"These long-ago brewers were practicing genetics without even knowing it," said Professor Gavin Sherlock of Stanford University School of Medicine.

"They've given us a very interesting opportunity to look at a relatively young, rapidly changing species, as well as some very good beer."

The story of lager begins with medieval Bavarians who forbade the brewing of beer in the summer months due to spoilage.

They chose instead to brew using ale yeast in the winter months -- colder temperatures inhibited the ale yeast and allowed hybridisation with another strain that thrived in the cold.

Analysis of lager yeast DNA reveals that hybridisation occurred not once but twice.

"It's possible that the ale strain provides a certain flavour profile, while the second strain conferred the ability to ferment at cooler temperatures," said Dunn.

"Mixing them together is a nice way for the yeast to double its genetic options."

Looking at the genetic research into lager yeast provides an insight into the history of brewing in Europe over the last 500 years.

One lineage is tied to the Carlsberg breweries in Denmark along with several breweries in the Czech Republic.

The other line is associated with breweries in the Netherlands, including Heineken.

By careful of analysis of how lager yeast's genetic profile has changed over the years, scientists can better determine what genetic traits please drinkers.

"When we look at the genes that have either been lost or amplified in copy number, we can make the case that some of them could be related to brewing," said Professor Sherlock.