Summary. Familial predisposition and patterns of genetic inheritance of eclampsia and pre‐eclampsia were investigated through three or four generations in 94 families from the homogenous island population of Iceland. The families descended from index women delivered in the years 1931–47 and who had either eclampsia (n = 38) or severe preeclampsia (n = 69). Inheritance was followed both through sons and daughters. The prevalence of pre‐eclampsia and eclampsia in daughters was significantly higher (23%) than that in daughters‐in‐law (10%). No difference was noted in the prevalence of these diseases by whether the daughter was born of an eclamptic or pre‐eclamptic mother or whether she was a first or later born daughter. There was a non‐significantly higher occurrence of pre‐eclampsia among grand‐daughters than in grand‐daughters‐in‐law. No difference was seen by whether granddaughters descended through sons or daughters. With increasing numbers of affected daughters or grand‐daughters the probability rose of finding more affected women in a family. Hypotheses of single recessive and dominant gene inheritance were compared and maximum likelihood estimates for gene frequency obtained. For a single recessive gene model this was 0.31 reflecting a population prevalence of 9.6%, whereas a dominant model with incomplete penetrance gave 0.14 at 48% gene penetrance, corresponding to a population prevalence of 0.9% homozygous expression of severe disease and 11% heterozygous expression of milder disease. Either genetic model could fit the data.
Among different populations, the shape of the age-incidence curve for breast cancer is strongly related to the overall incidence of breast cancer in the respective population. Data are available from Iceland for the period . These data show that breast cancer has increased very markedly in Iceland during this period, and that as the overall incidence has risen, so the age-incidence curve has changed in shape, the relation between the shape and the overall incidence being the same as that now observed in other countries. The change in shape is shown to be explicable entirely as a cohort phenomenon, each decade of birth cohort having an age-incidence curve of similar shape, but with different overall incidence. Data from some other regions of the world indicate that many of the present differences in the shape of the age-incidence curve may be the reflection of cohort phenomena.The break at the menopause in the steady increase of incidence of breast cancer with age, and the resumption of this steady increase a few years thereafter, was first noted by Clemmesen (1948). Clemmesen's hook, as it is called, has been the basis of much speculation on the aetiology of breast cancer. Epidemiologists have been further intrigued by a second feature o i the age-incidence pattern, namely the relation between the shape of the age-incidence curve in a given population and the overall incidence of breast cancer in that population. This relationship is best demonstrated graphically (Fig. 1). In a low-incidence country, such as Japan, the incidence decreases with age after the menopause. In intermediate-incidence countries (e.g. Finland) the post-menopausal incidence remains at the same level, whereas in high-incidence populations (e.g. Connecticut, U S A ) , the incidence increases with age after the menopausal hook. It has been suggested that such differences in behaviour indicate separate aetiologies for pre-and postmenopausal breast cancer (De Waard, 1969).The purpose of this paper is two-fold: first, to show that in Iceland the changes in the agespecific incidence curves associated with the increase in breast cancer incidence observed over the past 60 years mirror the differences between the age-incidence curves now seen among different countries (Fig. 1); second, to show that the change in the shape of these curves over the past 60 years can be explained solely as a cohort effect.
MATERIALThe material comes from two sources. The 661 cases diagnosed in the years 1910 to 1953 were collected by one of us (G.S.) by scrutiny of all death certificates, hospital and pathology records in the country and by visits and correspondence
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