Medical Genetics 4th Edition by Lynn B. Jorde -Test Bank

 

 

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Sample Test

Jorde: Medical Genetics, 4^th Edition

Chapter 3: Genetic Variation: Its Origin and Detection

 

Sample Problems & Essay Questions

 

Question 1: The H-Y antigen is encoded by a gene on the human Y chromosome. Suppose that a variant allele at the H-Y locus is found in one in 1,000 males in a population. What is the gene frequency of this allele in the male population?

 

Answer: 1/1000

 

Question 2: Continuing question 1, what is the gene frequency of this allele in the female population?

 

Answer: 0

 

Question 3: Hemochromatosis is a common autosomal recessive disorder of iron absorption. The gene frequency is approximately 0.05 (1/20) in some Northern European populations. If the penetrance of the disease-causing genotype is 0.5 in males and 0.2 in females, what is the expected incidence of the disease phenotype in the male and female populations?

 

Answer: Males: (1/20)^2 x 0.5 = 1/800

Females: (1/20)^2 x 0.2 = 1/2000

 

Question 4: List at least three explanations for the pedigree shown below for a Mendelian (single-gene) disease (for each explanation, a 2-3 word phrase is sufficient).

 

Answer: New Mutation
Incomplete penetrance
Autosomal recessive
X-linked recessive

Question 5: A 25-year-old woman has had three children with Down syndrome. Give two possible explanations (for each explanation, a 2-3 word phrase is sufficient).

 

Answer: Germline mosaicism, Robertsonian translocation, Reciprocal translocation involving distal 1/3 of 21q.

 

Question 6: A man who is a known heterozygous carrier of a cystic fibrosis mutation marries his half first cousin (see pedigree). What is the probability that this couple will produce a child with cystic fibrosis (autosomal recessive disorder)?

 

Answer: (1/2)^4 x 1/4 = 1/64

 

Question 7: A woman who has normal skin pigmentation has two sisters with oculocutaneous albinism, a fully penetrant autosomal recessive disease. What is the probability that this woman is a heterozygous carrier of the disease gene?

 

Answer: 2/3

 

Question 8: Consider a woman who is a known heterozygous carrier of a mutation that causes PKU (autosomal recessive). What is the probability that her two grandchildren (shown in the pedigree as individuals A and B) are both heterozygous carriers of this PKU-causing allele?

 

 

Answer: 1/2)^4 = 1/16

 

Question 9: Explain the difference between variable expression and incomplete penetrance. Use at least two disease examples to illustrate your discussion (4-5 sentences should be sufficient).

 

Answer: (NOTE: because I have included multiple examples, these answers are a little longer than I expect yours to be.)

A disease gene is said to have incomplete penetrance when individuals with the disease-causing genotype do not always develop the disease phenotype. A good example of incomplete penetrance is retinoblastoma, in which 10% of obligate carriers develop no eye tumors. Variable expression refers to variation in the degree of severity of a genetic disease. An example of variable expression is neurofibromatosis type 1 (which, by the way, has 100% penetrance). Some individuals who have the NF1 gene have only a few neurofibromas and cafe-au-lait spots, while others can have hundreds of neurofibromas, as well as malignancies, scoliosis, seizures, psudarthrosis, optic gliomas, and pheochromocytomas. Another example of a disease with variable expression is cystic fibrosis (CF), in which 15% of patients do not have pancreatic insufficiency. In addition, some CF patients have very severe lung disease and die within the first decade, while others can survive for several or more decades. (Other examples of diseases with variable expression include hemophilia A, osteogenesis imperfecta, and Marfan syndrome).

 

Question 10: Explain the concept of imprinting. Use disease examples to illustrate your points.

 

Answer: When a gene is imprinted, its expression varies depending on the parent from which it is inherited. Imprinting is strongly correlated with (and may be at least partially cause by) methylation of DNA sequences. Methylation is thought to inhibit the transcription of a gene, thus inactivating it. An example of imprinting is given by the Prader-Willi and Angelman syndromes, which are caused by alterations of genes on chromosome 15. Males normally transmit an active copy of the Angelman gene but an inactive (i.e., imprinted) copy of the Prader-Willi genes to their offspring. Males, in contrast, normally transmit an active copy of the Prader-Willi genes and an inactive (imprinted) copy of the Angelman gene to their offspring. A deletion of the paternally inherited region of chromosome 15 thus removes the active Prader-Willi genes and produces Prader-Willi syndrome. A deletion of the maternally inherited region of chromosome 15 removes the active Angelman gene and produces Angelman syndrome.

Another example of imprinting is given by Beckwith-Wiedemann syndrome, which can be caused by the presence of two copies of the paternally derived IGF2 (insulin-like growth factor 2) gene. This can occur as a result of uniparental disomy. Because the maternally derived IGF2 gene is imprinted (inactive), normal individuals have only one active copy of the gene(i.e., the single paternally inherited copy). Those having Two paternally inherited copies have twice the amount of IGF2 gene product, leading to some of the features of this disease (e.g., large size).

 

Question 11: Match the pedigree with the most likely mode of inheritance. Note that complicating factors, such as reduced penetrance, may be present. Assume that the gene frequency of the disorder in the general population is very low. These answers may be used more than once.

 

 

1.    autosomal dominant

2.    autosomal recessive

3.    X-linked recessive

4.    X-linked dominant

5.    mitochondrial

 

Answer: c

 

Question 12: Match the pedigree with the most likely mode of inheritance. Note that complicating factors, such as reduced penetrance, may be present. Assume that the gene frequency of the disorder in the general population is very low. These answers may be used more than once.

 

 

1.    autosomal dominant

2.    autosomal recessive

3.    X-linked recessive

4.    X-linked dominant

5.    mitochondrial

 

Answer: a