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302: Scientific basis of medicine > Genetics > Flashcards

Genetics Flashcards

1
Q

Consanguinity

A

When relatives who are second cousin/ closer, have children.

  • Child is inbred due to sharing common ancestor
  • Increases the likelihood of child inheriting recessive conditions

Due to genetic similarity being greater, the parents are more likely to have same copies of mutated genes
- More likely for child to inherit two copies of the same mutated gene shared by parents.

Probability of offspring inheriting 2 copies of ancestral allele is 0.0156 or higher (F inbred coefficient)

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2
Q

Relatedness of parents and inbreeding coefficient

A

The more related a parent is, the higher the inbreeding coefficient
- More likely to share identical genes

This increases poor outcomes of health as homozygous mutations are more likely to be shared.

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3
Q

Genomic imprinting

A

Process that causes genes on homologous chromosome to be expressed in a parent-of-origin/ parent-specific manner.
- Modification= some of genes in the allele from one parent is turned off

Causes of this is hypothesised in ‘parental conflict hypothesis’
- Imprinting occurs due to differing interest of each parent for evolutionary fitness

  • I.e. growth promotion more expressed for father
    Example
  • IGF2 is only expressed from allele inherited from father.
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4
Q

Imprinting centres

A

Areas of the genes that control genomic imprinting.

Methylation of the gene switches it off.

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5
Q

Prader willi syndrome

A

Disease caused by mutation of epigenetic genes - genomic imprinted genes

Paternal copies of the active gene is deleted whilst maternal copies are silent gene on chromosome 15.

  • Located on long arm (15q11-13)
  • Protein is unable to be produced

Phenotype

  • Mental retardation
  • Life expectancy around 30
  • Hypotonia
  • Obesity
  • Male hypogenitalism
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6
Q

Angelman syndrome

A

Imprinting genetic condition caused by deletion of maternally expressed genes on chromosome 15

  • Deletion on long arm (15q11-13)
  • Paternal gene is silent.

Phenotype

  • Mental and growth retardation retardation
  • Lack of speech
  • Hyperactivity
  • Inappropriate laughter
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7
Q

Autosomal dominant disorders

A

Disorders where mutations causes modification of the gene’s function/ expression.
- Inheriting the mutation means that the disorder is inherited.

Examples

  • Huntington’s chorea
  • Acondroplasia
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8
Q

Autosomal recessive disorders

A

Disorders where the mutation causes a loss of function for a gene product.
- Due to insertions/ deletions/ premature stop codons/ frameshift mutation.

Examples

  • SCD
  • CF
  • Phenylketonuria
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9
Q

Achondroplasia

  • Description
  • Mutation
A

Disorder that causes abnormality of cartilage formation.
- Due to mutation in fibroblast growth receptor 3 (FGR3)

Autosomal dominant disorder
- Although, most of the cases are ne novo mutations (7/8)

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10
Q

Retinoblastoma

  • Description
  • Inheritance
A

Tumour of the retina that arises due to mutation of Rb gene
- Inherited in autosomal dominant fashion

Has 90% pentrance
- 10% with the mutation are obligate carriers

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11
Q

Obligate carriers

A

Someone who carries a mutation for a disorder but does not express the mutation
- Therefore does not show phenotype for disorder.

The greater the proportion of obligate carriers for a condition, the less the penetrance is for the disorder.

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12
Q

Huntington’s disease

  • Description
  • Genetics
A

Progressive neurodegenerative disease caused by expansion of CAG repeats in the huntington gene, chromosome 4.

  • Polyglutamine disorder= excess glutamine production
  • Unstable repeat number= 36+
  • Repeat numbers increases with age.

Characterised by

  • Dementia
  • Severe depression
  • Chorea

Autosomal dominant disorder
- Age dependent penetrance: age of onset is lower +/ severity is worse in successive generations (anticipation)

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13
Q

Anticipation

A

A genetic disorder in which the symptoms become more apparent with each successive generation it’s passed on to.

  • Can also show an increase in severity of symptoms
  • Common in trinucleotide repeat disorders

Example: Huntington’s, myotonic dystrophy

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14
Q

Age dependent penetrance

A

Genetic mutation that only develops depending on the age.

Example:

  • Huntington’s onset is younger that more the CAG repeats increase
  • Male pattern baldness is X-linked and develops in later years.
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15
Q

Variable expression

A

The degree in which a genotype is phenotypically expressed.
- Multiple people with the same disease can have the same genotype but one may express more severe symptoms, while the other may appear normal.

Example

  • Neurofibromatosis
  • Marfan’s
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16
Q

Neurofibromatosis

  • Description
  • Mutation
  • Types
  • Genetics
A

Autosomal Dominant conditions that causes tumours of the nervous system.
- NF1 mutation on chromosome 17, a tumour suppressor gene

Types

  • Type 1, 2 and schwannomatosis
  • The severity of the parent’s disease does not affect how severe the condition will be in the child= variable expression.
  • Causes spots (cafe au lait) on skin, freckles and scoliosis.
17
Q

X-linked disorders

A

Mutations carried on the X-chromosome
- Which contains a lot more genes than Y.

