Question

The rb gene encodes a protein that inhibits the E2F transcription factor. When E2F is released from rb, it helps to increase expression of many genes, including those involved in DNA replication. Mutations in rb are associated with development of certain cancers, such as retinoblastoma. Given the following genotypes, please describe the expected phenotypes:

A) One copy of rb is defective, both copies of E2F are functional

B)Both copies of rb are defective, both copies of E2F are functional

C) Both copies of rb are defective, one copy of E2F is defective

D) Both copies of rb are defective, both copies of E2F are defective

Answer 1

Since Rb encodes a protein that inhibits a transcription factor E2F hence, Rb acts as a TSG. If any two of the allele is not mutated and functional then they will produce the protein which will inhibit the E2F and cancer will not form, however, the person shall be susceptible to cancer. Whereas, if both the alleles of Rb are mutated this will lead to complete absence of the TSG protein and no inhibition of the E2F will be seen. Therefore, when both the copies of RB will be defective then only E2F will be functional.

The answer is the option (B)