Retinoblastoma is an example of a childhood cancer that also follows Mendelian Genetics. It is almost exclusively present in children under the age of 5, which is why appointments with the optometrist are frequent in the early stages of life. Hypothetically, this would make sure that any tumours/problems found could be completely eradicated with a higher chance of saving the eye. The older a child is, the harder treatment will be. (I knew a guy who had retinoblastoma at age 7 and lost one of his eyes as a result)
There are two types of retinoblastoma, hereditary and non-hereditary. With hereditary retinoblastoma, mutations in the RB1 gene are inherited in an autosomal dominant pattern. That means that one copy of the altered gene is sufficiently present enough in each cell to risk cancer. Typically there is a family history, and both eyes are effected. A child could inherit it from one parent, or the altered gene could be the result of a new mutation made while still in utero. With non-hereditary retinoblastoma, normally only one eye is effected and there is no family history of the disease. The person would be born with two normal copies of RB1 genes, but in early childhood they would acquire mutations.
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