Block 7 Trinuc Repeat X Inact & Duchene MCQ's
.JPG "Block 7 Trinuc Repeat X Inact & Duchene MCQ")
Questions and Answers
- 1.
A single Barr body is typical for
- A.
A) a triple-X superfemale
- B.
B) an infertile patient with Klinefelter syndrome
- C.
C) A death row inmate with XYY "murderer chromosome"
- D.
A phenotypic female with androgen insensitivity syndrome ("testicular feminization")
- E.
A female with Turner syndrome
Correct Answer
B. B) an infertile patient with Klinefelter syndromeExplanation
The presence of a single Barr body indicates that only one X chromosome is active in a cell. In individuals with Klinefelter syndrome, there is an extra X chromosome (XXY), resulting in the presence of two X chromosomes. However, one of the X chromosomes becomes inactivated and forms a Barr body. Therefore, a single Barr body is typical for an infertile patient with Klinefelter syndrome.Rate this question:
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- 2.
A boy with Duchenne muscular dystrophy (DMD) was born to parents with no family history of the disease. The most likely explanation for this occurrence is
- A.
A CGG expansion that resulted in the disruption of the promoter of the dystrophin gene
- B.
Infidelity
- C.
A point mutation in the dystrophin gene
- D.
A recombination event in the dystrophin gene that gave rise to a frameshift mutation leading to an untranslatable mRNA
- E.
A translocation that resulted in the disruption of the dystrophin gene
Correct Answer
D. A recombination event in the dystropHin gene that gave rise to a frameshift mutation leading to an untranslatable mRNAExplanation
The most likely explanation for the occurrence of Duchenne muscular dystrophy (DMD) in a boy with no family history of the disease is a recombination event in the dystrophin gene that resulted in a frameshift mutation. This frameshift mutation would disrupt the reading frame of the gene, leading to the production of an untranslatable mRNA. This disruption in the gene's function would then result in the development of DMD. Other options such as CGG expansion, infidelity, and translocation do not typically lead to DMD.Rate this question:
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- 3.
A family is referred to a genetic specialist because of mild mental retardation in several of the school-age children. Laboratory evaluation demonstrates a specific chromosomal breakage site in metaphase studies of lymphocytes cultured with methotrexate. Which of the following chromosomes is most likely to be specifically affected?
- A.
X
- B.
Y
- C.
13
- D.
18
- E.
21
Correct Answer
A. XExplanation
The correct answer is X because it is the only option that represents a sex chromosome. Since the question mentions that several of the school-age children have mild mental retardation, it suggests that the condition is more common in males. This indicates that the genetic abnormality is likely located on the X chromosome, as males only have one X chromosome and any genetic abnormalities on it would be more impactful.Rate this question:
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- 4.
A 43-year-old man is brought to the general medicine clinic by his wife. She states that his memory has progressively deteriorated over the last several years, and that his personality has been changing. On examination, the physician notes abnormal, writhing movements of the man's limbs and hyperreactive patellar reflexes. An MRI of the head reveals a loss of volume in the neostriatum and cortex. A family history reveals that similar symptoms occurred in several members of the patient's family. Which of the following genetic mechanisms has been implicated in this disorder?
- A.
Expanded trinucleotide tandem repeat
- B.
Genomic imprinting
- C.
Large deletion in one gene
- D.
Single amino acid substitution
- E.
Translocation
Correct Answer
A. Expanded trinucleotide tandem repeatExplanation
The correct answer is expanded trinucleotide tandem repeat. This is because the patient's symptoms, such as memory deterioration, personality changes, abnormal movements, and hyperreactive reflexes, are consistent with Huntington's disease, which is caused by an expanded trinucleotide repeat (CAG) in the huntingtin gene. This repeat expansion leads to the production of a mutant huntingtin protein, which causes neurodegeneration in the neostriatum and cortex. A family history of similar symptoms further supports the involvement of a genetic mechanism, specifically the expanded trinucleotide tandem repeat.Rate this question:
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- 5.
