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Ch. 4 - Gene Interaction
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. 4 - Gene InteractionProblem 22a
Chapter 4, Problem 22a

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. Many genes are known to be involved in repair of UV-induced DNA damage, and several of these genes are implicated in XP. What genetic phenomenon is illustrated by XP?

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1
Understand the genetic basis of Xeroderma pigmentosum (XP): XP is an autosomal recessive condition caused by mutations in genes responsible for repairing UV-induced DNA damage.
Recognize that multiple genes are involved in the repair of UV-induced DNA damage. These genes encode proteins that function in nucleotide excision repair (NER), a pathway critical for correcting UV-induced DNA lesions.
Identify the genetic phenomenon illustrated by XP: Since mutations in any one of several genes can lead to the same condition (XP), this is an example of genetic heterogeneity. Genetic heterogeneity occurs when mutations in different genes produce the same phenotype.
Consider the implications of genetic heterogeneity: In XP, different patients may have mutations in different genes, but all exhibit the same clinical symptoms due to the shared disruption of the DNA repair pathway.
Conclude that understanding genetic heterogeneity is important for diagnosing and treating XP, as different mutations may require different therapeutic approaches or genetic counseling strategies.

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Key Concepts

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

Autosomal Recessive Inheritance

Autosomal recessive inheritance is a pattern of genetic transmission where two copies of a mutated gene, one inherited from each parent, are necessary for an individual to express a trait or disorder. In the case of Xeroderma pigmentosum (XP), individuals must inherit two defective copies of the genes responsible for DNA repair to exhibit symptoms, highlighting the importance of both alleles in the manifestation of the condition.
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Autosomal Pedigrees

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 XP, the deficiency in these repair pathways, particularly those that fix UV-induced damage, leads to an accumulation of mutations, resulting in skin lesions and increased cancer risk. Understanding these mechanisms is crucial for grasping how XP develops.
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Repair Pathways

Genetic Phenotype

A genetic phenotype refers to the observable physical or biochemical characteristics of an organism, determined by both genetic makeup and environmental influences. In XP, the phenotype is characterized by extreme sensitivity to UV light and the development of skin lesions and cancers, illustrating how specific genetic mutations can lead to distinct clinical manifestations.
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Related Practice
Textbook Question

Brachydactyly type D is a human autosomal dominant condition in which the thumbs are abnormally short and broad. In most cases, both thumbs are affected, but occasionally just one thumb is involved. The accompanying pedigree shows a family in which brachydactyly type D is segregating. Filled circles and squares represent females and males who have involvement of both thumbs. Half-filled symbols represent family members with just one thumb affected.

Is there evidence of incomplete penetrance in this family? Explain. 

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Textbook Question

A male and a female mouse are each from pure-breeding albino strains. They have a litter of 10 pups, all of which have normal pigmentation. The F₁ pups are crossed to one another to produce 56 F₂ mice, of which 31 are normally pigmented and 25 are albino.

Using clearly defined allele symbols of your own choosing, give the genotypes of parental and F₁ mice. What genetic phenomenon explains these parental and F₁ phenotypes?

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Textbook Question

A male and a female mouse are each from pure-breeding albino strains. They have a litter of 10 pups, all of which have normal pigmentation. The F₁ pups are crossed to one another to produce 56 F₂ mice, of which 31 are normally pigmented and 25 are albino.

What genetic phenomenon explains the F₂ results? Use your allelic symbols to explain the F₂ results.

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Textbook Question

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. 

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Textbook Question

Three strains of green-seeded lentil plants appear to have the same phenotype. The strains are designated G₁, G₂, and G₃. Each green-seeded strain is crossed to a pure-breeding yellow-seeded strain designated Y. The F₁ of each cross are yellow; however, self-fertilization of F₁ plants produces F₂ with different proportions of yellow- and green-seeded plants as shown below.

For what number of genes are variable alleles segregating in the G₁ x Y cross? The G₂ x Y cross? In the G₃ x Y cross? Explain your rationale for each answer. 

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Textbook Question

Three strains of green-seeded lentil plants appear to have the same phenotype. The strains are designated G₁, G₂, and G₃. Each green-seeded strain is crossed to a pure-breeding yellow-seeded strain designated Y. The F₁ of each cross are yellow; however, self-fertilization of F₁ plants produces F₂ with different proportions of yellow- and green-seeded plants as shown below.

Using the allele symbols A and a, B and b, and D and d to represent alleles at segregating genes, give the genotypes of parental and F₁ plants in each cross. 

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