In retinoblastoma, a mutation in one allele of the RB1 tumorsuppressorgene can be inherited from the germ line, causingan autosomal dominant predisposition to the developmentof eye tumors. To develop tumors, a somatic mutation in thesecond copy of the RB1 gene is necessary, indicating that themutation itself acts as a recessive trait. Given that the firstmutation can be inherited, in what ways can a second mutationalevent occur?
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Q: How can the retinoblastoma trait be inherited in a dominant fashion if the deletion of the RB gene…
A: Retinoblastoma is an eye cancer that begins in the retina.
In retinoblastoma, a mutation in one allele of the RB1 tumorsuppressor
gene can be inherited from the germ line, causing
an autosomal dominant predisposition to the development
of eye tumors. To develop tumors, a somatic mutation in the
second copy of the RB1 gene is necessary, indicating that the
mutation itself acts as a recessive trait. Given that the first
mutation can be inherited, in what ways can a second mutational
event occur?
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- Fragile-X syndrome causes the most common form of inherited intellectual disability. What is the chromosomal abnormality associated with this disorder? What is the phenotype of this disorder?Six months pregnant, an expectant mother had a routineultrasound that showed that the limbs of the fetus wereunusually short. Her physician suspected that the babymight have a genetic form of dwarfism called achondroplasia,an autosomal dominant trait occurring with a frequency of about1 in 27,000 births. The parents were directed to a genetic counselorto discuss this diagnosis. In the conference, they learnedthat achondroplasia is caused by a mutant allele. Sometimes itis passed from one generation to another, but in 80 percent ofall cases it is the result of a spontaneous mutation that arisesin a gamete of one of the parents. They also learned that mostchildren with achondroplasia have normal intelligence and a normallife span. What information would be most relevant to concluding whichof the two mutation origins, inherited or new, most likelypertains in this case? How does this conclusion impact on thiscouple’s decision to have more children?Six months pregnant, an expectant mother had a routineultrasound that showed that the limbs of the fetus wereunusually short. Her physician suspected that the babymight have a genetic form of dwarfism called achondroplasia,an autosomal dominant trait occurring with a frequency of about1 in 27,000 births. The parents were directed to a genetic counselorto discuss this diagnosis. In the conference, they learnedthat achondroplasia is caused by a mutant allele. Sometimes itis passed from one generation to another, but in 80 percent ofall cases it is the result of a spontaneous mutation that arisesin a gamete of one of the parents. They also learned that mostchildren with achondroplasia have normal intelligence and a normallife span. It has been suggested that prenatal genetic testing for achondroplasiabe made available and offered to all women. Wouldyou agree with this initiative? What ethical considerationswould you consider when evaluating the medical and societalconsequences of offering…
- In 1995, doctors reported a Chinese family in whichretinitis pigmentosa (progressive degeneration of theretina leading to blindness) affected only males. Allsix sons of affected males were affected, but all of thefive daughters of affected males (and all of thechildren of these daughters) were unaffected.a. What is the likelihood that this form of retinitispigmentosa is due to an autosomal mutationshowing complete dominance?b. What other possibilities could explain the inheritance of retinitis pigmentosa in this family? Whichof these possibilities do you think is most likely?Glucose-6-phosphate dehydrogenase deficiency/G6PDD (g) is an X-linked recessive conditionwherein the red blood cells of affected individuals undergo premature hemolysis. Fragile X syndrome(F), on the other hand, is an X-linked dominant mutation characterized by a mild to moderateintellectual disability. Amelogenesis imperfecta (A) is a congenital disorder affecting the formation ofthe teeth enamel making affected individuals at higher risk for dental cavities and related problems.Only male offspring inherit this condition. Jane is heterozygous for both X-linked traits like her mother. Her father is normal for both X-linked traits. James has a mother who suffers from G6PDD but not from fragile X-syndrome. His fatherdoes not exhibit any X-linked disorder but has amelogenesis imperfecta. A. What are the genotypes of the following: Jane: ______________________ James: ______________________Jane’s mother: _________________ James’ mother: ________________Jane’s father: __________________…The maternal-effect mutation bicoid (bcd) is recessive. Inthe absence of the bicoid protein product, embryogenesis isnot completed. Consider a cross between a female heterozygousfor the bicoid mutation (bcd+/ bcd-) and a homozygousmale(bcd-/ bcd-). How is it possible for a male homozygous for the mutationto exist?