A 3 year-old boy is brought to a physician because the mother notices that the child is engaging in less active play and tires easily. During physical examination, the pediatrician notices that the child's thighs are larger than normal for age and that the child cannot stand up without using his arms to help. Further studies demonstrate a defective dystrophin gene in the boy. Which of the following people in the child's family is most likely to also have this disease?
- A.
Father
- B.
Father's brother
- C.
Mother
- D.
Mother's brother
- E.
Sister
Correct Answer
D. Mother's brotherExplanation
X-Linked Recessive Inheritance: Females possessing one X-linked recessive mutation are considered carriers and will generally not manifest clinical symptoms of the disorder. All males possessing an X-linked recessive mutation will be affected (males have a single X-chromosome and therefore have only one copy of X-linked genes). All offspring of a carrier female have a 50% chance of inheriting the mutation. All female children of an affected father will be carriers (daughters possess their fathers' X-chromosome).
X-linked traits are maternally inherited from carrier mothers or from an affected father. Each son born to a carrier mother has a 50% probability of inheriting the X-chromosome carrying the mutant allele. There are a few Y-linked traits; these are inherited from the father
Duchenne muscular dystrophy (DMD) is a recessive X-linked form of muscular dystrophy, which results in muscle degeneration, difficulty walking, breathing, and death. The incidence is around 1 in 3,600 boys. Females and males are affected, though females are rarely affected and are more often carriers. The disorder is caused by a mutation in the dystrophin gene, located in humans on the X chromosome (Xp21).
Males have only one X chromosome, so one copy of the mutated gene will cause DMD. Fathers cannot pass X-linked traits on to their sons, so the mutation is transmitted by the mother.
If the mother is a carrier, and therefore one of her two X chromosomes has a DMD mutation, there is a 50% chance that a female child will inherit that mutation as one of her two X chromosomes, and be a carrier. There is a 50% chance that a male child will inherit that mutation as his one X chromosome, and therefore have DMDRate this question:
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- 6.
A 13-year-old boy is brought to a rural clinic because of poor school performance. His parents state that he did not begin talking until after three years of age, and still does not use language as effectively as his sister, who is 6-years-old. A careful family history reveals that a maternal grandfather was mildly retarded. The mother has two sisters, both of whom are apparently normal, but the mother admits that she did not do well in school, and dropped out at the age of 16. Physical examination of the child reveals large ears, a long, narrow face, and large testes. Which of the following genetic mechanisms most likely accounts for the observed findings in the son?
- A.
Trisomy 18
- B.
Genomic imprinting
- C.
Robertsonian translocation
- D.
Trisomy 13
- E.
Expanded trinucleotide repeat
Correct Answer
E. Expanded trinucleotide repeatExplanation
Fragile X syndrome (FXS), Martin–Bell syndrome, or Escalante's syndrome (more commonly used in South American countries), is a genetic syndrome that is the most common known single-gene cause of autism and the most common inherited cause of intellectual disability. It results in a spectrum of intellectual disability ranging from mild to severe as well as physical characteristics such as
an elongated face, large or protruding ears, and larger testes, behavioral characteristics such as stereotypical movements (e.g. hand-flapping), and social anxiety.
Fragile X syndrome is associated with the expansion of the CGG trinucleotide repeat
affecting the Fragile X mental retardation 1 (FMR1) gene on the X chromosome, resulting in a failure to express the fragile X mental retardation protein (FMRP), which is required for normal neural development. Depending on the length of the CGG repeat, an allele may be classified as normal (unaffected by the syndrome), a premutation (at risk of fragile X associated disorders), or full mutation (usually affected by the syndrome). A definitive diagnosis of fragile X syndrome is made through genetic testing to determine the number of CGG repeats. Testing for premutation carriers can also be carried out to allow for genetic counseling.Rate this question:
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- 7.
Although II-3 tested negative for the mutation seen in her brother, the physician is concerned regarding the persistent CK level of II-3. The pregnancy was tested again and showed a deletion of exon 45-48 considered consistent with being affected with DMD. The most likely explanation of the results is?
- A.
Non maternity
- B.
Non paternity
- C.
