I think its imporant to mention that who many claim to be from Quraish and/or Bani hashim, are not. Over the years claiming Hashimi roots has been done for either , religious/political influence or simply for monetary gain. (through al-khums or other indirect ways) It has become a common practice especially in areas where people arent that familiar with Arab tribes where its much easier to do. Therefore, the origins of many who claim to belong to Bani Hashim shouldnt be taken for granted.
Currently theres a few DNA projects being done on Quraish, Bani Tameem (tribe of Qatars ruling family ), Bani Khaled (previous rulers of the eastern province in Saudi Arabia) and other prominent tribes.
Research is being done on both Adnani Arabs (decendants of Isma'eel aka Ishmael, son of Abraham, who Quraish, Bani Tameem and other tribes go back to) and Qahtani Arabs as well. Im pretty sure once we have definitive scientific information to refer to we're going to be in for a few surprises especially regarding those who claim that they are from Quraish and Bani Hashim in particular for reasons related to politics, and in some cases religion as well. (I'll try keeping politics and religion in this post to a minimum).
For instance, many scholars and historians have said that the Fatimides are most certainly not decendants of Ali Bin Abi Talib, and therefore call them the "Ubaidis" instead of Fatimides. According to them, the real name of the founder of the dynasty is "Saeed Bin Al hussain Bin Ahmad Bin Abdallah Bin Maymoon Al Qaddda Bin Desaan Al Shanawee Al Ahwazee" who went by the name Ubaidillah and claimed to be a decendant of Ali.
The famous historian, Al Dhahabi (born 1274) said in "Siyar A'laam Al Nobala":
"There are several opinions concerning the lineage of al-Mahdi, which may be summed up by noting that he was neither Hashimi nor Fatimi"
He also stated in Tareekh Al Islam 'The genealogists and scholars denied his claims concerning his lineage" In another book he said "Al Mahdi Ubaidulla lied about being a decendant of Jafar Al Sadiq ( a well known decendant of Ali ) "
Al Suyooti (born 1445) in his book "The history of Caliphs" (Tareekh Al Khulafaa') mentioned this and said that he was a son of a Magian. (Magians being from Persia) Al Baqillani, Ibn Katheer, Al Nowairee, Al Ghazali, have also said similar things about their lineage.
Judges and scholars from Baghdad at the time also collectivly stated this opinion in a letter.
"Bin" meaning "son of", a decendant of Ali whos full name going back to Ali is proved and established as "Akhu Muhsin, Muhammad Bin Ali Bin Al hussain Bin Ahmad Bin Isma'eel Bin Muhammad Bin Jaffar Al Sadiq Bin Muhammad Bin Ali Bin Alhussain Bin Al hussain Bin Ali Bin Abi Talib said about the real name of the founder of the dynasty :
"Ubaidullah Al Mahdi is Saeed Bin Al hussain Bin Ahmad Bin Abdallah Bin Maymoon Al Qaddda Bin Desaan Al Thanawee, and is originally a Magian" (Masr Min Ahd Bonat Al Qahira p 81)
Granted, theres others that dispute this for various reasons and have their own quotes to provide.
However, the lineage of Ubaidis (or Fatimids), and many others today (local leaders etc, not necessarily royals) , is most certainly not a matter of consensus. Some Arabs that belong to tribes and well known families give exceptional importance and care in marriage and other issues to geneology and authentic roots, and its much harder to make unsupported claims regarding geneology among them as it would be in say, Indonesia.