Malformations in Northeastern Brazil

Abstract

The double positive binomial distribution is applied to malformations in northeastern Brazil under complete ascertainment. It leads to the conclusion that the 871 clinically examined families from normal parents may be divided into 2 groups: the major one, comprising 92% of the families, in which the malformation rate is less than 2%; and the remaining families, in which the malformation rate is an order of magnitude greater and approaches 1/4. Only the high-risk group is significantly affected by inbreeding. The best estimates of its parameters are: the inbred load at examination, B = 0.443[plus or minus] 0.186; the con-sanguinity ratio, B/A = 61; the selection cofficient against homozygotes before examination, s = 0. 74; the inbred load before selection, and the total frequency per gamete of contributory genes, B/(l-s) = 1. 69; the number of loci, n = 500; the mean gene frequency per contributory locus, Q = 0. 0034; and the total mutation rate per gamete, U = 1251X 10-5. On this and other evidence, it is inferred that most of the effect of low levels of inbreeding on morbidity and mortality in nearly panmictic populations is due to rare oligogenes largely maintained by recurrent mutation, and not to detrimentals with low pene-trance (phenodeviants).