Somatic mosaicism of III-2
- D.
Laboratory error in the testing of II-3
- E.
De Novo mutation in III-2
Correct Answer
D. Laboratory error in the testing of II-3Explanation
The most likely explanation for the results is a laboratory error in the testing of II-3. This is because II-3 initially tested negative for the mutation seen in her brother, but the pregnancy test showed a deletion of exon 45-48, which is consistent with being affected with DMD. This discrepancy suggests that there may have been an error in the testing process for II-3, leading to the incorrect initial result.Rate this question:
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- 8.
Triplet repeat expansions (CGG) in Fragile X syndrome interferes by?
- A.
Decreasing mRNA synthesis
- B.
Increasing mRNA synthesis
- C.
Creating splicing defect
- D.
Gain of function mutation
- E.
Dominant negative mutation
Correct Answer
A. Decreasing mRNA synthesisExplanation
Prevalence 1 in 4000 in males and 1 in 8000 in females
FRM1 gene encodes for a RNA-binding protein termed FMRP that is expressed in many cells including neurons. Loss of function mutation
99% of the FRM1 mutations are due to expansions of a (CGG)n repeat located at the 5’ end of the gene CGG Repeat expansion
(slide 17- trinuc repts-Blanch)Rate this question:
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- 9.
The following pedigree shows a family segregating for Duchenne Muscular Dystrophy (DMD). Individual II-1 was found to have a deletion of exons-45-48 of the dystrophin gene. Individual 11-3 is currently pregnant with a boy. She shows an elevated CK but she tested negative for this mutation. The risk for the pregnancy 111-2 to have DMD is closest to?
- A.
0%
- B.
10%
- C.
50%
- D.
75%
- E.
100%
Correct Answer
A. 0%Explanation
If mother tested negative for allele, she cannot pass on what she does not have.Rate this question:
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- 10.
A dominantly inherited trait affects a child and his grandmother, but neither parent. This best illustrates which of the following principles?
- A.
Variable expressivity
- B.
New mutation
- C.
Somatic mosaicism
- D.
Non-penetrance
Correct Answer
C. Somatic mosaicismExplanation
50% of the females who inherit a full mutation will have Fragile X
Full mutation are mitotically unstable, some patients have a mixture of cells ranging from premutations to full mutation (somatic mosaicism)
Female carriers of premutations (but not full mutation) are at risk (20%) of premature ovarian failure (POF)
POF is NOT early menopause, women develop symptoms similar to menopause. Women can still be pregnant because their ovaries may occasionally work
Male carriers of an expanded , but unmethylated premutation of the Fragile X gene are at risk of Tremor/ Ataxia Syndrome (FXTAS)
FXTAS manifests progressive cerebellar ataxia and tremor as well as Parkinsonism
Affected individuals are mostly men over the age of 50
(slide 20- trinuc repts-Blanch)Rate this question:
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- 11.
Characteristic of Fragile X syndrome
- A.
The gene is located at the end of the short arm of the X and the Y chromosomes
- B.
Males and females are affected equally
- C.
The mutation involves expansion of a triplet repeat and anticipation occurs when transmitted by a male
- D.
The mutation involves expansion of a triplet repeat and anticipation occurs when transmitted by a female
- E.
The mutation exerts a gain of function effect
Correct Answer
D. The mutation involves expansion of a triplet repeat and anticipation occurs when transmitted by a femaleExplanation
Question 11
Anticipation corresponds to progressively earlier onset and increased severity in successive generations
Mild symptoms in the first generation to severe symptoms in successive generation
What is the general mechanism ?
Stepwise expansion of unstable triplet repeats over generations
Anticipation is a hallmark of several trinucleotide repeat expansion diseases
(slide 9- trinuc repts-Blanch)Rate this question:
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- 12.
Evidence that Huntington disease is due to a "gain of function" mutation in the gene which codes for the huntington protein, comes from:
- A.
The fact that disease-causing mutations in this gene involve expansions rather than contractions of a trinucleotide repeat.
- B.
