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The genetic architecture of facial variation

ArticlesPosted by djmcrouch Thu, July 12, 2018 12:12:46

The following is a non-technical summary of our work on facial feature genetics together with some musings on future possibilities, with some genealogical ideas. It is reproduced here from the Lost Cousins newsletter for which I contributed it in 2017.

Additional research has since been published by other groups, which I may comment on later.

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Most of us at some point will have met a pair of identical twins, and been astonished by how similar they are in most respects. This is especially so when considering their physical appearance; including height, weight and the pigmentations of their hair, eyes and skin, and this is due them sharing 100% of their DNA sequence. Perhaps most noticeable of all is the similarity in the structure of their facial features, and this tells us that differences in facial appearance between individuals must be overwhelmingly genetic. In other words, they are due to DNA differences, rather than being a result of one's upbringing or some effect of the external environment.

Genes are also disproportionately shared between lower degree relatives and, accordingly, it is widely understood that facial similarity is on the whole lower between cousins than it is between siblings, and lower still between second cousins, and so on. Degree of likeness seems to manifest as the number of shared or extremely similar facial features, and these often appear to be inherited from particular ancestors, for example when someone is described as having 'their mother's eyes'. This is in contrast with other traits such as height, for which people appear to conform roughly to the average between their two parents, after correcting for their sexes and any year-on-year average increase in height due to improved population health.

This is probably due to the particular genetic mechanisms at work. A person's height is the product of a large number of genes acting in concert, each with a small influence. As we inherit, on average, 25% of our genes from each grandparent, roughly 25% of one's height-influencing genes are also likely to be inherited from each of them. On the other hand, I propose that a facial feature is likely to be under strong influence of a single gene variant. As each individual has two versions of each variant (termed alleles), these must have descended from just two of their grandparents. This model, based on the strong effects of small numbers of genes, can explain the inheritance pattern of facial features that we tend to observe in families, as it implies that individuals will tend to take after a limited number of relatives that they share at least one of their two alleles with.

In theory, then, it should be possible to locate particular genes that have strong influences on facial appearance, and eventually to understand their biological functions. Only recently has this become feasible due to advances in a) our ability to establish people's DNA sequence information (or 'genotypes') from blood or saliva samples on a large scale, and b) the camera technology that allows one to obtain accurate 3D images of faces. In a new publication, our research group describes work that has resulted in the discovery and verification of 3 genetic variants that have strong effects on facial features, influencing the spacing between the eyes, the protrusion of the face and prominence of the chin. This represents one of the first steps towards uncovering the overall genetic architecture of the human face, which one has to presume remains largely mysterious due to the huge amount of facial variation that exists between people.

Understanding these and other genes' influences on appearance serves a number of purposes, most obvious being those in forensic science; for example producing e-fit images for suspects from DNA samples they have left at crime scenes. But there are also medical applications. Treatments for those congenital illnesses which have accompanying dysmorphic facial features currently rely on plastic surgeons coming to, inevitably, fairly subjective decisions about the desired appearance for the patient, largely based on the average facial characteristics that exist within the appropriate ethnic background. It would be more desirable to estimate, in quantitative fashion, what the patient would have looked like, if they did not have their particular condition; giving a more accurate objective for the surgical outcome. In theory this can be achieved by interrogating their DNA sequence, provided that a reasonable number of the genetic causes of appearance have been established.

There has long been an interest in reconstructing the outward physical appearance of people based on their skeletal remains; often for forensic purposes, but also in archaeology. Presently this is done by remodelling soft tissue structure, either by hand, over a cast of the underlying skull, or by using 3D computer artistry. These techniques suffer from being relatively subjective, as the distribution of soft tissue can only be estimated approximately. It is now possible to extract accurate DNA profiles from skeletons that are thousands of years old, and this suggests the intriguing possibility of using information on face-influencing genes to build up a picture of what particular individuals from the past would have looked like. From a genealogical perspective, one could in theory even reconstruct the genomes of ancestors, computationally, by piecing together DNA segments shared between living descendants. Facial appearances of these individuals from the past, for whom no biological samples are available, could then be predicted using the remnants of their genomes carried by those living in the present day.




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