We perfom both segregation and linkage analysis for an hypothesized breast cmxr suscqt&ility gene u d e r a s w l e analyze these families, we present a breast cancer penetrance model a sirrgle parameter for each gemtype. lmis is in corrtrast to previms models that require two or more parameters for each genotype. W advantage of this newly develcpd model is the interpretability of the parameters as age-specific relative risks for breast cancer. locus msdel allawing Iygl-genetic cases of breast cancer. To pedigree Analysis Paage (IIassteat and cartwright, 1981) w a s used for bath segregation d linkage analysis. Ascertairmrerrt c o d o n for segregation analysis was made by dividhq the likelihood by the pzchbility of the segregation model sonsists of one autoscnadl locus w i t h two alleles (A and a), 'A' being amsidered the susceptibility allele. Mendelian segregation and Haxdy-weinberg eqilibrium assumed. under a daninant model individuals w i t h genotypes AA and Aa are expcted to have higher age-specific rates for breast cancer than genotype aat urder a recessive model individuals w i t h genutype AA are expect& to have higher age-specific rates for breast cancer than individuals w i t h genotype Aa or aa. Cases of breast cancer associated with the m n -' le genotype(s) are termed Isporadic' or 'non-gmetic' cases. event, as described in axmpson and tannings (1979). Ihe To define the penetrance, we assume for e a d ~ genotype tbat the five year ageand sex-specific incidence rates are praportiondl to the C O -' Utah ageand sex-specific O 1987 Man R. Liss, Inc.