Genetics Flashcards
Describe how to prepare a karyotype for staining
Get 5ml of heparinised venous blood
Isolate the white cells
Culture in the presence of phytohaemagglutinin (stimulates T lymphocyte growth and differentiation)
After 48 hrs, add colchicine (causes mitosis arrest at metaphase)
Place in hypotonic saline
Place onto slide
Fix and stain
Is paternal age a risk factor for increased aneuploidy
No - paternal age has no effect
But primary spermatocytes undergo about 23 mitotic divisions per year and potentially will accumulate defects through this
What is non disjunction and what does it cause
Chromosome duplicates are not evenly distributed between daughter cells - due to problems with spindle fibres
Results in monosomy or trisomy
What does the darkness of bands on chromosome (after gisema chromosome staining ) represent
Different amounts of compaction
Dark (heterochromatin) - more compact DNA, fewer genes
Light (euchromatin) - more open/less compact, more genes
When does crossing over of chromosomes occur
Prophase 1
Give examples of Mendelian/monogenic diseases
Haemophilia
Sickle cell anaemia
Cystic fibrosis
Thalassaemia
Give examples of complex diseases
Diabetes CVD Cancer Asthma Hypertension Mental health diseases etc
What are SNPs
DNA variations that occur when a single nucleotide is changed
They are the most common form of variation in the human genome
GWAS look for SNPs between people with the same trait - although many SNPs have been found to be associated with many diseases, the actual level of increased risk caused by SNPs is usually low (correlation not causation)
What conditions put you at high risk of CVD
type 2 diabetes
Obesity
High cholesterol
High blood pressure
How much DNA do monozygotic and dizygotic twins share
Monozygotic - identical - share 100% of DNA
Dizygotic - non identical- share 50% of DNA
What is heritability
The estimate of the genetic contribution to increased risk of a disease
How to study/measure heritability
1) Twin studies: with monozygotic and dizygotic twins. Dizygotic twins will have differences due to environment and genes, we can assume their environments are the same as they’re siblings so differences are mostly due to genes
2) GWAS (genome wide association study): looks at a large group of people and their genomes and looks for an association in the group between specific genetic conditions and particular diseases. - does this by looking for SNPs
What is missing heritability and what could be reasons for it
Gap between what is known about the heritability of a disease and what the GWAS show
Rare variants (SNPs that are so rare the GWAS doesn’t identify them)
Low frequency variants with intermediate effect (cause a small effect)
Interactions (between susceptibility genes)
Miscalculated estimate of heritability
Diagnosis (accuracy and precision)
What is pharmacogenetics
The study of variability in drug response due to genetic differences
(Variations in enzymes can affect drug metabolism)
Difference between monogenic and complex diseases
Monogenic: clear Mendelian inheritance pattern, caused by an individual gene with limited environmental influence
Complex diseases: genetic predisposition but no clear inheritance pattern, phenotype is determined by the interaction of many genes with the environment
3 different types of chromosomes
Sub meta centric chromosome: long arm and short arm
Meta centric chromosome: two arms of the same length
Acrocentric chromosome: one long arm, short arm has been replaced with non coding satellite
What are the long and shirt arms of the chromosome called
P arm - short arm
P telomere - non coding bit of DNA at top of short arm
Q arm - long arm
Q telomere - non coding bit of DNA at bottom of long arm
What is the P value used for genome wide association studies
10^-6 or 10^-7
What are the types of autosomal dominant mutations
Gain of function - gain of new function, enhancing the activity of the protein
Dominant negative effect - formation of a new abnormal protein which disrupts the functioning of normal proteins
Insufficient - mutation causes half the amount of protein to be made, which is insufficient for normal functioning
Give examples of autosomal dominant diseases
Huntington’s disease
- mutation in HTT gene causes CAG base repeat, forming abnormal Huntingtin protein which clumps together - forms toxic aggregates which disrupt normal cell functioning
- depression, lack of concentration, involuntary jerking, problems swallowing
- genetic anticipation —> with each generation, age of onset decreases and severity of symptoms increases
Osteogenesis imperfecta
- mutations in some genes encoding for type 1 collagen
- type 1: insufficient protein formed
- type 2/3/4: production of abnormal protein with altered structure that interferes with function of normal protein
- hearing loss, breathing problems, short height, blue tinge of sclera
What type of mutations do autosomal recessive mutations tend to be
Loss of function mutations
What increases risk of inheriting autosomal recessive conditions
Consanguinity
Example of autosomal recessive condition
Cystic fibrosis
- mutation in CFTR gene in chromosome 7 causes defecting chloride ion channels which result in build up of sticky mucus outside of cells
- lungs: increased secretion, more prone to chest infections and breathing problems
