Occasional families show a considerable number of cases of this common disorder. A simple Mendelian mechanism could not be proved, however. Indeed, some (Burch et al., 1964) could not demonstrate significant familial aggregation.
Lynn et al. (1995) conducted family studies and segregation analyses of RA based on consecutive patients with RA ascertained without regard to family history or known risk factors. Included in the analyses were first-degree relatives from 135 simplex and 30 multiplex families. A highly penetrate recessive major gene, with a mutant allele frequency of 0.005, was identified as the most parsimonious genetic risk factor. Significant evidence for heterogeneity in risk for RA was observed for proband gender but not for proband age at onset. Kaplan-Meier risk analysis demonstrated significant evidence for differences in the distribution of risk among first-degree relatives. Although both proband gender and age at onset were identified as important risk factors, proband gender appeared to be the more important determinant of risk, with relatives of male probands having the greatest cumulative risk for RA. For future genetic analyses, Lynn et al. (1995) suggested that families with an excess of affected males having a young age at onset might be most informative in identifying the putative recessive gene and its modifiers.
Hasstedt et al. (1994) studied 28 pedigrees ascertained through pairs of first-degree relatives with RA. RA was confirmed in 77 pedigree members, including probands; the absence of disease was verified in an additional 261 pedigree members. Members of the pedigrees were typed serologically for HLA. Analyses supported the existence of an HLA-linked RA susceptibility locus, estimated the susceptibility allele frequency as 0.0216, and estimated the lifetime penetrance as 41% in male... [continues]
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