evil and should be abolished if I look hard enough. But such a vague and harmful claim such as that the "DNA composition" of "different castes" must be taken with the deepest suspicion, because it is very ill-defined to the point of falling into wishful thinking from the start.
After all, what is meant by DNA composition, and what does define a caste? Dalits, Ksatryas and Brahmans are interfertile, they are the same species. Many generations of mutual isolation may have made some genetic traits more common among some groups, but by that token I might say that, say, Finns have a different "DNA composition" from Turks, and that is just an attempt at making racism and ethnocentrism look respectable.
As for castes, it is plain to see that they are a social construct, not a god-given mandate.