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Chapter 19: Problem 8

A couple with European ancestry seeks genetic counseling before having children because of a history of cystic fibrosis (CF) in the husband's family. ASO testing for CF reveals that the husband is heterozygous for the \(\Delta 508\) mutation and that the wife is heterozygous for the \(R 117\) mutation. You are the couple's genetic counselor. When consulting with you, they express their conviction that they are not at risk for having an affected child because they each carry different mutations and cannot have a child who is homozygous for either mutation. What would you say to them?

Short Answer

Expert verified
Answer: The probability of the couple having an affected child is 25%, as the child could inherit one mutated allele from each parent resulting in a compound heterozygote for the disease. This occurs even though the parents carry different cystic fibrosis mutations (\(\Delta 508\) and \(R 117\)).

Step by step solution

01

Understanding cystic fibrosis mutations

Cystic fibrosis is an autosomal recessive genetic disorder, meaning that a person must inherit two copies of the mutated gene (one from each parent) to be affected. In this case, the husband is heterozygous for the \(\Delta 508\) mutation, and the wife is heterozygous for the \(R 117\) mutation. Being heterozygous means that they each carry one normal allele and one mutated allele.
02

Examine the possible outcomes for child's genotype

To understand the possible outcomes for the couple's potential children, we need to examine all possible combinations of the two alleles from each parent. Using the Punnett square, we can see the following possible genotypes for the couple's child: 1. Normal allele from both parents (unaffected) 2. Normal allele from father and \(R 117\) mutation from mother (unaffected carrier) 3. \(\Delta 508\) mutation from father and normal allele from mother (unaffected carrier) 4. \(\Delta 508\) mutation from father and \(R 117\) mutation from mother (affected)
03

Calculating the probability of having an affected child

From the four possible genotypes listed above, only one of them results in an affected child (combination of both mutated alleles). The probability of this outcome is \(\frac{1}{4}\), as there is a 50% chance for each parent to pass on their mutated allele, and to have an affected child, both mutated alleles must be inherited simultaneously (0.5 x 0.5 = 0.25).
04

Explaining the risk to the couple

Contrary to the couple's belief, they can indeed have a child who is affected by cystic fibrosis, even if they carry different mutations. Their child would inherit one mutated allele from each parent, resulting in a compound heterozygote for the disease. They have a 25% chance of having an affected child, a 50% chance of having an unaffected child who is a carrier of the mutation, and a 25% chance of having an unaffected child with no mutated alleles. It is important to discuss these risks with the couple and help them make informed decisions regarding family planning.

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Most popular questions from this chapter

In March 2010 Judge \(R\). Sweet ruled to invalidate Myriad Genetics' patents on the \(B R C A 1\) and \(B R C A 2\) genes. Sweet wrote that since the genes are part of the natural world, they are not patentable. Myriad Genetics also holds patents on the development of a direct-to-consumer test for the \(B R C A 1\) and \(B R C A 2\) genes. (a) Would you agree with Judge Sweet's ruling to invalidate the patenting of the \(B R C A 1\) and \(B R C A 2\) genes? If you were asked to judge the patenting of the direct-to-consumer test for the BRCA1 and BRCA2 genes, how would you rule? (b) J. Craig Venter has filed a patent application for his "firstever human made life form." This patent is designed to cover the genome of \(M\). genitalium. Would your ruling for Venter's "organism" be different from Judge Sweet's ruling on patenting of the \(B R C A 1\) and \(B R C A 2\) genes?

In this chapter, we focused on a number of interesting applications of genetic engineering, genomics, and biotechnology. At the same time, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions: (a) What experimental evidence confirms that we have introduced a useful gene into a transgenic organism and that it performs as we anticipate? (b) How can we use DNA analysis to determine that a human fetus has sickle- cell anemia? (c) How can DNA microarray analysis be used to identify specific genes that are being expressed in a specific tissue? (d) How are GWAS carried out, and what information do they provide? (e) What are some of the technical reasons why gene therapy is difficult to carry out effectively?

As genetic testing becomes widespread, medical records will contain the results of such testing. Who should have access to this information? Should employers, potential employers, or insurance companies be allowed to have this information? Would you favor or oppose having the government establish and maintain a central database containing the results of individuals' genome scans?

The Genetic Testing Registry is intended to provide better information to patients, but companies involved in genetic testing are not required to participate. Should company participation be mandatory? Why or why not? Explain your answers.

The first attempts at gene therapy began in 1990 with the treatment of a young girl with a genetic disorder abbreviated SCID. What does SCID stand for? In the context of SCID, what does ADA stand for?
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