Unit 2 - 11. Molecular Methods in the Diagnosis of Genetic Diseases

Question 1

6 uses of genetic tests

Answer 1

1. Diagnostic testing
2. Predictive testing
3. Carrier testing
4. Prenatal testing
5. Preimplanatiation testing
6. Newborn screening

Question 2

1. Diagnostic testing

Answer 2

• noninvasive way to confirm or rule out a diagnosis of a genetic disorder in a symptomatic individual (may not always be the best method of diagnosis)

Question 3

2. Predictive testing

Answer 3

• offered to asymptomatic individuals with family history to predict whether eventual development of symptoms is certain or likely if present genetic mutation
• controversial - treatment may not be available

Question 4

3. Carrier testing

Answer 4

• offered to individual with family history or have a higher risk (ethnic/race) to identify carriers of a gene in recessive disorder
• allows reproductive choices

Question 5

4. Prenatal testing

Answer 5

• offered when there is increased risk of having a child with genetic disorder
• performed on fetal samples obtained by amniocentesis, chronic villus sampling (piece of placenta), and etc.

Question 6

5. Preimplantation testing

Answer 6

• offered to couples with high chance of having a child with serious genetic disease
• performed on early embryos from in vitro fertilization
• only unaffected embryos are selected for implantation

Question 7

6. Newborn Screening

Answer 7

• legally mandated (varies by state)
• performed routinely at birth to identify newborns with genetic disorders to begin treatment as early as possible
• ex: PKU

Question 8

Direct Testing

Answer 8

• testing for specific gene mutation that is directly related to the disease
• used on monogenic disorders and requires that the mutated gene has been identified
• preferred over indirect testing

Question 9

Indirect Testing

Answer 9

• tracing the inheritance of a mutated gene in a family along with known genetic markers which are located within (intragenic) or close to (extragenic) a target gene = Linkage analysis
• performed when direct testing is not possible because disease gene has not been identified or due to a number of mutations

Question 10

Cyctic Fibrosis

Answer 10

= autosomal recessive

Mutation: at CFTR gene on long arm of chromosome 7 which codes for a protein that regulates chloride ions across plasma membrane
- most common mutation = delta F508 - deletion of 3-base codon for phenylalanine at 508

Detection: Direct testing using PCR and gel electrophoresis
- Diagnostic, carrier, and prenatal testing via fluorescence-based testing

Question 11

Duchenne Muscular Dystrophy

Answer 11

= X-linked recessive

Mutation: mutation in gene that codes for the muscle protein, dystrophin
- can be a number of mutations but most are deletions

Detection: PCR amplifications

Question 12

Huntington Disease

Answer 12

= autosomal dominant

Mutation: multiple CAG repeats in gene coding for Huntington protein

Detection: PCR

Question 13

Fragile X Syndrome

Answer 13

= X-linked dominant

Mutation: multiple CGG repeats at FMR1 gene (>200)

Detection: Southern Blot

Question 14

RFLP

Answer 14

• restriction fragment length polymorphism
• DNA finger printing/profiling
• one type of marker
• genetic variations in length of DNA fragments resulting from restriction endonucleases
• mutations here may effect a restriction endonuclease site

Question 15

Linkage Analysis

Answer 15

• used to determine whether a family member is likely to carry a disease gene based on pattern of markers inherited

Question 16

Limitations to Linkage Analysis

Answer 16

1. Family members must be willing to participate
2. Family must be informative (carriers are heterozygous for mutant allele)
3. Recombination of extragenic markers can affect accuracy (the further the marker, the less accurate)