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EMS - GiM > (2) Categories and impact of genetic disease > Flashcards

(2) Categories and impact of genetic disease Flashcards

1
Q

What are the 3 points of the triangle representing aetiology of disease?

A

100% environmental

Single gene

Polygenic

For any condition, the balance of genetic and environmental factors can be represented by a point somewhere within the triangle

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2
Q

What type of condition is achondroplasia?

A

Single gene but different heights

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3
Q

What type of condition is stroke?

A

Multifactorial with environment and polygenic factors - for some there may be single gene influence (Cadasil)

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4
Q

On a scale from 100% genetic to 100% environmental, what are the characteristics of diseases nearer the 100% genetic end?

A
  • rare
  • simple genetics
  • unifactorial
  • high recurrence rate
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5
Q

On a scale from 100% genetic to 100% environmental, what are the characteristics of diseases nearer the 100% environmental end?

A
  • common
  • complex genetics
  • multifactorial
  • low recurrence rate
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6
Q

Give an example of a 100% genetic disease

A

Duchenne muscular dystrophy (single gene - mutation in dystrophin gene)

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7
Q

Give an example of a 100% environmental disease

A

Scurvy

tuberculosis also near this end of the scale

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8
Q

Is osteogenesis imperfecta (OI) more genetic or environmental?

A

In the middle but nearer the 100% genetic end of the scale

Bone fragility caused by genes, expression different on environment

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9
Q

Give 5 classifications of genetic disorders

A
  • multifactorial/complex
  • single gene
  • chromosomal
  • mitochondrial
  • somatic mutations
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10
Q

What are multifactorial/complex genetic disorders?

A

The interaction of multiple genes (genetic predisposition) in combination with environmental factors eg. type II diabetes, ischaemic heart disease

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11
Q

What is a single gene genetic disorder?

A

A mutation in a single gene = Mendelian inheritance - AD, AR, XL eg. cyctic fibrosis

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12
Q

What is a chromosomal genetic disorder?

A

An imbalance or rearrangement in chromosome structure eg. aneuploidy, deletion, translocation

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13
Q

What is a mitochondrial genetic disorder?

A

A mutation in mitochondrial DNA

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14
Q

What is a somatic mutation genetic disorder?

A

Mutation(s) within a gene(s) in a defined population of cells that results in disease eg. breast cancer

Not born with the mutation, mutation not found in the blood but found in the tumour

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15
Q

What are the 4 single gene modes of inheritance?

A
  • autosomal dominant
  • autosomal recessive
  • X-linked
  • mitochondrial
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16
Q

What are the characteristics of autosomal dominant (AD) inheritance?

A
  • a trait or disease runs from one generation to the next
  • males and females equally affected
  • offspring of affected person has a 50% chance of inheriting the mutation
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17
Q

What is the chance of the offspring of an affected parent inheriting an AD disorder?

A

50% (1 in 2)

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18
Q

Autosomal dominant disordes are usually related to what?

A
  • structural proteins
  • receptors
  • transcription factors
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19
Q

Give some examples of autosomal dominant disorders that are caused by gene mutations

A
  • myotonic dystrophy
  • marfan syndrome
  • huntington disease
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20
Q

Give an example of an autosomal dominant disorder that is cause by chromosomal deletion or duplication

A

22q11 deletion syndrome

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21
Q

Are affected individuals of AD disorders homozygous or heterozygous?

A

They are heterozygous for the mutation (one copy of mutation/one normal gene)

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22
Q

What does ‘penetrance’ mean?

A

The frequency with which a specific genotype is expressed by those individuals that possess the mutation, usually given as a percentage.

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23
Q

What is incomplete penetrance?

A

Not all relatives who inherit the mutation develop the disorder

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24
Q

Give an example of a mutation that has incomplete penetrance

A

BRCA1 mutations - 80% life time chance of developing breast cancer

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25
Q

Penetrance may alter with which factor?

A

Age - eg. Huntington disease by 80 years has 100% penetrance

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26
Q

What is ‘expressivity’?

