Genetic researchers crossed a critical threshold last year in their ability to understand complex diseases, posting a number of new discoveries that advanced knowledge of ailments caused by small contributions from multiple genes, the environment, and other causes.

Assistant Professor of Medicine Mark J. Daly, a researcher at Harvard-affiliated Massachusetts General Hospital, said April 10 that though science had become adept in recent years at finding the roots of diseases caused by single genes, more complex ailments such as type 2 diabetes and Crohn’s disease had eluded analysis.

As technology has advanced, however, so has the ability to analyze many genes at once — between populations of people afflicted with the disease and those who are not — for associations with specific diseases. In 2007, Daly said, science reached a watershed moment, with several studies revealing the presence of multiple genes that have small effects on several diseases.

“We have clearly crossed a critical threshold in our ability to discover genes for common diseases,” Daly said.

Daly made his comments as the opening speaker of a two-day symposium sponsored by the Harvard University Center for AIDS Research, the Broad Institute of MIT and Harvard, and the Harvard Initiative for Global Health. The symposium, called “Human Genomics: Advances and Opportunities in HIV/AIDS Research,” was held at the New Research Building at Harvard Medical School on April 10 and at the Broad Institute on April 11.

The symposium, held annually, brought together researchers in a variety of fields to discuss recent advances in genetic studies. Speakers discussed topics including natural selection’s effect on the genome, population structure and ancestry in genetic disease association studies, and pharmacogenomics.

Daly was introduced by Professor of Medicine Bruce Walker, director of the Harvard University Center for AIDS Research. The symposium topic was chosen, Walker said, because it was clear that genomics was playing a huge role in disease research.

Though genomics has been said to herald a day of personalized medicine, where treatments are tailored to each person’s unique genetic and physical makeup, in actuality, genomics’ most important use will be to understand the biological causes of diseases in humans, Daly said.

A development that helped researchers, Daly said, was that humans turned out to not be very genetically diverse. While geographic diversity often means genetic diversity, humanity’s spread around the globe was so rapid that there wasn’t time for a lot of genetic diversity to develop.

While technology has allowed researchers to find many new genes implicated in complex diseases, these new genes tend to explain just a small part of the disease’s heredity. These ailments, such as Crohn’s disease, result from a complex interplay between environment and several different genes that each affect whether and how someone develops an illness.

“We began to find quite a significant number of disease genes, each of which explains a small part of heritability of Crohn’s disease,” Daly said.

Though each of these genes plays just a small role in the disease, Daly said, they are all critical in our understanding of the ailment. The insight gained by studying them could lead to a novel approach to treatment.

“What matters in these studies is the insight into disease mechanism,” Daly said, adding that after decades of toil to understand the genetic roots of disease, the work is now bearing fruit. “We’ve clearly gotten over some hump of identifying genes that underlie complex diseases.”