In this sample, the SC phenotype best fit a negative binomial distribution, which was identified by applying PROC COUNTREG and PROC SGPLOT in SAS (http://support.sas.com/rnd/app/da/glimmix.html). By specifying a negative binomial distribution and a logarithmic link function, we parametrically modeled the observed trait distribution and included relevant covariates described above. Association with SC was analyzed for each SNP using a dose-effect model (number of minor alleles present in each individual) as implemented in PROC GLIMMIX from SAS. To control for relatedness, the test was placed in a general linear mixed model (GLMM) framework26 using an independent working correlation matrix where each family was a separate cluster.
