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Y2 GENETICS IN MEDICINE > Basics of Clinical Genetics > Flashcards

Basics of Clinical Genetics Flashcards

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Biology 101 flashcards
1
Q

What are the 3 points of the aetiology triangle?

A
  1. 100% environmental
  2. single gene
  3. polygenic
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2
Q

What is true about the cause of any disease, relating to the aetiology triangle?

A

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

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3
Q

What is achondroplasia and what causes it?

A

a form of short-limbed dwarfism

it is caused by a single gene, but different heights

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4
Q

What are the characteristics of a disease that is mainly caused by genetics?

A
  1. rare
  2. genetics are simple
  3. unifactorial
  4. high recurrence rate
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5
Q

What are the characteristics of a disease that is mainly caused by environmental factors?

A
  1. common
  2. genetics are complex
  3. multifactorial
  4. low recurrence rate
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6
Q

What is the difference between unifactorial and multifactorial?

A

Unifactorial means a condition is dependent on a single gene

Multifactorial means a condition is dependent on multiple factors, both genetic and environmental

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7
Q

What is the definition of a multifactorial/complex genetic disorder?

A

the interaction of multiple genes (genetic predisposition) in combination with environmental factors

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8
Q

What are 2 common examples of multifactorial conditions?

A

type II diabetes

ischemic heart disease

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9
Q

What is meant by a single gene genetic disorder?

What pattern does it show?

A

a condition caused by a mutation in a single gene

shows Mendelian inheritance

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10
Q

What is an example of a single gene disorder?

A

cystic fibrosis

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11
Q

What is meant by a chromosomal genetic disorder?

A

an imbalance or rearrangement in chromosome structure

e.g. aneuploidy, deletion, translocation

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12
Q

What is meant by a mitochondrial genetic disorder?

A

a condition caused by a mutation in mitochondrial DNA

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13
Q

What is meant by a genetic disorder caused by a somatic mutation?

A

mutation (s) within a gene (s) in a defined population of cells that results in disease

e.g. breast cancer

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14
Q

What are the 4 single gene modes of inheritance?

A
  1. autosomal dominant
  2. autosomal recessive
  3. x-linked
  4. mitochondrial
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15
Q

What is the nomenclature used to visualise inheritance patterns?

A

A = healthy gene/chromosome

a = gene/chromosome with disease mutation

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16
Q

What would someone with an AA genotype be?

A

a healthy person with 2 “normal” genes

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17
Q

What would someone with an Aa genotype be?

A

a healthy person with 1 normal gene and 1 gene with the disease mutation

this person is a carrier

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18
Q

How is a disease with autosomal dominant inheritance passed between generations?

A

the trait/disease runs from one generation to the next

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19
Q

Are males or females more likely to be affected by an autosomal dominant condition?

A

males and females are equally affected

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20
Q

What is the chance of the offspring of an individual with an autosomal dominant condition inheriting the condition?

A

the offspring has a 1 in 2 (50%) chance of inheriting the mutation

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21
Q

What types of molecule does the mutation usually affect in autosomal dominant inheritance?

A

structural proteins, receptors and transcription factors

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22
Q

What are examples of conditions caused by autosomal dominant inheritance?

A
  1. myotonic dystrophy
  2. Marfan syndrome
  3. Huntingdon disease
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23
Q

What is DiGeorge syndrome as an example of autosomal dominant inheritance?

A

It is caused by chromsome deletion (22q11 deletion syndrome)

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24
Q

What is meant by ‘penetrance’?

A

the extent to which a particular gene/set of genes is expressed in the phenotypes of individuals carrying it

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25
Q

How is penetrance measured?

A

the percentage of carriers of a gene/set of genes showing the characteristic phenotype

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26
Q

What is meant by incomplete penetrance?

A

not all relatives who inherit a mutation will develop the disorder and express the characteristic phenotype

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27
Q

How may penetrance change throughout a lifetime?

A

Penetrance may alter with age

e.g. by 80 years, Huntingdon disease shows 100% penetrance

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28
Q

What is meant by expressivity?

