Segregation analysis in hereditary retinoblastoma.

Segregation analysis was performed in 211 nuclear families belonging to 166 pedigrees of hereditary retinoblastoma found in a number of series which have been gathered from the literature. Bilaterally affected carriers appear homogeneous. The segregation ratio in their offspring is 0.49 and the proportion of bilateral cases among affected offspring is 0.87. Both unilaterally affected and unaffected carriers appear heterogeneous. The very low segregation ratio (0.08) in the offspring of unilateral carriers who are not detected through an affected child, suggests the possibility of two types of carriers. "high" and "low transmitters." The proportions of "low transmitters" was estimated as 0.14 among all familial unilateral carriers and as 0.45 among all detected unaffected carriers. Unilateral and unaffected "high transmitters" give a significantly lower segregation ratio than bilaterally affected carriers. On the one hand, the existence of these two different types of carriers provides arguments in support of the hypothesis of delayed mutation. On the other hand, the differences in penetrance among "high transmitters." according to their phenotype, supports the hypothesis of host resistance. Under the two-mutation hypothesis, the possibility that the mutation rate is variable among individuals and partly genetically determined, is suggested.