The fact that there is a correlation between numbers of trinucleotide repeats in this gene and the age of onset of Huntington disease
- C.
The larger molecular weight protein coded by the mutant gene being inactive
- D.
The finding that a translocation involving a breakpoint in this gene does not lead to Huntington disease
Correct Answer
B. The fact that there is a correlation between numbers of trinucleotide repeats in this gene and the age of onset of Huntington diseaseExplanation
Gene located on chromosome 4p called “huntingtin” is ubiquitously expressed
Gene product confers toxicity to the cell: Gain of function mutation
Disease causing mutations expansion of (CAG)n repeat in exon1 encoding for polyglutamine
Normal HD alleles have 10- 26 CAG repeats whereas mutant alleles have more than 36 repeats
Mutable alleles are from 27—35 CAG repeats and all the patients inherit this allele from their father
Alleles from 36-39 CAG repeats show reduced penetrance, meaning that some patients will develop the disease and some will not
(slide 25- trinuc repts-Blanch)Rate this question:
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- 13.
Fragile X syndrome is caused by the dramatic expansion of a CGG sequence in the 5' untranslated region of the FMR1 gene. Which of the following is the consequence of this expansion?
- A.
Too much FMR1 gene product is synthesized
- B.
The expansion causes a frameshift mutation, and the resulting gene product cannot function
- C.
The 5' untranslated region is hypermethylated when the repeats reach a certain size, leading to the loss of transcription of the gene.
- D.
Expansion of the trinucleotide repeats produces a toxic "gain of function" mutation
- E.
The expansion causes a nonsense mutation and the gene product is not produced
Correct Answer
C. The 5' untranslated region is hypermethylated when the repeats reach a certain size, leading to the loss of transcription of the gene.Explanation
The expansion of the CGG sequence in the 5' untranslated region of the FMR1 gene leads to hypermethylation of this region when the repeats reach a certain size. Hypermethylation of the 5' untranslated region results in the loss of transcription of the gene. This means that the FMR1 gene is not transcribed into mRNA, and therefore, no FMR1 gene product is synthesized.Rate this question:
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- 14.
SRY, like other genetic switches, has a DNA-binding domain, consistent with its function during gonadal development as a:
- A.
Receptor
- B.
Transcription factor
- C.
Metabolic enzyme
- D.
Structural protein
Correct Answer
B. Transcription factorExplanation
PAR1 at distal of Xp, there is a obligatory cross-over within a 2.6 Mb region for proper pairing and segregation of X and Y
The Sex-Determining Region (SRY) is a gene that encodes a transcription factor that is a member of the high mobility group HMG-box family of DNA binding proteins
SRY is just proximal to PAR1 in Y specific region. Unequal recombination could lead to XX males and XY females (see next slide)
The main function of the SRY protein is to initiate male sex determination and male development.
(slide 15- X-inact-Blanch)Rate this question:
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- 15.
The mother of a boy with molecularly proven Duchenne muscular dystrophy has mild weakness and an elevated serum creatine kinase (CK). What most likely accounts for this?
- A.
X-inactivation (Lyonization)
- B.
She has testicular feminization syndrome
- C.
She is 45,X
- D.
She is 47,XXX
- E.
She is 47,XXY
Correct Answer
A. X-inactivation (Lyonization)Explanation
There are an estimated 2000 - 5000 genes on the X chromosome.
Females normally have two X chromosomes; males, an X and a Y
To "balance out" the sex chromosome to autosome ratio in males and females,
one X chromosome in the female is inactivated. (All X's in excess of one are inactivated in all individuals).
This "balancing" is called "dosage compensation" and the theory behind X inactivation is named the "Lyon hypothesis" after the geneticist Mary Lyon.
The elements of the Lyon hypothesis are:
a) In normal females only one of the 2 X chromosomes is genetically active. The other is inactivated.
b) X inactivation occurs early in embryogenesis.
c) Either X chromosome; the maternal or the paternal is inactivated. The choice is random and independent in each cell.
d) Once X inactivation occurs in a cell, it is irreversible (in somatic cells) and all of the descendants of that cell will have the same X inactivated.