A

Variation in expression - the extent to which a heritable trait is manifested by an individual

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27
Q

Give 2 examples of genetic diseases that vary in expressivity

A
  • Marfan syndrome = 100% penetrance but some have aortic dilatation, lens dislocation, stretch marks etc, others do not
  • BRCA1 mutation = some may have ovarian cancer, some may have breast cancer, some both and some not at all (as it is not 100% penetrant)
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28
Q

What is ‘anticipation’?

A

The symptoms of a genetic disorder become apparent at an earlier age as it is passed from one generation to the next. In most cases, there is an increased in the severity of the symptoms too

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29
Q

Give 2 examples of genetic disorders that show anticipation

A
  • myotonic dystrophy

- huntington’s disease

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30
Q

What is a new dominant/ de novo mutation?

A

A new mutation that has occurred during gametogenesis or in early embryonic development

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31
Q

Are the parents of those with new dominant/ de novo mutations affected?

A

No

The parents are not affected and the mutation is not detected in their blood cells. The child is the first to be affected in the family but can pass the mutation on to their own children

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32
Q

What are the characteristics of autosomal recessive (AR) inheritance?

A
  • disease seen in one generation
  • does not tend to pass from one generation to the next and so parents usually unaffected
  • offspring of affected individuals has low risk of disease unless in consanguineous relationship
  • relatives may be asymptomatic carriers of the disease
  • affects males and females equally
  • gene mutations, not chromosomes
33
Q

Are those with autosomal recessive disorders homozygous or heterozygous?

A

Affected individuals are homozygous or compound heterozygous (two different mutations but they work together to give the disease)

34
Q

What is the risk of offspring being affected if both parents are carriers of an AR disease?

A

1/4 (25%)

35
Q

What is the risk to unaffected offspring being a carrier? (when both parents are heterozygous carriers of AR disorder)

A

2/3

36
Q

Give some examples of autosomal recessive disorders

A
  • cystic fibrosis
  • many of the metabolic disorder
  • haemachromatosis
  • sickle cell disease
37
Q

Who is affected by X-linked inheritance?

A
  • males affected
  • females may be unaffected, mildly affected or fully affected
  • males usually more severely affected than females
38
Q

Why is male to male transmission not possible in X-linked disorders?

A

Fathers pass on their Y chromosome to their son

Daughters all carriers as they definitely get their dad’s X chromosome

39
Q

Give some examples of X-linked disorders

A

Gene mutations and chromosome deletions/duplications

  • duchenne muscular dystrophy
  • fragile X syndrome
  • red/green colour-blindness
  • haemophilia
40
Q

What is the chance of a so having haemophilia given that the father is affected with haemophilia?

A

All sons are unaffected (no male to male transmission)

All daughters are carriers

41
Q

In XL carrier females, the phenotype is very variable. What are the 2 main factors influencing expression of phenotype?

A
  • X inactivation

- XL dominant vs. XL recessive inheritance

42
Q

What is another name for X-inactivation?

A

Lyonisation

43
Q

What is X-inactivation/lyonisation?

A

The process of random inactivation of one of the X chromosomes in cells with more than one X chromosome

Compensates for the presence of the double X gene dose

44
Q

What is a Barr body?

A

The inactive X chromosome rendered inactive during lyonisation

(the condensed X chromosome)

45
Q

Is X inactivation normal?

A

It is normal and occurs when there is 2 or more X chromosomes

Occurs in early embryogenesis

46
Q

Which X is silenced during X-inactivation?

A

Random

47
Q

Does the silenced X chromosome remain inactive?

A

Once inactivated, an X chromosome remains inactive throughout the lifetime of the cell and all its descendants

48
Q

Which genes are switched off on the inactivated X chromosome?

A

Most, but not all genes are switched off on the inactivated X chromosome

49
Q

What does skewed X-inactivation mean?

A

There is a random preference for the “normal” X chromosome to be inactivated - significant phenotype

50
Q

What is tissue variability in terms of X-inactivation?

