GENETICISTS are peeling back almost a thousand years of Scottish history to trace the men alive today whose DNA marks them out as belonging to the royal Stuart bloodline.

Around half of men with the Stuart or Stewart surname - regardless of the spelling - are believed to carry a unique marker in their Y chromosome which identifies them as the direct descendants of a 13th Century nobleman who fought alongside William Wallace and whose subsequent lineage includes the Stuart monarch, James VI of Scotland.

Researchers are now keen to recruit as many men as possible to build an accurate picture of the Stuarts' genetic dynasty in modern Scotland.

In the run up to Father's Day on June 21, Scotland's DNA is offering cut price genetic testing to encourage as many men as possible to have their ancestry unlocked.

The offer is only open to men as women do not carry the all-important Y chromosome where the S781 royal marker is found.

It was pinpointed after geneticists on the long-running project received a DNA sample from the current Duke of Buccleuch, a direct descendent of Sir John Stewart of Bonkyll, a military commander killed during the Battle of Falkirk in 1298.

By comparing it against the DNA of a direct descendent of Sir John's brother, they were able to prove that the S781 marker originated in Sir John, because his brother's descendants did not inherit the marker but those of both his sons, including the Duke of Buccleuch, did.

"This is the first time ever that we've been able to link a genetic marker to a named historical person," said Dr Jim Wilson, chief scientist at Scotland's DNA. "Scientists in Oxford and Cambridge have identified a Y marker that they think marks the lineage of Genghis Khan but that's based on circumstantial evidence.

"But S781 being Sir John of Bonkyll's marker, there's no two ways about it."

Markers refer to "errors in copying" when DNA is passed down the generations. Each of us inherit some six billion letters of DNA, three billion from each parent.

The unique sequence is made up of a combination of four letters: A, C, G and T, shorthand for the molecules adenine, cytosine, guanine and thymine.

Occasionally, small mistakes occur in the code that see letters switch places. This is known as a single nucleotide polymorphism - or marker.

The Scotland's DNA team are also keen to hone in on other royal bloodlines. They believe they may have uncovered the genetic marker for the Bruce line using a sample from a man in Shetland who traces his family tree back to Robert the Bruce's grandson, although Dr Wilson stressed this remained a "work in progress".

They have also found significant genetic overlaps among men who share the Irvine and MacGregor surnames, indicating a shared ancestor in the last 1000-1500 years.

In many cases, a dominant genetic legacy is all that remains of once powerful men.

"It implies that there were these patriarchs, these powerful men, who had the opportunity to have a lot of children," said Dr Wilson. "Why are there are lot of Stuarts in Scotland today? - it's because a lot of the early bearers of the surname Stuart were noblemen who, because they were powerful, had access to many women in order to have many children and, in particular, many sons.

"So you get this runaway process whereby that guy's Y chromosome gets spread into the population and overwhelms the Y chromosomes of his peers.

"The more data we have in the database the more able we are to accurately count how many there are and where they are, and see if there are any other clusters lurking that we haven't picked out."