(slides 6,7- X-inact-Blanch)Rate this question:
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- 16.
If there is a family history of genetic disorders, knowing the gender of an unborn child can be important because
- A.
Male children are more likely to have autosomal defects show up in their phenotypes
- B.
Female children are more likely to have autosomal defects show up in their phenotypes
- C.
Male children are more likely to have X-linked traits show up in their phenotype
- D.
Answer A and C are correct
Correct Answer
C. Male children are more likely to have X-linked traits show up in their pHenotypeExplanation
Knowing the gender of an unborn child can be important in the case of a family history of genetic disorders because male children are more likely to have X-linked traits show up in their phenotype. This is because males have only one X chromosome, and if that chromosome carries a recessive gene for a disorder, it is more likely to be expressed in their phenotype. On the other hand, females have two X chromosomes, so even if one carries a recessive gene, the other X chromosome may carry a dominant gene that masks the effects of the recessive gene.Rate this question:
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- 17.
An exchange of fragments of chromatids between non-homologous chromosomes may occur during the first meiotic division. This chromosomal structural abnormality is called:
- A.
Deletion
- B.
Inversion
- C.
Nondisjunction
- D.
Segregation
- E.
Translocation
Correct Answer
E. TranslocationExplanation
Translocation is the correct answer because it refers to the exchange of fragments of chromatids between non-homologous chromosomes during the first meiotic division. This chromosomal structural abnormality can result in genetic disorders and can lead to the rearrangement of genetic material.Rate this question:
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- 18.
A 4-year-old boy is brought to the physician by his parents because of a 4-month history of difficulty running and frequent falls. His parents report that his calves have been gradually increasing in size during this period. Examination shows diffusely enlarged muscles of the calves and lumbar lordosis. Sensation is intact. He has difficulty arising from a supine position. Which of the following is the most likely diagnosis?
- A.
Duchenne muscular dystrophy
- B.
Juvenile rheumatoid arthritis
- C.
Lumbosacral radiculopathy
- D.
Rhabdomyosarcoma
- E.
Spina bifida
Correct Answer
A. Duchenne muscular dystropHyExplanation
The patient's presentation of difficulty running, frequent falls, gradually increasing size of the calves, and lumbar lordosis is consistent with Duchenne muscular dystrophy. This X-linked recessive disorder primarily affects boys and is characterized by progressive muscle weakness and wasting. The diffusely enlarged muscles of the calves, known as pseudohypertrophy, result from the replacement of muscle fibers with fibrous and fatty tissue. The difficulty arising from a supine position is due to weakness of the proximal muscles. Sensation is intact, ruling out other conditions such as radiculopathy or spina bifida. Juvenile rheumatoid arthritis and rhabdomyosarcoma are less likely diagnoses given the clinical presentation.Rate this question:
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- 19.
A 28 year-old African American female presents to your OB/GYN clinic to discuss her Risk to be carrier for Duchenne/Becker muscular dystrophy. She is currently 9 weeks pregnant. The patient reports an innocent mummur which was evaluated by a cardiologist and does not require follow-up. She has otherwise normal medical history and normal pregnancy to date. The patient has one healthy 2 year-old daughter who has a history of recurrent ear infections but otherwise healthy. Patient reports having a 25 year-old sister who is in good health and does not have yet children. The patient has a maternal uncle who died at the age of 16, due to a form of muscular dystrophy with progressive, severe “muscle weakness” and respiratory difficulty. The patient also reports that her maternal aunt has a 37 year-old healthy daughter, a 34 year-old healthy son and a son who died at the age of 18. This cousin to your patient was considered by the family to have the same disease as the uncle. The cousin was unable to walk, and died for cardiac and respiratory complications. Your patient’s partner does not have a family history of any form of muscular dystrophy and the couple denies any consanguinity What is the most likely diagnosis for your patient’s family?
- A.
Down syndrome
- B.
Duchenne Muscular dystrophy
- C.
Myotonic Dystrophy
- D.
