A 2-year-old boy is found to have bilateral retinal tumors. Molecular studies demonstrate a germline mutation in one allele of the Rb gene. Which of the following genetic events best explains the mechanism of carcinogenesis in this patient?

Retinoblastomas are malignant ocular tumors of young children. In cases of hereditary retinoblastoma:- An affected child inherits one defective Rb allele together with one normal gene. This heterozygous state is sufficient to prevent the development of retinoblastoma. However, if the remaining normal Rb allele is inactivated by deletion or mutation, the loss of its suppressor function leads to the appearance of a neoplasm. This genetic process is referred to as loss of heterozygosity. Diagnosis: Retinoblastoma