I first verified that f3(Brahmins_from_Uttar_Pradesh_M; Orcadian, North_Kannadi) is negative, using qp3Pop:
Source 1 Source 2 Target f_3 std. err Z SNPs
result: Orcadian North_Kannadi Brahmins_from_Uttar_Pradesh_M -0.007882 0.000359 -21.951 463297
The exponential fit can be seen below:
This is about a thousand years younger than the signal observed for the South Indian Brahmin group. A possible explanation has to do with the fact that South Indian Brahmins migrated to South India, and hence did not intermarry with successive waves of invaders into India in historical times. Uttar Pradesh, on the other hand, received multiple invasions from the direction of Central Asia:
Most of the invaders of North India passed through the Gangetic plains of what is today Uttar Pradesh. Control over this region was of vital importance to the power and stability of all of India's major empires, including the Maurya (320–200 BC), Kushan (100–250 CE), Gupta (350–600 CE), and Gurjara-Pratihara (650–1036 CE) empires. Following the Huns invasions that broke the Gupta empire, the Ganges-Yamuna Doab saw the rise of Kannauj. During the reign of Harshavardhana (590–647 CE), the Kannauj empire reached its zenith.It will be interesting to see whether a young admixture signal also exists in my 5-strong sample of Jatts, since that population has traditions of "Scythian" origins.