Updated: 7/10/2018

Genetic Principles

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Principles
  • Codominance
    • both allelic copies are expressed
      • e.g.) blood groups (A, B, AB)
  • Variable expression
    • nature and degree of phenotype vary from 1 individual to another with the same mutation
      • e.g.) 2 patients with neurofibromatosis may have varying disease severity
  • Incomplete penetrance
    • not all individuals with a mutant genotype have diseased phenotype
      • explanation for a dominant disease "skipping" a generation
    • penetrance can be calculated by ( # with symptoms) / (# with disease genotype)
      • must be figured into recurrance calculations
        • if parents have a 50% chance of giving defective gene but the penetrance is 50%
          • 0.5 x 0.5 x100 = 25% of recurrence
    • observed in recessive and dominant diseases
  • Pleiotropy
    • single mutation has diverse effects upon several organ systems
      • e.g.) PKU
  • Anticipation 
    • changes in disease presentation in succeeding generations
      • ↑ severity
      • earlier onset 
    • caused by trinucleotide expansion
      • region of repeating triplets expands in each generation
      • e.g.) Huntington's disease, fragile X, myotonic dystrophy, Friedreich ataxia
  • Loss of heterozygosity
    • "two-hit model"
    • individual inherits or develops a mutation in one copy of gene
      • disease occurs when the complementary allele is lost
    • e.g. tumor suppressor diseases (Li-Fraumeni, retinoblastoma)
  • Dominant negative mutation
    • mutant gene product antagonizes wild-type gene product
      • exerts a dominant effect
      • e.g.) common in multimeric proteins where one mutant subunit can change function of entire enzyme
  • De novo mutation
    • genetic disease in an individual with no familial history
    • recurrence risk for offspring of same parents is low
  • Locus heterogeneity 
    • different mutations can produce the same phenotype
      • e.g., marfanoid habitus caused by
        • Marfan's syndrome, MEN 2B, homocystinuria
  • Heteroplasmy
    • presence of both normal and mutated mitochondrial (mt)DNA in the same cell
      • results in variable expression in mitochondrial inherited disease   
  • Uniparental disomy 
    • offspring receives both copies of a chromosome from 1 parent
      • no copies from the other parent
      • causes disease if the chromosome is usually imprinted
        • see Epigenetics topic
  • Polygenic inheritance 
    • multiple genes are responsible for inheritance of a disease
    • e.g.) androgenic alopecia
  • Heritability
    • can measure the relative effect of genetic vs. environmental factors on a phenotype
      • calculated by phenotypic relationship between dizygotic (DZ) and monozygotic (MZ) twins
      • heritability = (CMZ-CDZ) / (1-CDZ)
        • where C = concordance
          • prevalence of disease in both twins
          • entirely environmental disease should have CMZ = CDZ
          • entirely genetic disease should have CMZ=1.0 and CDZ=0.5
            • siblings share 50% of their genes

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(M1.BC.15.72) A 3-year-old is brought to the pediatrician by his mother because she is concerned about recent changes to his behavior. She states that he has seemed to regress in his motor development and has been having occasional brief episodes of uncontrollable shaking. During the subsequent work up, a muscle biopsy is obtained which demonstrates red ragged fibers and a presumptive diagnosis of a genetic disease made. The mother asks if her other son will be affected. What should be the physician's response?

QID: 106531
1

There is a 100% he will be affected, but the severity may be different

60%

(74/124)

2

There is a 25% chance he will be affected

9%

(11/124)

3

He will be unaffected

4%

(5/124)

4

There is a 100% he will be affected, and the severity will be the same

13%

(16/124)

5

There is a 50% chance he will be affected

11%

(14/124)

M 1 D

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(M1.BC.15.72) A 35-year-old male is brought to the physician by his wife who is concerned because he has begun to demonstrate odd behavior which has worsened over the past several months. She states that he has become very aggressive and at times will have sudden, jerky movements which he is unable to control. The patient states that his father had the same problem which he died of at age 69. The patient had a recent, "cold," with fevers, chills and, "throat pain," which resolved on its own, "some time ago." Which of the following is true of this disease?

QID: 106511
1

A mutation in ATP7B on chromosome 13 is responsible

10%

(3/29)

2

Erythema marginatum is a complication associated with this disease

0%

(0/29)

3

Overactivity of dopamine in the mesolimbic pathway is the underlying pathology

7%

(2/29)

4

Underactivity of dopamine in the nigrostriatal cortex is the underlying pathology

14%

(4/29)

5

It demonstrates anticipation

66%

(19/29)

M 1 E

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(M1.BC.15.73) A mother brings her 3-year-old daughter to the pediatrician because she is concerned about her development. She states that her daughter seemed to regress in her motor development. Furthermore, she states she has been having brief episodes of uncontrollable shaking, which has been very distressing to the family. During the subsequent work-up, a muscle biopsy is obtained which demonstrates red ragged fibers and a presumptive diagnosis of a genetic disease is made. The mother states that she has another 6-year-old son who does not seem to be affected or have any similar symptoms. What genetic term explains this phenomenon?

QID: 106546
1

Allelic heterogeneity

16%

(8/51)

2

Phenotypic heterogeneity

22%

(11/51)

3

Genetic heterogeneity

20%

(10/51)

4

Heteroplasmy

33%

(17/51)

5

Locus heterogeneity

6%

(3/51)

M 1 D

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(M1.BC.14.1) A mother from rural Louisiana brings her 4-year-old son to a pediatrician. Her son is intellectually disabled, and she hopes that genetic testing will help determine the cause of her son's condition. She had previously been opposed to allowing physicians to treat her son, but his impulsive behavior and learning disabilities are making it difficult to manage his care on her own. On exam, the child has a long, thin face with a large jaw, protruding ears, and macroorchidism. The physician also hears a high-pitched holosystolic murmur at the apex of the heart that radiates to the axilla. Which of the following trinucleotide repeats is most likely affected in this individual?

QID: 106988
1

CTG on chromosome 19

11%

(4/36)

2

CAG on chromosome 4

3%

(1/36)

3

GAA on chromomsome 9

6%

(2/36)

4

CGG on the sex chromosome X

72%

(26/36)

5

CTG on chromosome 8

3%

(1/36)

M 1 C

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(M1.BC.14.27) An 8-year-old male child presents to the pediatrician for slight exercise intolerance, headaches, and difficulty with hearing. The patient's father explains that the patient's mother had similar symptoms, however hers were much more severe and also included vision loss and seizures; the mother passed away two years ago as she developed significant dementia and profound weakness. The child was given an exercise test, and after 5 minutes of walking his blood lactate levels were 6 mmol/L (nl < 2 mmol/L). A muscle biopsy was performed and a Gömöri trichrome stain was done on the sample (Figure A). What genetic principle best explains the difference in symptoms seen between the mother and the child?

QID: 107022
FIGURES:
1

Codominance

8%

(3/39)

2

Incomplete penetrance

23%

(9/39)

3

Pleiotropy

5%

(2/39)

4

Anticipation

10%

(4/39)

5

Heteroplasmy

49%

(19/39)

M 1 D

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