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Y2 GENETICS IN MEDICINE > Basics of Clinical Genetics > Flashcards

Basics of Clinical Genetics 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
How is penetrance measured?
the percentage of carriers of a gene/set of genes showing the characteristic phenotype
26
What is meant by incomplete penetrance?
not all relatives who inherit a mutation will develop the disorder and express the characteristic phenotype
27
How may penetrance change throughout a lifetime?
Penetrance may alter with age e.g. by 80 years, Huntingdon disease shows 100% penetrance
28
What is meant by expressivity?
The variation in expression of a gene/set of genes It is the extent to which a heritable trait is manifested by an individual
29
What is meant by anticipation?
the symptoms of a genetic disorder become apparent at an earlier age as it is passed from one generation to the next
30
What are examples of diseases that show anticipation? How do the symptoms vary as the disease is passed from one generation to the next?
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
31
What is meant by a 'de novo' or 'new dominant' mutation?
a new mutation that has occurred during gametogenesis or in early embryonic development
32
How can a parent's blood cells be tested for a de novo mutation?
THEY CAN'T the parents are NOT affected so the mutation is not detected in their blood cells
33
If someone has been affected by a de novo mutation, can it be passed on to their offspring?
YES They are the first to be affected in the family, but can pass the mutation on to their offspring
34
How is a disease with autosomal recessive inheritance passed between generations?
The disease is seen in one generation It does NOT pass from one generation to the next (parents are usually unaffected)
35
What is the risk of the offspring of an individual with an autosomal recessive condition inheriting the condition?
They have a low risk of inheriting the condition unless in a consanguineous relationship
36
How does AR inheritance affect chromosomes?
It doesn't - it involves gene mutations only
37
If 2 carriers for an AR condition have children, what is the chance that their child will have the condition?
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
What are the 4 main autosomal recessive conditions?
1. cystic fibrosis 2. many of the metabolic disorders 3. haemachromatosis 4. sickle cell disease
39
What are the differences in the ways males and females are affected by X-linked disorders?
Males are affected - usually more severely than females Females may be unaffected or mildly through to fully affected
40
What types of things can cause X-linked disorders?
1. gene mutations 2. chromosome deletions 3. chromosome duplications
41
What are 4 examples of common X-linked conditions?
1. Duchenne muscular dystrophy 2. Fragile X syndrome 3. Red/green colour-blindness 4. haemophilia
42
What would the xX and xY genotypes mean in terms of an X-linked disorder?
xX - unaffected carrier female xY - affected male (they only have 1 X chromosome)
43
If a male has an X-linked disorder, what are the chances of his child inheriting it?
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
How severe are the symptoms experienced by female carriers for an X-linked condition?
Most female carriers are asymptomatic or have mild symptoms Sometimes they can have significant symptoms
45
What are the 2 main factors influencing expression of the phenotype in a carrier female for an X-linked disorder?
1. X inactivation | 2. X-linked dominant vs. X-linked recessive inheritance
46
Why is prenatal testing not used for X-linked disorders?
Often cannot predict a female phenotype accurately on prenatal testing
47
What is meant by X inactivation? What is an alternative name for this process?
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
Why does X inactivation occur?
to compensate for the presence of the double X gene dose
49
When does X inactivation occur? Which cells does it occur in?
It occurs in early embryogenesis It occurs in all cells where there is 2 or more X chromosomes
50
What happens to the inactivated X chromosome throughout life?
It remains inactive throughout the lifetime of the cell and all its descendants
51
Are any genes still active once an X chromosome has been inactivated?
Most of the genes are switched off on the inactivated X chromosome, but NOY ALL
52
In a female carrier for an X-linked condition, what proportion of X chromosomes express the normal gene?
approximately 50% of cells express the normal gene This is due to X-inactivation
53
What is meant by 'skewed X-inactivation'?
there is a random preference for "normal" X chromosome to be inactivated this leads to a significant phenotype
54
What is meant by 'tissue variability'?
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
What are 2 examples of X-linked dominant conditions?
1. Rett syndrome it is lethal in males, so phenotype only seen in females 2. Fragile X syndrome
56
What mutation causes Rett syndrome? What does it involve?
Mutation in the MECP2 gene It is a genetic brain disorder that becomes apparent around 6-18 months in females
57
What are the typical symptoms of Fragile X syndrome?
developmental problems such as learning disabilities and cognitive impairment males are usually affected more severely than females
58
What are examples of X-linked recessive conditions?
1. red-green colour blindness 2. Duchenne muscular dystrophy 3. haemophilia
59
When will a girl be affected by an X-linked recessive condition?
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
What are 2 examples of diseases caused by mitochondrial inheritance?
1. maternally inherited diabetes | 2. deafness
61
What is significant about mitochondrial inheritance and how mitochondria are inherited?
the sperm head does NOT contain any mitochondria ALL mitochondria are inherited from the mother
62
How many genes are found in mitochondrial DNA? How many copies of these genes are present?
there are only 27 genes within mitochondrial DNA every mitochondrium has many copies of each gene
63
If a mother is affected by a mitochondrial inherited condition, what % of her offspring will be affected?
ALL the children will receive the mutation Mitochondria are inherited from the mother's egg
64
If a father is affected by a mitochondrial inherited condition, what % of his offspring will be inherited?
ALL of the children will be UNaffected
65
How does the severity of mitochondrial inherited conditions vary between relatives?
The severity of the phenotype varies highly between relatives This is due to the highly variable expressivity
66
When drawing a family pedigree, what should be recorded?
1. full names 2. dates of birth 3. maiden names
67
What is the first stage when drawing a family pedigree?
Build up the tree from the bottom It starts with the affected child and their siblings
68
What is the second stage when drawing a family pedigree?
# Choose one parent Ask about their siblings, children and then parents
69
How do you show that someone is affected when drawing a family pedigree?
The symbol is coloured/filled in for an affected individual
70
How do you show that someone has died when drawing a family pedigree?
Draw a line through the symbol
71
How do you show that someone is male or female when drawing a family pedigree?
Circles are used for females Squares are used for males
72
How do you show a stillborn baby of unknown sex on a family pedigree?
a diamond with a line through it
73
How do you show therapeutic and spontaneous (miscarriage) abortions on a family pedigree?
therapeutic abortion is a triangle with a line through it miscarriage is a triangle with NO line through it
74
What does a double line between individuals show on a family pedigree?
a consanguineous couple this is a couple who are "blood" relatives
75
How would you show an asymptomatic carrier of a recessive gene mutation (Aa) on a family pedigree?
With a dark black circle in the centre of the symbol
76
How would you show identical and non-identical twins on a family pedigree?
The symbols sprout from the same branch Identical twins have a line between the 2 symbols
77
How would you show an unaffected person who's sex is unknown?
a diamond that is not coloured in
78
What is the risk of a congenital birth defect in consanguineous couples?
5-6% if there is no history of the genetic condition | 2-3% normal
79
What is the risk of consanguineous couples having children?
they potentially share recessive gene mutations
80
What is the final stage of drawing a family pedigree?
1. create a key with the symbols used | 2. date and sign the pedigree

Y2 GENETICS IN MEDICINE (11 decks)

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