Familial aggregation of disease Relative risk Prevalence of the disease among the relatives of an affected person Prevalence of the disease in the general population Concordance and allele sharing among relatives Genotype of sib #1 A1A4 A2A3 A2A4 AlA2 Degree of Relationship and Alleles in Common AlA3 Proportion of Alleles in A1 Relationship to proband Common with Proband M A2A3 First-degree relative 12 Second-degree relative A2A40 Third-degree relative 1/8 0. 25 (2 shared alleles)+0.5(1 shared allele +0.25 (0 shared alleles)=1 shared allele (average). One-of-two alleles shared 50% shared allelesFamilial aggregation of disease • Relative risk • Concordance and allele sharing among relatives Prevalence of the disease among the relatives of an affected person Prevalence of the disease in the general population lr = 0.25 (2 shared alleles) + 0.5 (1 shared allele) + 0.25 (0 shared alleles) = 1 shared allele (average). One-of-two alleles shared = 50% shared alleles