Critical Thinking Questions

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1. State possible advantages and disadvantages of DNA microarray tests performed shortly after birth to identify susceptibilities and inherited diseases that will likely affect the individual later in life.

2. A young couple is devastated when their second child is born and has PKU. Their older child is healthy and no one else in the family has PKU. How is this possible?

3. A balding man undergoes a treatment that transfers some of the hair from the sides of his head, where it is still plentiful, to the top. Is he altering his phenotype or his genotype?

4. Bob and Joan know from a blood test that they are each heterozygous (carriers) for the autosomal recessive gene that causes sickle cell disease. If their first three children are healthy, what is the probability that their fourth child will have the disease?

5. A DNA microarray test includes several genes that cause cancer or increase sensitivity to substances that cause cancer. It also includes genes that confer high risk of b. A cell with an extra chromosome is trisomic. A cell with a missing chromosome is monosomic. Individuals with trisomies are more likely to survive to be born than those with monosomies.

c. Aneuploidy results from nondisjunction, in which a chromosome pair does not separate, either in meiosis I or meiosis II, producing a gamete with a missing or extra chromosome. At fertilization, a monosomic or trisomic zygote results.

3. Prenatal tests a. Maternal serum marker tests indirectly detect a small fetal liver, which can indicate a trisomy.

b. Amniocentesis samples and examines fetal chromosomes in amniotic fluid.

c. Chorionic villus sampling obtains and examines chorionic villus cells, which descend from the fertilized egg and therefore are presumed to be genetically identical to fetal cells.

d. Fetal cell sorting obtains and analyzes rare fetal cells in the maternal circulation.

Gene Therapy (page 993)

Gene therapy corrects the genetic defect causing symptoms.

1. Two approaches to gene therapy a. Heritable gene therapy alters all genes in an individual and therefore must be done on a gamete or fertilized egg. It is not being pursued in humans but is useful in research on other species.

c. Nonheritable gene therapy replaces or corrects defective genes in somatic cells, often those in which symptoms occur.

2. Tools and targets of gene therapy a. Healing genes are sent into cells in viruses, liposomes, blasted in, or as naked DNA.

b. Gene therapies are being tested in various tissues and to treat cancer.

addictive behaviors. The test is being developed to assess the risk that an individual who smokes will develop lung cancer. Do you think that such a test would be valuable or that it might be abused? Cite a reason for your answer.

6. In Hunter syndrome, lack of an enzyme leads to build up of sticky carbohydrates in the liver, spleen, and heart. The individual is also deaf and has unusual facial features. Hunter syndrome is inherited as an X-linked recessive condition. Intellect is usually unimpaired and life span can be normal. A man who has mild Hunter syndrome has a child with a woman who is a carrier (heterozygote).

a. What is the probability that a son inherits the syndrome?

b. What is the chance that a daughter inherits it?

c. What is the chance that a girl would be a carrier?

7. Amelogenesis imperfecta is an X-linked dominant condition that affects deposition of enamel onto teeth. Affected males have extremely thin enamel layers all over each tooth. Female carriers, however, have grooved teeth that result from uneven deposition of enamel. Explain the difference in phenotype between the sexes for this condition.

8. Why are medium-brown skin colors more common than very white or very black skin?

9. Why are there fewer Y-linked traits than there are Y-linked genes?

10. A woman aged forty receives genetic counseling before having an amniocentesis performed. She understands that her risk of carrying a fetus that has trisomy 21 Down syndrome is 1 in 106, but she is confused when the counselor explains that the risk of "any aneuploid" is 1 in 66. What does this mean?

11. Can a person who has been successfully treated for CF with an aerosol nongermline gene therapy still transmit the disease-causing allele to offspring? Cite a reason for your answer.

12. Parkinson disease is a movement disorder in which neurons in a part of the brain (the substantia nigra) can no longer produce the neurotransmitter dopamine, which is not a protein. Although Parkinson disease is not usually inherited, it may be treatable with gene therapy. What are two difficulties in developing gene therapy for Parkinson disease?

13. Cirrhosis of the liver, emphysema, and heart disease are all conditions that can be caused by a faulty gene or by a dangerous lifestyle habit (drinking alcohol, smoking, following a poor diet). When gene therapies become available for these conditions, should people with gene-caused disease be given priority in receiving the treatments? If not, what other criteria should be used for deciding who should receive a limited medical resource?

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