Condition mainly affects males and is inherited from the mother

Examples

  • Haemophilia
  • Fragile X
18
Q

Haemophilia

A

X-linked, recessive disorder characterised by a lack of clotting factors.

  • Leading to the tendency to bleed.
  • Most common in males. If female requires homozygous inheritance of mutation, which is very unlikely.

Haemophilia A
- Lack of factor 8

Haemophilia B
- Lack of factor 9

19
Q

Fragile X

A

X-linked disorder characterised by CGG expansion in FMR1 gene
- Most common single-gene cause of autism/ inherited mental retardation

Normal range of repeats =<54

Premutation= 55-200
- Reduced penetrance

Fragile X allele= 200-1300 repeats

Repeats increases as it is passed through the female line, in which the severity increases with each inheritance.

20
Q

Lyonization

A

X-chromosome inactivation
- Occurs early in embryogenesis in females to avoid overexpression of X genes

X chromosome from mother/father is randomly selected and condensed to be inactive= bar body

This can lead to a female inheriting a condition and not expressing it, but showing reduced penetrance.

21
Q

Germiline mosaic

A

Where during embryogenesis, mutation occurs in the germline cells.
- This leads to the parent not having the condition, but can pass on the mutation to their offspring.

Example
- 15% of Duchenne muscular dystrophy occurs from germline mosaics

22
Q

Mitochondrial inheritance

A

Mutation occurs in mitochondrial and is almost exclusively pass on from the mother
- As the sperm loses mitochondria during fertilization

23
Q

Genetic tests

  • Samples
  • Types
A

Done through blood/ saliva/skin sample.

Prenatal= amniocentesis/ chorionic villus sampling

Types
- Diagnostic tests= identifies/ exclude specific genetic conditions.

  • Carrier tests= sees if individual carries mutations
  • Predictive/ presymptomatic test= family history of serious, later onset genetic condition (i.e Huntington’s disease)
  • Exome tests= looks at expression of all genes, looking for the genetic change causing the disorder
  • Whole genome test- Looks at entire genome, including non-coding regions
  • New born blood spot screening tests= screens for multiple conditions (i.e. SCD, CAH, MCAD)
24
Q

Locus heterozygeneity

A

Mutation that occurs at different genetic loci but cause the same/ similar phenotype.

Example
- Haemophilia A/B
-

25
Q

Alleic heterogeneity

A

Where a single phenotype can be produced by multiple mutations on the same locus of a chromosome

Examples

  • Beta-thalassemia
  • Phenylketonuria
  • Achondroplasia
26
Q

Promotor nucleotide sequence

A

TATA

27
Q

Start codon

A

ATG

28
Q

Splice site nucelotide sequence

A

GT

AG

29
Q

Stop codons (3)

A

TAG

TAA

TGA

30
Q

Classes of mutation (5)

A

Deletions

Insertions

Single base substitution

  • Missense: substitution of amino acid
  • Nonsense: insertion of premature stop codon.
  • Splice site; formation of splice site (GT, AG)

Frameshift

Dynamic mutations

31
Q

Cystic fibrosis

  • Most common mutation
  • Location and inheritance of mutation
A

Delta-F508 mutation (most common mutation in whites)

  • Deletion of phenylalanine oat 508th amino acid
  • CTT deletion
  • Loss of function for CFTR

Location on Chromosome 7
- Inherited in autosomal recessive fashion

32
Q

Phenylketonuria

  • Features
  • Mutation/ defect
  • Inheritance
A

Features

  • Severe learning difficulties
  • Fair skin
  • Eczema
  • Epilepsy

Enzyme affected
- Phenylalanine hydroxylase

Autosomal recessive

33
Q

Phenylketonuria mutations

- Most common

A

Exon 7

- 143-410 catalytic domain

34
Q

Compound heterozygotes

A

Having two or more different recessive alleles at the same gene locus.

Seen in Phenylketonuria, CF
- Tend to have a less severe form of the condition

35
Q 
Inbreeding coefficient
of child for:
- Siblings
- Half-siblings
- Uncle/niece
- First cousins
- Second cousins
A

Siblings
- 1/4

Half-siblings
- 1/8

Uncle/niece
- 1/8

First cousins
- 1/16

Second cousins
- 1/32

36
Q

Chromosome 15 genomic imprinting

A

In a bialleic gene
- maternal expressed gene is active on the maternal chromosome, while the paternally expressed gene is silent

  • Paternally expressed gene is active on the paternal chromosome, whilst the maternally expressed gene is silent.
37
Q

Huntington’s disease CAG repeats

  • Chromosome location
  • Stable repeats
  • Unstable repeats
A

Located on chromosome 4

Stable repeats
- 6-35

Unstable
- 36+

36-39 repeats= uninformative test result, reduced penetrance range.

40+ is positive test result.

302: Scientific basis of medicine (28 decks)

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