NEW YORK (GenomeWeb News) – Genetic studies of admixed populations are providing new insights into the genetic basis for the wide range of facial traits observed in humans.
Pennsylvania State University biological anthropologist Mark Shriver and his team are using genetic analyses and a variety of three-dimensional facial mapping techniques to understand how genes and genetic ancestry influence normal human facial traits. That, in turn, may provide information about human natural selection and sexual selection and provide new resources for forensic investigations.
"I think there's a valid and useful place for this type of science," Shriver told GenomeWeb Daily News.
While some research has been done to try to understand the genetics of cranio-facial anomalies, scientists still don't understand how genes and genetic ancestry influence the normal variation in human facial features.
In a review appearing in Forensic Science International: Genetics in early 2007, researchers from Spain's Institute for Legal Medicine and National Genotyping Centre noted that, at that time, "[a] search of the literature has not revealed any published work on normal variation of continuous features contributing to human facial morphology."
"The accepted and established facial similarity between zygotic twins when compared with dizygotic twins, indicates high heritability for these features," senior author Angel Carracedo and colleagues wrote.
In an effort to understand how genetics and genetic ancestry relate to the normal human facial feature variation, Shriver and his colleagues started by looking at 254 African-American individuals whose ancestors were West African and European. The relative genetic contribution of each group was determined using DNA testing. Researchers excluded individuals with high East Asian or Native American ancestry from the study to simplify their analysis.
"We chose to look at African Americans because they were a large enough and available admixed population," Shriver said in a statement. "Looking at admixed populations shows us the influence genes have and how they relate to physical features."
The idea is that researchers can measure landmarks on the human face — such as the tip of the nose, the tip of the chin, and the outer corner of the eye — and correlate inter-landmark distances with genetic ancestry. So far they are using three-dimensional and two-dimensional photos, eye photos, and other techniques to assess three-dimensional phenotypes.
Eventually, Shriver said, they hope to be able to do more advanced mesh analyses of the face. Discrete and continuous variable approaches then allow them to correlate facial traits with genetic variables.
Shriver, who presented the work and some preliminary results at the recent American Association for the Advancement of Science meeting in Chicago, said the study is yielding interesting results.
The team has already found a handful of genes that seem to influence different facial features. "I think we've got compelling evidence for six genes that we tested," Shriver said. The team's data also suggests that different parts of the face are influenced by different genes.
By looking at admixture percentages, the team was able to determine how genetically determined admixture and ancestral genes affected facial traits. The researchers also drew on information from the Online Mendelian Inheritance in Man database that were related to the genetics associated with conditions that affect the face.
"This type of study, done on admixed populations, shows that each person is a composite of their ancestors and that the range of facial features is a continuum," Shriver said in a statement.
Shriver said the work may improve researchers' understanding of human evolutionary biology, including how humans perceive each others' faces and the role of facial features in human natural selection and sexual selection.
And beyond what the work reveals about human biology, Shriver insists that it has forensic applications as well. "I've always been interested in forensic applications of evolutionary genetic information," he said.
If things pan out the way he thinks they will, genetic information from a crime scene may provide information about the perpetrator's face that could be used to prop up other evidence in court or provide useful hints during investigations.
Because there are so many different human populations around the world and a huge amount of variation in facial features, he cautioned, it may take decades to describe and understand all normal human facial variation. Still, Shriver is confident that researchers can collect enough information to be useful for forensics in the US in the next few years.
The forensic applications of this type of genetic-facial analysis are "right there to be had," Shriver said, adding, "Our initial results seem to show that not all of our genes have small effects." Even so, more research is necessary to validate the initial studies and assess facial features in additional populations.
The team has already collected samples from more than 3,500 subjects from around the world — including 1,400 individuals from Pennsylvania, 720 from Europe, 850 from Brazil, and 500 from the Cape Verde Islands. Eventually, they plan to collect about 8,000 samples.
Shriver noted that such studies may also help people understand that although humans are genetically diverse, the parts of the genome that have undergone rapid evolution in different human populations usually affect superficial traits such as facial features.