Researchers have sourced William’s Indian ancestry to Eliza Kewark, his great-great-great-great-great grandmother, who was assumed to be Armenian, but now has been revealed as an Indian by genetic research.Chaubey et al. (2008) is an article that touches upon the subject of R30b.
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“Through genealogy we traced two living direct descendants of Eliza and by reading the sequence of their mtDNA, we showed not only that they matched, but also that it belongs to a haplogroup called R30b, thus determining Eliza Kewark’s haplogroup,” the research team revealed.
The haplogroup, which is a group of related ancestral lineages, in this case was revealed to be rare and found only in South Asia. Other related branches of R30a and R30* are also entirely South Asian.
“This confirms therefore that the mtDNA of Eliza Kewark of Surat was of Indian heritage. R30b is rare even in India, where roughly 0.3 per cent of people carry this lineage,” the researchers revealed.
Personally, I wouldn't be so quick in discounting the traditional genealogical story. A lineage that occurs at a frequency of 0.3% will almost certainly be missed in any small sample if it occurs at similar trace frequencies in other populations.