Chin protrusion is described as protruding lower jaw or receding upper jaw. Chin protrusion causes a “long face” and makes the other parts of the face (nose and eyes) to look disproportionately small. Since people with disproportionate jaws may have problems with speaking, eating and breathing, they may need orthognathic surgery in severe cases.
In 2015, researchers reported the successful identification of genes influencing outer ear (pinna) morphology using a three-point ordinal phenotyping approach. To extend this methodology to other aspects of facial variation, they performed a genome-wide association scan (GWAS) for facial features from the lower, middle and upper face.
The study sample ( ~6,000 Latin Americans) was part of the Consortium for the Analysis of the Diversity and Evolution of Latin America (CANDELA) cohort. 14 facial traits such as chin protrusion, nose bridge breadth and nose wing breadth on an ordinal scale which categorized by receding, normal, and pronounced were evaluated using facial photographs. Other than reproducing the associations detected with ordinal traits, the GWAS of quantitative traits detected a genome-wide significant association to chin protrusion (Fig.1) for markers in rs3827760 in (Ectodysplasin A receptor) EDAR gene. The derived G allele of rs3827760 encodes a functional substitution in the intracellular death domain of EDAR (370A) and is associated with reduced chin protrusion. EDAR is part of the EDA signaling pathway, which specifies prenatally the location, size, and shape of ectodermal appendages such as hair follicles, teeth and glands. The G allele at rs3827760 is not present in Europeans and Africans but is seen at high frequency in East Asians and is essentially fixed in Native Americans. This SNP has been associated in East Asians with characteristic tooth morphologies, hair type and sweat gland density. In the previous study, it was found that rs3827760 impacts on aspects of pinna morphology, including: lobe size and attachment, ear protrusion and helix rolling.
To sum up, the effect of EDAR on chin protrusion adds to the developing picture of this gene having a multitude of effects in populations with East Asian and Native American ancestry, the 370A allele having been associated so far with: increased sweat gland density, straight hair, lower beard and eye-brow thickness, increased incisor shoveling and a range of pinna features. It will be interesting to further examine the role these gene regions might play in the evolutionary diversification of facial features in mammals, including the appearance of derived features in archaic and modern humans and their potential involvement in the evolution of adaptive features of facial anatomy. Read more about the study here:
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