A

There is a random preference for the X chromosome with the mutation to be active in crucial tissue group eg. muscle in Duchenne muscular dystrophy

51
Q

Are XL dominant or XL recessive disorders more common?

A

XL recessive

XL dominant conditions are rare

52
Q

Give an example of an XL dominant condition

A

Rett syndrome

Lethal in males, phenotype only in females

Females only need gene on one of their Xs to show phenotype

53
Q

What type of inherited disorder is fragile X syndrome?

A

XL dominant

Females can be asymptomatic to fully symptomatic (due to X-inactivation pattern)

54
Q

Give 2 XL dominant disorders

A
  • Rett syndrome

- Fragile X syndrome

55
Q

Give examples of XL recessive disorders

A
  • red-green colour blindness
  • haemophilia
  • duchenne muscular dystrophy
56
Q

Are girls affected by XL recessive disorders?

A

Carrier girls usually unaffected BUT can have significant symptoms because of X-inactivation switching off the normal X

Girls are fully affected if they inherit the mutation from mum and dad

57
Q

Give examples of mitochondrial inheritance disorders

A
  • maternally inherited diabetes and deafness
58
Q

Where do we inherit mitochondrial DNA from?

A

All mitochondrial DNA comes from the egg

The sperm head does not have any mitochondria

All out mitochondria are inherited from our mother

59
Q

Who is affected by mitochondrial inheritance disorders?

A

Males and females are affected equally

Rare

60
Q

How many genes are there within mitochondrial DNA?

A

27 genes

Every mitochondrion has many copies of each gene

61
Q

How are offspring affected by mitochondrial inheritance?

A

An affected mother will give all of her children the mutation

All the children of an affected man will be unaffected

Highly variable expressivity and therefore severity of phenotype between relatives

62
Q

Why is family history taking important?

A
  • helps make a diagnosis
  • helps clarify risk for the patient
  • helps understand the patient’s point of view
  • helps identify who else may be at risk
  • helps understand the patient’s support network
63
Q

How are males and females represented in a family pedigree?

A

Female = circle

Male = square

64
Q

How do you show a person has died in a family pedigree?

A

Draw a line through the circle/square

65
Q

What information should you record about each person on a family pedigree?

A
  • name
  • maiden name
  • date of birth
66
Q

What does a line through a line connecting two people mean?

A

The couple has split up

67
Q

What are 2 questions you shouldn’t forget to ask when taking a family pedigree? (concerning half siblings and step siblings)

A
  • “do the children all have the same dad?” - half siblings

- “ do either of you have any other children from other relationships?” - step siblings

68
Q

What are 2 questions you shouldn’t forget to ask when taking a family pedigree? (concerning miscarriages)

A
  • “have you had any other pregnancies?”

- “do you know if anyone in the family has had any miscarriages?”

69
Q

How should you represent a person affected by the condition in question?

A

Colour in the square/circle

70
Q

How is a miscarriage (spontaneous abortion) represented in a family pedigree?

A

Triangle

71
Q

How is a therapeutic abortion represented in a family pedigree?

A

Triangle with a line through it

72
Q

How is a stillborn baby of unknown sex represented?

A

Diamond with a line through it

73
Q

How is an asymptomatic carrier of a recessive gene mutation represented in a family pedigree?

A

A coloured in circle in the middle of the square/circle

74
Q

What does a double line connecting 2 people mean in a family pedigree?

A

Consanguineous couple

75
Q

What is consanguinity?

A
  • couples who are “blood” relatives
  • potentially share recessive gene mutations
  • seen in all ethnic groups
76
Q

How does consanguinity change the risk of congenital birth defects?

A

Risk of congenital birth defects = 5-6% compared to 2-3% in the general population

Given that there is no family history of genetic condition

77
Q

How is a person of unknown sex represented in a family tree?

A

A diamond

78
Q

How are twins represented in a family tree?

A

(look at image of this slide 33)

79
Q

What does EDD mean in a family pedigree?

A

Expected date of delivery

EMS - GiM (14 decks)

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