A

The variation in expression of a gene/set of genes

It is the extent to which a heritable trait is manifested by an individual

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29
Q

What is meant by anticipation?

A

the symptoms of a genetic disorder become apparent at an earlier age as it is passed from one generation to the next

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30
Q

What are examples of diseases that show anticipation?

How do the symptoms vary as the disease is passed from one generation to the next?

A

myotonic dystrophy, huntington’s disease

there is an increase in the severity of the symptoms as they are passed from one generation to the next

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31
Q

What is meant by a ‘de novo’ or ‘new dominant’ mutation?

A

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

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32
Q

How can a parent’s blood cells be tested for a de novo mutation?

A

THEY CAN’T

the parents are NOT affected so the mutation is not detected in their blood cells

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33
Q

If someone has been affected by a de novo mutation, can it be passed on to their offspring?

A

YES

They are the first to be affected in the family, but can pass the mutation on to their offspring

34
Q

How is a disease with autosomal recessive inheritance passed between generations?

A

The disease is seen in one generation

It does NOT pass from one generation to the next

(parents are usually unaffected)

35
Q

What is the risk of the offspring of an individual with an autosomal recessive condition inheriting the condition?

A

They have a low risk of inheriting the condition unless in a consanguineous relationship

36
Q

How does AR inheritance affect chromosomes?

A

It doesn’t - it involves gene mutations only

37
Q

If 2 carriers for an AR condition have children, what is the chance that their child will have the condition?

A

25% chance (1 in 4) that child will be affected

There is a 50% chance (1 in 2) that the child will be a carrier

There is a 25% chance that they will be unaffected and not a carrier

38
Q

What are the 4 main autosomal recessive conditions?

A
  1. cystic fibrosis
  2. many of the metabolic disorders
  3. haemachromatosis
  4. sickle cell disease
39
Q

What are the differences in the ways males and females are affected by X-linked disorders?

A

Males are affected - usually more severely than females

Females may be unaffected or mildly through to fully affected

40
Q

What types of things can cause X-linked disorders?

A
  1. gene mutations
  2. chromosome deletions
  3. chromosome duplications
41
Q

What are 4 examples of common X-linked conditions?

A
  1. Duchenne muscular dystrophy
  2. Fragile X syndrome
  3. Red/green colour-blindness
  4. haemophilia
42
Q

What would the xX and xY genotypes mean in terms of an X-linked disorder?

A

xX - unaffected carrier female

xY - affected male (they only have 1 X chromosome)

43
Q

If a male has an X-linked disorder, what are the chances of his child inheriting it?

A

All the daughters will be carriers as they inherit the mutated gene from the father

All the sons are unaffected as there is NO male to male transmission

44
Q

How severe are the symptoms experienced by female carriers for an X-linked condition?

A

Most female carriers are asymptomatic or have mild symptoms

Sometimes they can have significant symptoms

45
Q

What are the 2 main factors influencing expression of the phenotype in a carrier female for an X-linked disorder?

A
  1. X inactivation

2. X-linked dominant vs. X-linked recessive inheritance

46
Q

Why is prenatal testing not used for X-linked disorders?

A

Often cannot predict a female phenotype accurately on prenatal testing

47
Q

What is meant by X inactivation?

What is an alternative name for this process?

A

Also called Lyonisation

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

48
Q

Why does X inactivation occur?

A

to compensate for the presence of the double X gene dose

49
Q

When does X inactivation occur?

Which cells does it occur in?

A

It occurs in early embryogenesis

It occurs in all cells where there is 2 or more X chromosomes

50
Q

What happens to the inactivated X chromosome throughout life?

A

It remains inactive throughout the lifetime of the cell and all its descendants

51
Q

Are any genes still active once an X chromosome has been inactivated?

A

Most of the genes are switched off on the inactivated X chromosome, but NOY ALL

52
Q

In a female carrier for an X-linked condition, what proportion of X chromosomes express the normal gene?

A

approximately 50% of cells express the normal gene

This is due to X-inactivation

53
Q

What is meant by ‘skewed X-inactivation’?

