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Medical Genetics 4th Edition by Lynn B. Jorde -Test Bank
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,Jorde: Medical Genetics, 4th EditionChapter 1: Background and History
Sample Problems & Essay Questions
Question 1: Your class, which has 100 individuals, has been typed for a locus that has
three possible alleles, labeled 1, 2, and 3. The genotypes and their counts are:
1,1 10
1,2 5
1,3 15
2,2 20
2,3 20
3,3 30
Based on these genotype counts, which is the gene frequency of allele 1?
Answer: 20+5+15 = 40 = 0.2
200 200
Question 2: If an X-linked recessive disorder affects approximately 1/1,000,000 females
(all homozygotes) in a population, what is the expected frequency of affected males in the
population?
Answer: q= √(1/1,000,000) = 1/1,000
Question 3: Two individuals who both have achondroplasia (autosomal dominant
disorder) mate. What is the occurrence risk for this disorder in their offspring?
Answer: 0.75
Question 4: Suppose that you have done a carrier test for PKU (autosomal recessive
disorder) in a population, and you discover that the heterozygote carrier frequency is
1/500. Based on this information, what proportion of the population will be affected with
PKU? (Note: your answer does not have to be exact.).
Answer: 2pq=1/500 2q=1/500 q=1/1000 q2=1/1,000,000
Question 5: A woman with an X-linked dominant disorder mates with a phenotypically
normal male. On average, what proportion of this couple's daughters will be affected with
the disorder?
Answer: 0.50
Question 6: A woman who is a heterozygous carrier of an X-linked recessive disease
gene mates with a phenotypically normal male. The disease gene has a penetrance of
Mosby items and derived items © 2010, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
,Sample Problems & Essay Questions 2
80%. On average, what proportion of this couple's sons will be affected with the
disorder?
Answer: 0.8 x 0.5 = 0.4
Question 7: In the accompanying pedigree, a man is affected with tyrosinase-negative
oculocutaneous albinism (autosomal recessive). Based on this information alone, what is
the probability that his grandson (labeled A) and his great-granddaughter (labeled B) are
both heterozygous carriers of the albinism gene?
Answer: 1/8
Question 8: A woman has an autosomal dominant form of postaxial polydactyly.
Assuming that you have not examined any other family members to determine whether
they have postaxial polydactyly, what is the probability that her second cousin (labeled A
in the pedigree) is also affected with this fully penetrant disorder?
Answer: (1/2)^6 + (1/2)^6 = 1/32
Question 9: Discuss the concept of pleiotropy. Use at least two disease examples to
illustrate the concept. (This answer should require no more than about five sentences.)
Mosby items and derived items © 2010, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
, Sample Problems & Essay Questions 3
Answer: Pleiotropy describes a common situation in which a single gene defect produces
multiple phenotypic problems. There are many pleiotropic genes, some examples of
which are:
Marfan syndrome: fibrillin mutation causing ocular, skeletal, and cardiovascular
problems (especially mitral valve prolapse and dilation of ascending aorta).
Cystic fibrosis: mutations in the CFTR gene reduce chloride ion transport across the
apical surface of some epithelial cells. This leads to elevated sweat chloride, pancreatic
insufficiency (in most cases), and production of thick mucus in the airway.
Neurofibromatosis type 1: mutations in the NF1 gene (encoding neurofibromin) can
cause Lisch nodules, neurofibromas, skeletal defects (e.g., pseudarthrosis, sphenoid wing
dysplasia), malignancies, hypertension, cafe-au-lait spots.
Osteogenesis imperfecta: Mutations in type 1 procollagen lead to brittle bones, hearing
loss, blue sclerae (in some cases).
(Down syndrome was also accepted as an answer, although Down syndrome is not a
single-gene disorder.)
Question 10: Hereditary diseases often present with no previous family history of the
disorder. Briefly describe three situations in which you would be most likely to observe a
genetic disorder for which there is no previous family history of the disease phenotype
(three brief sentences should be sufficient).
Answer: Some possibilities:
Autosomal recessive diseases usually have no previous history (consanguinity may be
present, but usually is not for relatively common autosomal recessive disorders).
New mutation transmitted by one of the parents.
Germline mosaicism in one of the parents.
Most chromosome abnormalities (aneuploidies, translocations, deletions, duplications)
have no previous family history.
Reduced penetrance can produce no previous history, although the penetrance would
have to be very low.
Anticipation is a possibility, although all diseases in which anticipation has been
observed are dominant and thus they usually do have a previous family history.
Mosby items and derived items © 2010, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
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