We conducted a family-based GWAS and identified genome-wide significant association (p=5.4×10−8) between SC and SNPs in chromosome 15q14. However, the Q-Q plot for the COGA family-based sample suggests genomic inflation factor of 1.25. One possible explanation is the presence of polygenic inheritance29. The SC trait is likely due to many loci of small effect. Because of this and the high LD in the dense map, it is possible that there are relatively few genomic regions free of loci with small effect moderately inflating GIF. A second possibility is that the inflation may be due to the presence of very large families in our sample. Since our association test controls for relatedness by treating each family as a separate cluster, this may be insufficient in families where distantly related individuals should not be clustered into the same class. After correcting for the genomic inflation factor of 1.25 using the genomic controls method, none of the SNPs associated with SC reached genome-wide significance.
