Dentinogenesis imperfecta: severe expression in a probable homozygote

A family wherein 25 individuals, of whom 23 are affected and 2 are reputed to be affected with dentinogenesis imperfecta, is presented. The distribution of the patients which spans 5 generations in this pedigree accords well with autosomal dominant transmission of the disorder. A consanguineous marriage of 2 second cousins both of whom are affected has resulted in 4 offspring: unaffected, 2 affected with moderate severity and one with markedly severe involvement. These are held to correspond to 3 genotypic classes: homozygous lor the normal allele, heterozygotes, and homozygous for the mutant allele of dentinogenesis imperfecta, respectively. Relevant examples in other dominantly transmitted disorders of individuals affected with unusual severity, encountered among the progeny of matings between more typically affected parents and were thus presumed to be homozygotes, are also described. To our knowledge no instance of homozygosity for the dentinogenesis imperfecta gene has previously been described.