It is like a tree, at the base there is only one trunk [ancestor], then the branches [sons], and then these branches also have sub-branches [grandsons], so on and so forth. You can treat the Q1a as the trunk of the tree and looking just at the trunk it is not possible to tell what kind of a fruit it bears.
Jatt ethnic group is in no way as old as these basic haplogroups are, so this problem cannot be solved with basic haplogroups, we need to know the branches and sub branches, etc to separate the wheat from the chaff. In the case of a distinct haplogroup, say an African A00 haplogroup if found in a Jatt will immediately tell one that the guy is in no way a Jatt, but in the case of a shared haplogroup like R1a, one will have to look into its sub branches. Y-Dna results of all groups who use gathwala gotra be needed to find out if they are related or not, gotra can be adopted but the Y-Dna can't be adopted.
There is also a little bit of this H1a found among Jatts and one source of this could be the Bheel and Kol etc. who were promoted to the status of Rajputs by Brahmins to use them against Jatts and Gujjars, and these Rajputs of these origins now may have found their way into Jatts and Gujjars by acceptance etc., or are just posing as Jatts when in fact they are the rajputs of Bheel or Kol origin etc.
http://www.anthrogenica.com/showthre...sts-99-or-more
There is also a cheap shotgun approach that can be used to test Y-Dna at YSEQ.
Also not everybody have to test either, only distantly related people have to test, the related people can pool their funds and choose only one of them to test.