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Week 1 - Medical Genetics > Week 1 - Medical Genetics > Flashcards

Week 1 - Medical Genetics Flashcards

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1
Q

What is an allele?

A

A variant of a gene that occupies the same locus on a chromosome.

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2
Q

List the typical Mandelian inheritance.

A
  • autosomal dominant
  • autosomal recessive
  • x-linked dominant
  • x-linked recessive
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3
Q

What is autosomal dominant inheritance?

A

A copy of the allele of one of the autosomal chromosomes is sufficient for a phenotypic expression.

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4
Q

Features of autosomal dominant inheritance?

A
  • equal chances among male and female offsprings
  • vertical mode of transmission
  • incomplete penetrance ( not everyone with mutation will show the clinical features. Eg: BRCA2 mutation indicates only risk of Breast cancer ; needs FGFR2 mutation)
  • variable expression ( different manifestation although mutation is present )
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5
Q

What is a modifier gene?

A

A gene that affects the phenotypic expression of another gene.

Eg: FGFR2 and BRCA2

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6
Q

Examples of autosomal dominant inheritance?

A
  1. Achondroplasia (FBGF3 constant activation)
  2. Hungtington’s disease
  3. Myotonic dystrophy
  4. Inherited breast, colorectal cancer
  5. Hypertrophic cardiomyopathy
  6. Long QT syndrome
  7. Adult polycystic kidney disease
  8. Marfan’s syndrome
  9. Familial hypercholesterolnaemia
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7
Q

Define autosomal recessive inheritance.

A

A mode of inheritance which the phenotypic expression is present when both copies of the alleles are identical (homozygousity).

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8
Q

Features of autosomal recessive inheritance.

A
  • equal chances of male and female offsprings being affected
  • horizontal pattern
  • consanguinity in the family
  • both parents must be carriers for an offspring to be affected
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9
Q

What is compound heterozygousity?

A

A condition when 2 heterogenous recessive alleles are present and causes a disease

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10
Q

Examples of autosomal recessive inheritance?

A
  1. Sickle cell anaemia
  2. Congenital adrenal hyperplasia
  3. Cystic fibrosis
  4. Phenylketonuria
  5. Haemochromatosis
  6. Alpha 1 Antitrypsin deficiency
  7. Spinal muscular dystrophy
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11
Q

Define x-linked recessive inheritance.

A

Mutation of a gene on the X chromosome causes the phenotype to be expressed in males whom are hemizygous for the gene mutation and females whom are homozygous

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12
Q

Define lyonization/ x - inactivation

A

The process of turning off one of the 2 x chromosomes in females during early embryonic stage.

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13
Q

What is skewed x inactivation?

A

The process of turning off different amount of paternal and maternal x chromosomes.

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14
Q

What are Barr bodies?

A

Inactivated X chromosomes.

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15
Q

Features of X-linked recessive inheritance.

A
  1. No male-male transmission (knight’s move)
  2. Mothers are obligate carriers
  3. Sometimes carriers manifest the phenotype due to skewed x inactivation.
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16
Q

Example of x linked recessive inheritance.

A

Duchenne muscular dystrophy

17
Q

What is x linked dominant inheritance?

A

Phenotypes that are expressed in heterozygous females.

18
Q

Features of x linked dominant inheritance?

A
  1. Vertical pattern
  2. Males and females have equal chance of being affected.
  3. No male to male transmission
19
Q

Examples of x linked dominant inheritance?

A
  1. Vitamin D resistant rickets/ x linked hyperphosphataemia (PHEX gene - affects bones, bowing of leg)
  2. Incontinentia pigmenti* - skin rash then pigmentation ( NEMO gene partial deletion)
  3. Rett’s syndrome* (MECP2 mutation - child development to stop at 6th - 18th months of age with regression)
  • Male lethality - spontaneous abortion
20
Q

What is Knight’s move?

A

Male in F1 to male in F3 transmission via a carrier daughter

21
Q

Compare and contrast autosomal dominant and recessive transmission.

A

Similarity :

  1. Males and females have equal chances of being affected.
  2. Severity in males and females are equal.

Dissimilarities:

  1. AD: vertical pattern ; AR: horizontal pattern
  2. AD: heterozygous manifests disease; AR: homozygous / compound heterozygous manifest disease
  3. AD: offsprings have 50% chance of acquiring if one parent is affected; AR: offsprings have 25% chance of acquiring if one parent is a carrier
  4. AD: not related to consanguinity ; AR: associated with consanguinity
  5. AD: variable expression and incomplete penetrance; AR: expression is constant in family
22
Q

Compare and contrast X linked recessive and X linked dominant inheritance.

A

Similarities:

  • no M to M transmission
  • uniform severity in males

Differences:

  1. Pedigree pattern : XR: knight’s move ; XD: vertical pattern
  2. Affected progeny : XR: all daughters are carriers; XD: all daughters are affected
  3. Sex ratio XR: M&raquo_space; F; XD: F:M = 2:1

Week 1 - Medical Genetics (5 decks)

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