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The genealogic and genetic data on retinoblastoma were reviewed and interpreted according to the model of delayed mutation; then applications of the model to specific situations in genetic counseling were considered. Patients with multiple congenital abnormalities and systemic chromosome aberrations are regarded as belonging to a different category of retinoblastoma cases than the more common patients without such abnormalities. The model of delayed mutation is considered for the latter group of patients. According to the model, mutation at the retinoblastoma locus can be delayed or complete and can occur during meiotic or mitotic cell division. Genotypically, three clases of individuals can be identified in retinoblastoma families: homozygous normal, heterozygous for the premutated allele, and heterozygous for the (fully) mutated allele; the other possible combinations of individuals have apparently not been observed. There is to date no evidence to suggest incomplete penetrance of the mutant allele, but 14% of individuals who have the mutant gene are "only" unilaterally affected. Carriers produce normal, affected and carrier offspring in the empiric proportion of, respectively, 54.5%, 36.4% and 9.1%. Most difficulties in genetic counseling arise because affected individuals may have inherited the premutated or the mutated allele and because unaffected individuals may have inherited the normal or the premutated allele. These aspects were considered for individuals presenting as sporadic-unilateral, sporadic-bilateral, familial-unilateral and familial-bilateral cases, and the empiric risk figures for various situations were quoted from the literature.

1. Expression and nonexpressin of genetic information may be viewed in relationship to the biologic structures that express or do not express the genetic information. We suggest defining expressivity as the quality of expression of genetic information in cells, tissues, organs, etc, of individuals, defining penetrance as the quality of expression of genetic information in an individual organism as a whole, and using a new term, phenotrance, to describe the quality of expression of genetic information in generations. 2. Decreased phenotrance may be indicated by "incompletely dominant" inheritance, by conditions for which "dominant" as well as "recessive" inheritance has been reported, or by disorders with sporadic occurrence in most and familial occurrence in some instances. The human conditions with decreased phenotrance that we have studied indicate that there are different types of decreased phenotrance. 3. The mechanisms for decreased phenotrance in man may correspond to certain genetic mechanisms that have been studied in lower organisms, such as delayed mutation, replicating instabilities, controlling elements, extrachromosomal inheritance, and others.

From one personal patient and 13 reported in the literature, arthrodentoosteodysplasia (ADOD) is defined as a heritable disorder of connective tissue with the main clinical manifestations of laxity of joints, early loss of teeth, and multiple osteolytic lesions, including acroosteolysis on roentgenographic examination. These lesions probably represent "pseudoosteolysis" with faulty primary bone formation rather than true osteolysis of previously normal bone. ADOD is an example of relational pleiotropism with most clinical manifestations representing secondary effects and deformities. The cranial sutures frequently remain uncalcified and contain multiple wormian bones in unusual places. Secondary deformities may be progressive and affect primarily the skull, spine, fingers and fingernails. Pathologic fractures are clinically the most important manifestation of ADOD. In one family the mother and four of her six children were affected. The other nine case reports describe sporadic instances. ADOD is presumed to be caused by an autosomal dominant gene, the sporadic cases representing new mutations.