Alignment of GSF and NSS genotyping samples. The average ARR homozygote (ARR/ARR), heterozygote (ARR/X), and non-ARR (X/X) frequencies in the NSS sample, which only contains animals over 18 months of age, was aligned with the GSF sample by exclusion of the birth cohorts which are not present in the NSS sample. In detail: the NSS of 2008 (of animals over 18 months) does not contain animals from the 2007 birth cohort; the NSS of 2007 does not contain animals from the 2006 and 2007 birth cohorts; etc. GSF '99-02: genotype frequencies of birth cohorts 1999 - 2002; GSF '99-03: genotype frequencies of birth cohorts 1999 - 2003; etc. NSS 2005: genotype frequencies of the National Surveillance Sample from 2005; NSS 2006: genotype frequencies of the National Surveillance Sample from 2006; etc.