1Department of Anthropology, The Pennsylvania State University, University Park, PA 16802, 2Department of Physiology and Institute for Genetic Medicine, Johns Hopkins University School of Medicine Baltimore, MD 21205, 3Department of Basic Medical Science, School of Medicine, University of Missouri-Kansas City, Kansas City, Missouri 64108, 4Center for Functional Anatomy and Evolution, The Johns Hopkins University School of Medicine, Baltimore, MD 21205
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Down syndrome (DS), resulting from trisomy 21 (Ts21), is a common live-born human aneuploidy. The phenotypic expression of Ts21 produces variable, though characteristic, facial morphology. It has been argued that Ts21 phenotypes result from amplified developmental instability (DI), a disruption of developmental homeostasis from cumulative small dosage effects of many genes. Based on this argument, DS facial features should exhibit more fluctuating asymmetry (FA, a measure of DI) than typical faces, and patterns of FA should reveal which facial regions are most affected during development. To address this hypothesis, we acquired a sample of 3D facial images (N=220) consisting of DS individuals (n=55), their siblings (n=55), and unaffected sibling pairs (n=110), aged 4-12 yrs. 3D coordinate data from 20 landmarks were used to estimate asymmetry of all bilateral linear distances of the face. A nonparametric bootstrapping procedure was used to test for local differences in FA of facial features between each sibling sample after correcting for size differences. We compared the DS sample to their siblings. The unaffected sibling sample was divided into two samples age-matched to the DS and DS sibling sample, and we compared two samples to each other. We then compared the differences from each comparison. Our results suggest that facial features are affected differentially in the DS sibling sample as evidenced by more statistically significant differences in FA across the face and larger average values of FA. These results lend support to the amplified DI hypothesis, and provide evidence of how DI changes across the face.
Funding: Grant sponsor: NIH; Grant numbers: R01-HD038384, R01-DE018500, R01-DE018500-S1; Grant sponsor: NSF; Grant number: GRF-053135.