- Ehler-Danlos syndrome is a rare disorder caused by a mutation ina gene that encodes a protein called collagen (type 3 A1). Collagenis found in the extracellular matrix that plays an important role inthe formation of skin, joints, and other connective tissues. Peoplewith Ehler-Danlos syndrome have extraordinarily flexible skin and very loose joints. The pedigree below contains several individualsaffected with this syndrome, shown with black symbols. Based onthis pedigree, does the syndrome appear to follow autosomalrecessive, autosomal dominant, X-linked recessive, or X-linkeddominant inheritance? Explain your reasoning.Tay–Sachs disease is caused by loss-of-function mutations ina gene on chromosome 15 that encodes a lysosomal enzyme.Tay–Sachs is inherited as an autosomal recessive condition.Among Ashkenazi Jews of Central European ancestry, about1 in 3600 children is born with the disease. What fraction ofthe individuals in this population are carriers?Ehler-Danlos syndrome is a rare disorder caused by a mutation ina gene that encodes a protein called collagen (type 3 A1). Collagenis found in the extracellular matrix that plays an important role inthe formation of skin, joints, and other connective tissues. Peoplewith Ehler-Danlos syndrome have extraordinarily flexible skin andvery loose joints. The following pedigree contains several individualsaffected with this syndrome, shown with black symbols. Basedon this pedigree, does the syndrome follow autosomal recessive,autosomal dominant, X-linked recessive, or X-linked dominantinheritance? Explain your reasoning.
- A female patient 19 years old, whose symptoms areanemia and internal bleeding due to a massive buildupof leukemic white blood cells, is diagnosed withchronic myelogenous leukemia (CML). Karyotypeanalysis shows that the leukemic cells of this patientare heterozygous for a reciprocal translocation involving chromosomes 9 and 22. However, none of thenormal, nonleukemic cells of this patient contain thetranslocation. Which of the following statements istrue and which is false?a. The translocation results in the inactivation (loss offunction) of a tumor-suppressor gene.b. The translocation results in the inactivation (loss offunction) of an oncogene.c. There is a 50% chance that any child of this patientwill have CML.d. This patient is a somatic mosaic in terms of thekaryotype.e. DNA extracted from leukemic cells of this patient,if taken up by normal mouse tissue culture cells,could potentially transform the mouse cells intocells capable of causing tumors.f. The normal function of the…A DNA variant has been found linked to a rare autosomal dominant disease in humans and can thus beused as a marker to follow inheritance of the diseaseallele. In an informative family (in which one parentis heterozygous for both the disease allele and the DNA marker in a known chromosomal arrangementof alleles, and his or her mate does not have the samealleles of the DNA variant), the reliability of such amarker as a predictor of the disease in a fetus is related to the map distance between the DNA markerand the gene causing the disease.Imagine that a man affected with the disease(genotype Dd) is heterozygous for the V1and V2forms of the DNA variant, with form V1on the samechromosome as the D allele and form V2on the samechromosome as d. His wife is V3V3dd, where V3isanother allele of the DNA marker. Typing of the fetusby amniocentesis reveals that the fetus has the V2andV3variants of the DNA marker. How likely is it thatthe fetus has inherited the disease allele D if thedistance…The pedigree here shows the inheritance of a human disease knownas familial hypercholesterolemia. This disorder is characterized by an elevated level of serumcholesterol in the blood. Though relatively rare, this geneticabnormality can be a contributing factor to heart attacks. At themolecular level, this disease is caused by a defective gene thatencodes a protein called low-density lipoprotein receptor (LDLR).In the bloodstream, serum cholesterol is bound to a carrier proteinknown as low-density lipoprotein (LDL). LDL binds to LDLR,which enables cells to absorb cholesterol. When LDLR is defective,it becomes more difficult for the cells to absorb cholesterol. Thisexplains why the blood level of cholesterol remains high. Basedon the pedigree, what is the most likely pattern of inheritance ofthis disorder?