That is from this post of John Hawks, which has several other interesting things to say too, as the following quote shows:
A selected gene is like garlic mustard. We may say that only a few members of the Roman elite intermarried with Britons. But if a single Roman married a Briton, carrying an advantageous gene, that gene has the chance to grow exponentially. That chance is not a guarantee, any more than a single garlic mustard seed is a guarantee. A single copy of an advantageous gene still has a very high probability of being lost by chance. But selected genes have a much higher chance of spreading than neutral ones. A very slight amount of long-distance gene flow can cause a selected gene to spread vastly faster than diffusion across a population.
Besides that, in this case, the history is incomplete. Roman legions occupied Britain for more than 400 years. Those legions were not only Italian, but included soldiers from across the empire, including in one famous instance thousands of Sarmatians. Sarmatians carried with them genes from the steppes of Central Asia, much farther than Rome. Soldiers were stationed for years, and many left the service and became local merchants, landowners, or minor nobles. They were not celibate. For that matter, neither were the early Latin clergy…
A must-read post. Have fun!