Huntinton disease
Correct Answer
B. Duchenne Muscular dystropHyExplanation
Based on the patient's family history, the most likely diagnosis for her family is Duchenne Muscular Dystrophy. The patient's maternal uncle had progressive muscle weakness and respiratory difficulty, which are characteristic symptoms of Duchenne Muscular Dystrophy. Additionally, her cousin had similar symptoms and died from cardiac and respiratory complications. Duchenne Muscular Dystrophy is an X-linked genetic disorder that primarily affects males, but carriers (such as the patient) can also have symptoms or be at risk of passing the condition to their children.Rate this question:
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- 20.
A 28 year-old African American female presents to your OB/GYN clinic to discuss her Risk to be carrier for Duchenne/Becker muscular dystrophy. She is currently 9 weeks pregnant. The patient reports an innocent mummur which was evaluated by a cardiologist and does not require follow-up. She has otherwise normal medical history and normal pregnancy to date. The patient has one healthy 2 year-old daughter who has a history of recurrent ear infections but otherwise healthy. Patient reports having a 25 year-old sister who is in good health and does not have yet children. The patient has a maternal uncle who died at the age of 16, due to a form of muscular dystrophy with progressive, severe “muscle weakness” and respiratory difficulty. The patient also reports that her maternal aunt has a 37 year-old healthy daughter, a 34 year-old healthy son and a son who died at the age of 18. This cousin to your patient was considered by the family to have the same disease as the uncle. The cousin was unable to walk, and died for cardiac and respiratory complications. Your patient’s partner does not have a family history of any form of muscular dystrophy and the couple denies any consanguinity Before any testing is performed what is your patient’s risk to be carrier for this condition?
- A.
100%
- B.
50%
- C.
25%
- D.
0%
Correct Answer
C. 25%Explanation
The patient's risk to be a carrier for Duchenne/Becker muscular dystrophy is 25%. This is because she has a maternal uncle who died from the disease and a cousin who also had the disease. The fact that her sister and partner do not have a family history of muscular dystrophy does not change her risk, as the disease can be inherited from either the mother or the father. Therefore, there is a 25% chance that she is a carrier for this condition.Rate this question:
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- 21.
A 28 year-old African American female presents to your OB/GYN clinic to discuss her Risk to be carrier for Duchenne/Becker muscular dystrophy. She is currently 9 weeks pregnant. The patient reports an innocent mummur which was evaluated by a cardiologist and does not require follow-up. She has otherwise normal medical history and normal pregnancy to date. The patient has one healthy 2 year-old daughter who has a history of recurrent ear infections but otherwise healthy. Patient reports having a 25 year-old sister who is in good health and does not have yet children. The patient has a maternal uncle who died at the age of 16, due to a form of muscular dystrophy with progressive, severe “muscle weakness” and respiratory difficulty. The patient also reports that her maternal aunt has a 37 year-old healthy daughter, a 34 year-old healthy son and a son who died at the age of 18. This cousin to your patient was considered by the family to have the same disease as the uncle. The cousin was unable to walk, and died for cardiac and respiratory complications. Your patient’s partner does not have a family history of any form of muscular dystrophy and the couple denies any consanguinity Based on your patient’s risk to be a carrier, what is her chance that the pregnancy could be affected ?
- A.
100%
- B.
50%
- C.
25%
- D.
6%
- E.
12.5%
- F.
0%
Correct Answer
D. 6%Explanation
Explantion 25% divided by 4Rate this question:
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.JPG "Although II-3 tested negative for the mutation seen in her brother, the physician is concerned regarding the persistent CK level of II-3. The pregnancy was tested again and showed a deletion of exon 45-48 considered consistent with being affected with DMD. The most likely explanation of the results is?")
.JPG "The following pedigree shows a family segregating for Duchenne Muscular Dystrophy (DMD). Individual II-1 was found to have a deletion of exons-45-48 of the dystrophin gene. Individual 11-3 is currently pregnant with a boy. She shows an elevated CK but she tested negative for this mutation. The risk for the pregnancy 111-2 to have DMD is closest to?")
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Mar 21, 2023Quiz Edited by
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