Xeroderma pigmentosum (XP) is an autosomal recessive condition characterized by moderate to severe sensitivity to ultraviolet (UV) light. Patients develop multiple skin lesions on UV-exposed skin, and skin cancers often develop as a result. XP is caused by deficient repair of DNA damage from UV exposure.


A series of 10 skin-cell lines was grown from different XP patients. Cells from these lines were fused, and the heterokaryons were tested for genetic complementation by assaying their ability to repair DNA damage caused by a moderate amount of UV exposure. In the table below, '+' indicates that the fusion cell line performs normal DNA damage mutation repair, and '−' indicates defective DNA repair. Use this information to determine how many DNA-repair genes are mutated in the 10 cell lines, and identify which cell lines share the same mutated genes. 

Verified step by step guidance

1. Step 1: Understand the concept of genetic complementation. Genetic complementation occurs when two different mutations in separate genes are combined in a heterokaryon (a fused cell with nuclei from different sources), and the resulting cell is able to perform a normal function. This indicates that the mutations affect different genes.
2. Step 2: Analyze the table provided in the problem. Look at the '+' and '−' results for each fusion cell line. A '+' result indicates that the fused cell lines complement each other, meaning their mutations are in different genes. A '−' result indicates that the mutations are in the same gene.
3. Step 3: Group the cell lines based on their complementation results. Identify which cell lines share the same mutated gene by observing which pairs consistently result in '−' when fused. These cell lines belong to the same complementation group.
4. Step 4: Count the number of distinct complementation groups. Each complementation group corresponds to a unique DNA-repair gene that is mutated in the cell lines.
5. Step 5: Summarize the findings by listing the number of DNA-repair genes mutated and grouping the cell lines that share the same mutated gene. This will provide the final answer to the problem.

Key Concepts

Xeroderma Pigmentosum (XP)

Xeroderma pigmentosum is a genetic disorder caused by mutations in genes responsible for repairing DNA damage induced by ultraviolet (UV) light. It is inherited in an autosomal recessive manner, meaning that an individual must inherit two copies of the mutated gene to exhibit symptoms. Patients with XP are highly sensitive to UV light, leading to skin lesions and an increased risk of skin cancers due to the accumulation of unrepaired DNA damage.

DNA Repair Mechanisms

DNA repair mechanisms are cellular processes that identify and correct damage to the DNA molecules that encode an organism's genome. In the context of XP, the nucleotide excision repair (NER) pathway is particularly relevant, as it is responsible for removing UV-induced DNA lesions. Understanding these mechanisms is crucial for analyzing how mutations in specific genes can lead to the defective repair observed in XP patients.

Genetic Complementation

Genetic complementation is a method used to determine whether two mutations that produce a similar phenotype are in the same or different genes. In the experiment described, cell lines from different XP patients were fused to create heterokaryons, allowing researchers to assess whether the presence of a functional gene from one cell line could compensate for the defective gene in another. This approach helps identify the number of mutated DNA-repair genes and their interactions in the context of XP.