I've copied this from my own web site:
>>Was Peter III the father of Paul?
Rumor has attached the possible father as being Serge Saltikov.
It is believed that Catherine II "the Great" never disclosed the facts about Paul I's conception.
Due to the new discoveries through DNA, it appears that Saltikov's claim may be null and void. See the following information: http://en.wikipedia.org/wiki/List_of_haplogroups_of_historical_and_famous_figures#Rurik_of_Novgorod
>>All Russian emperors from at least Nicholas I to Nicholas II "Holstein-Gottorp-Romanov"
The haplotype of Nicholas II of Russia has been predicted to belong to R1b.
It matched a member of another line of Nicholas I's descendants. So, all the Emperors from Nicholas I to Nicholas II shared this Y-DNA. It can also be said that this result is German-specific, so Paul I was most likely the real son of his official father Peter III, and not the son of a lover, as was speculated.[citation. needed][dubious – discuss]
It also allowed to validate the remains of Alexei, son of Nicholas II.<<
>>...likely the real son of his official father Peter III...<< That may be an obvious leap. I think I'd say, Paul I's father was from a "German-specific" which covers a group rather than just Peter III.
Frederick II "the Great" was always plotting so who knows.
Read more: http://agrbear.hyperboards.com/action/view_topic/topic_id/962#ixzz30IZQmqYu
If this report is reliable: It is probably safe to say that Peter III is probably the father of Paul I
unless someone can produce a possible candidate, who's background was the same or similar to Peter III's and was in contact with Catherine during the time of conception.