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Breast parenchymal patterns, as visible on mammograms, are determined by the relative amount of radiodense, glandular dysplastic tissue (DY). High percentages of DY are related to higher breast cancer risk. Previous studies reported heritable influences on DY of 32-67%, depending on the family relationship that was studied.We assessed heritability in 466 sister-, 25 dizygotic twin- and 26 monozygotic twin-pairs participating in a population-based breast cancer screening program; the DOM project (Diagnostic Investigation Mamma Carcinoma).The heritability was estimated for non-twin sisters, dizygotic and monozygotic twins seperately by computing correlations between siblings from the dichotomous DY-score (high risk versus low risk). This was done using methods based on the number of shared genes per sibtype.Heritability estimates were 38, 34 and 88% for sisters, dizygotic twins and monozygotic twins respectively. Heritability estimates from models that combine monozygotic twins with dizygotic twins or sisters indicated that dominant gene effects, genetic interactions or gene-environment effects could be involved. Parity appeared to have an effect on the heritabile influence with estimates ranging from 90% in sisters that were both nulliparous, to 2% in sisterpairs discordant for nulliparity. These result indicate a substantial genetic influence on DY, but with a possible modifying ability of other factors, such as parity.