- GI tract: blockage of pancreatic duct which stops pancreatic enzymes reaching gut lumen. Causes impaired absorption and malnutrition
In which conditions are all daughters of an affected father affected, but no sons
X linked dominant
Example of X linked recessive condition
Haemophilia
- mutation in genes encoding clotting factor 7 (causes haemophilia A) or in clotting factor 9 (causes haemophilia B)
- excessive bleeding, brushing, swelling, impaired clotting
Example of X linked dominant condition
X linked hypophosphataemia
- mutation in PHEX gene causing overproduction of FGF21
- inhibits kidney phosphate resorption
- results in vitamin D rickets
Example of Y linked condition
Retinis pigmentosa
- mutation in RPY gene
- cell in retina produce defective protein
- trouble seeing at night and tunnel vision
Why can you get large variation in the presentation of mitochondrial conditions even in the same family
Mitochondria have many copies of the same gene, some of which may be mutant and some may be normal - “heteroplasmy”
- during cell division, mtDNA replicates and sorts Randi ky amongst mitochondria and amongst daughter cells
- mitochondria can gain or lose mutant genomes as a result of Rabin segregation
- so each daughter cell may receive a different proportion of mitochondria carrying normal and mutant DNA
- only those with number of mutant genomes above a certain threshold will express the disease
- and the severity of the disease expressed is increased the more affected mitochondria there are
Example of mitochondria condition
Leber’s hereditary optic neuropathy (LHON)
- degeneration of retinal ganglion cells
- loss of visual acuity, central vision, colour vision
What 3 qs should you ask when reading pedigrees
Is it inherited from mothers only, fathers only or both?
Can unaffected parents have affected child? and what about the other way around?
Is there any sex predilection?
What is a karyotype
Chromosome set of an individual species in terms of number an structure if the chromosomes
What are the 3 types of chromosomal abnormalities
Numerical
Structural
Mocaicism
Describe numerical chromosomal abnormalities
Normal = disomy
Abnormal = monosomy, trisomy, tetrasomy
Caused by non disjunction and uneven distribution of chromosomes between daughter cells, due to problems with spindle fibres
Give examples of numerical chromosomal abnormalities
Turners syndrome (X monosomy) Klinefelter syndrome (XXY trisomy) Down syndrome (21 trisomy)
Why does maternal age increase risk of Down syndrome
Progressive degeneration of the factors that hold homologous chromatids together
Describe structural chromosomal abnormalities
Mistakes during DNA replication and crossing over result in changes to DNA banding sequence
Single chromosome abnormalities:
- deletion (of a band)
- insertion (of a band)
- inversion (2 adjacent bands are switched). Paracentric = below centromere. Petricentric = on either side of the centromere
Two chromosome abnormalities (during crossing over)
- insertion ( one chromosome gives in band to another chromosome)
- translocation ( bands are exchanged in wrong way), the junction between the 2 new genetic sequences codes for an abnormal protein eg in leukaemia
- robertsonian translocation: acrocentric chromosomes, the satellites of 2 chromosomes are lost and 2 long arms join together. So no coding DNA is lost but a new chromosome is formed with twice the DNA than before. Doesn’t cause problems in the person but can cause problems in their offspring
What is mocaicism
Presence of 2 or more populations of cells with different genotypes
Due to
1) non disjunction in embryonic development
2) loss of extra chromosome in early development
The later in development it occurs, the milder the phenotype
Examples of mocaicism
Williams syndrome - 7q11.23 deletion
7q11.23 duplication syndrome
Why does a null mutation cause a more mild phenotype in OI but a more severe phenotype in muscular dystrophy
In OI1, the null mutation reduces the level of expression of collagen A1 but there is suffice not to allow some functional collagen to be produced
In DMD there is complete lack of functional dystrophin
Why does deletion of a section of collagen cause a more severe phenotype of OI whilst deletion of a section of dystrophin causes a milder phenotype of muscular dystrophy
Deletion of a large section of collagen comprises its function a great deal
Collagen helix starts forming from both ends therefore missing section causes large structural change
Deletion of a section in middle of dystrophin only mildly comprises function. Some reduced binding to cytoskeleton but can still bind to a degree and can still bind to the membrane proteins
What is the two hit hypothesis
Situations where a form of genetic damage is not sufficient to enable cancer to develop
Eg High degree of exposure to UV light may be required to initiate melanoma development
Describe how a colon cancer can grow
Colon cancer may begin with a defect in a tumour suppressor gene (APC) that allows excessive cell proliferation. The proliferating cells then tend to acquire additional mutations involving DNA repair genes, other than tumour suppressor genes (p53) and many growth related genes (K-ras)
Over time the accumulated damage can yield a highly malignant, metastatic tumour
How can hereditary cancers be passed on
Inherited mutations of proto oncogenes that cause oncogenes to be turned on or activated
