Kofi Agbo HB1, Medicine Date:26th November,2018 Genetics Assignment Index No. - 3433718 1. Discuss possible reasons why colorectal cancer is an adult cancer whereas retinoblastoma affects children. Ans. Colorectal cancer can arise as a result of epigenetic alterations in genes, a subset of these genes called driver genes, believed to play a role in cancer. In the CpG islands of protein-coding genes, the epigenetic alterations are as a result of hypermethylation or hypomethylation of these genes. Abnormal methylation is a normal characteristic of aging, and thus the risk of developing colon cancer increases with aging, hence why majority of colorectal cancer cases are found in adults. In retinoblastoma cases however, when the eyes are developing to maturity, they form immature progenitor cells called retinoblasts which later become retina cells . A mutation in the RB1 gene in these retinoblasts causes retinoblastoma. There being 2 alleles of the RB1 gene, with the mutation being recessive, there must be a change in both copies in a retinoblast for retinoblastoma to develop. Given that both mutations must occur in the early stages of development in order for cancer to take place, this cancer is therefore most likely to occur in children. 2. Propose a theory why so few hereditary cancer syndromes inherited as autosomal dominant diseases, are caused by activated oncogenes, whereas so many are caused by germline mutations in a tumor-suppressor gene. Ans. Autosomal dominant cancers with regards to oncogenes are generally suppressed by tumour suppressor genes. This would decrease the frequency of autosomal dominant diseases in relation to oncogene mutations than to mutations in tumour suppressor genes, since there’s no check on cancers caused by mutated suppressor genes.