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Ch. 3 - Cell Division and Chromosome Heredity
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 3 - Cell Division and Chromosome HeredityProblem C.3d
Chapter 3, Problem C.3d

For the retinal cancer retinoblastoma, the inheritance of one mutated copy of RB1 from one of the parents is often referred to as a mutation that produces a 'dominant predisposition to cancer.' This means that the first mutation does not produce cancer but makes it very likely that cancer will develop.


What is the genotype of a normal cell in the retina in a person who has sporadic retinoblastoma? What is the normal cell genotype if the person has hereditary retinoblastoma? Explain the reason for the difference between the genotypes.

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Understand that retinoblastoma involves mutations in the RB1 gene, which is a tumor suppressor gene. Cancer develops when both copies of RB1 in a retinal cell are inactivated (the 'two-hit hypothesis').
For sporadic retinoblastoma, both mutations occur in retinal cells after conception (somatic mutations). Therefore, the normal retinal cells in this person have two normal RB1 alleles (genotype: RB1+/RB1+), and cancer arises only after two somatic mutations in the same cell.
For hereditary retinoblastoma, the person inherits one mutated RB1 allele from a parent (germline mutation). Thus, all cells, including normal retinal cells, have one mutated and one normal RB1 allele (genotype: RB1+/RB1-).
The difference in genotype arises because hereditary retinoblastoma involves a germline mutation present in every cell, predisposing cells to cancer with only one additional somatic mutation, whereas sporadic retinoblastoma requires two somatic mutations in the same cell to develop cancer.
Summarize that the key concept is the difference between germline (hereditary) and somatic (sporadic) mutations, which explains why normal retinal cells have different genotypes in these two forms of retinoblastoma.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Two-Hit Hypothesis

The two-hit hypothesis explains that both copies of a tumor suppressor gene, like RB1, must be inactivated for cancer to develop. In hereditary retinoblastoma, one mutated copy is inherited (first hit), and a second mutation occurs somatically (second hit). In sporadic cases, both mutations happen in the same retinal cell during the person's life.
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Translation:Wobble Hypothesis

Genotype Differences in Hereditary vs. Sporadic Retinoblastoma

In hereditary retinoblastoma, normal retinal cells carry one mutated RB1 allele and one normal allele, predisposing them to cancer after a second mutation. In sporadic retinoblastoma, normal retinal cells have two normal RB1 alleles initially, and cancer arises only after both alleles acquire mutations in the same cell.
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Cancer Mutations

Role of Tumor Suppressor Genes in Cancer

Tumor suppressor genes like RB1 regulate cell growth and prevent tumor formation. Mutations that inactivate both gene copies remove this control, leading to uncontrolled cell division and cancer. Understanding this helps explain why one inherited mutation increases cancer risk but does not cause cancer alone.
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Cancer Mutations
Related Practice
Textbook Question

For the retinal cancer retinoblastoma, the inheritance of one mutated copy of RB1 from one of the parents is often referred to as a mutation that produces a 'dominant predisposition to cancer.' This means that the first mutation does not produce cancer but makes it very likely that cancer will develop.

Using RB1⁺ for the normal wild-type allele and RB1⁻ for the mutant allele, identify the genotype of a cell in a retinoblastoma tumor.

Textbook Question

From a piece of blank paper, cut out three sets of four cigar-shaped structures (a total of 12 structures). These will represent chromatids. Be sure each member of a set of four chromatids has the same length and girth. In set one, label two chromatids 'A' and two chromatids 'a.' Cut each of these chromatids about halfway across near their midpoint and slide the two 'A' chromatids together at the cuts, to form a single set of attached sister chromatids. Do the same for the 'a' chromatids. In the second set of four chromatids, label two 'B' and two 'b.' Cut and slide these together as you did for the first set, joining the 'B' chromatids together and the 'b' chromatids together. Repeat this process for the third set of chromatids, labeling them as 'D' and 'd.' You now have models for three pairs of homologous chromosomes, for a total of six chromosomes. Align the chromosomes as they might appear at metaphase I of meiosis.

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Textbook Question

Examine the following diagrams of cells from an organism with diploid number 2n=6, and identify what stage of M phase is represented.

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Textbook Question

Our closest primate relative, the chimpanzee, has a diploid number of 2n = 48. For each of the following stages of M phase, identify the number of chromosomes present in each cell.

End of mitotic telophase

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