A

there is a random preference for “normal” X chromosome to be inactivated

this leads to a significant phenotype

54
Q

What is meant by ‘tissue variability’?

A

there is a random preference for the X chromosome with the mutation to be active in a crucial tissue group

e.g. muscle in Duchenne muscular dystrophy

55
Q

What are 2 examples of X-linked dominant conditions?

A
  1. Rett syndrome

it is lethal in males, so phenotype only seen in females

  1. Fragile X syndrome
56
Q

What mutation causes Rett syndrome?

What does it involve?

A

Mutation in the MECP2 gene

It is a genetic brain disorder that becomes apparent around 6-18 months in females

57
Q

What are the typical symptoms of Fragile X syndrome?

A

developmental problems such as learning disabilities and cognitive impairment

males are usually affected more severely than females

58
Q

What are examples of X-linked recessive conditions?

A
  1. red-green colour blindness
  2. Duchenne muscular dystrophy
  3. haemophilia
59
Q

When will a girl be affected by an X-linked recessive condition?

A

Carrier girls are usually unaffected, but can have significant symptoms due to X-inactivation

Girls will be fully affected if they inherit the mutation from both their mum and dad

60
Q

What are 2 examples of diseases caused by mitochondrial inheritance?

A
  1. maternally inherited diabetes

2. deafness

61
Q

What is significant about mitochondrial inheritance and how mitochondria are inherited?

A

the sperm head does NOT contain any mitochondria

ALL mitochondria are inherited from the mother

62
Q

How many genes are found in mitochondrial DNA?

How many copies of these genes are present?

A

there are only 27 genes within mitochondrial DNA

every mitochondrium has many copies of each gene

63
Q

If a mother is affected by a mitochondrial inherited condition, what % of her offspring will be affected?

A

ALL the children will receive the mutation

Mitochondria are inherited from the mother’s egg

64
Q

If a father is affected by a mitochondrial inherited condition, what % of his offspring will be inherited?

A

ALL of the children will be UNaffected

65
Q

How does the severity of mitochondrial inherited conditions vary between relatives?

A

The severity of the phenotype varies highly between relatives

This is due to the highly variable expressivity

66
Q

When drawing a family pedigree, what should be recorded?

A
  1. full names
  2. dates of birth
  3. maiden names
67
Q

What is the first stage when drawing a family pedigree?

A

Build up the tree from the bottom

It starts with the affected child and their siblings

68
Q

What is the second stage when drawing a family pedigree?

A

Choose one parent

Ask about their siblings, children and then parents

69
Q

How do you show that someone is affected when drawing a family pedigree?

A

The symbol is coloured/filled in for an affected individual

70
Q

How do you show that someone has died when drawing a family pedigree?

A

Draw a line through the symbol

71
Q

How do you show that someone is male or female when drawing a family pedigree?

A

Circles are used for females

Squares are used for males

72
Q

How do you show a stillborn baby of unknown sex on a family pedigree?

A

a diamond with a line through it

73
Q

How do you show therapeutic and spontaneous (miscarriage) abortions on a family pedigree?

A

therapeutic abortion is a triangle with a line through it

miscarriage is a triangle with NO line through it

74
Q

What does a double line between individuals show on a family pedigree?

A

a consanguineous couple

this is a couple who are “blood” relatives

75
Q

How would you show an asymptomatic carrier of a recessive gene mutation (Aa) on a family pedigree?

A

With a dark black circle in the centre of the symbol

76
Q

How would you show identical and non-identical twins on a family pedigree?

A

The symbols sprout from the same branch

Identical twins have a line between the 2 symbols

77
Q

How would you show an unaffected person who’s sex is unknown?

A

a diamond that is not coloured in

78
Q

What is the risk of a congenital birth defect in consanguineous couples?

A

5-6% if there is no history of the genetic condition

2-3% normal

79
Q

What is the risk of consanguineous couples having children?

A

they potentially share recessive gene mutations

80
Q

What is the final stage of drawing a family pedigree?

A
  1. create a key with the symbols used

2. date and sign the pedigree

Y2 GENETICS IN MEDICINE (11 decks)

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