Session 10 Flashcards
How are chromosomes analysed?
Living cells are used to source chromosomes. Chromosomes are harvested from cells whilst they are at metaphase of the cell cycle.
How are chromosomes grouped in a karyotype?
According to size and shape: grouped A-G and ordered by size. Exception is chromosome 22 is bigger than chromosome 21, and X chromosome is considered group C and the Y chromosome is considered group G
What is G-banding?
Staining method for chromosomes. Metaphase a are exposed to trypsin to digest proteins, then stained with Romanowski type dye to produce dark (gene poor) and light (gene rich) bands.
When is amniocentesis used?
From 15 weeks of gestation onwards.
When is chorionic villus sampling used?
At 11-12 weeks of gestation.
What is Down’s syndrome?
Trisomy 21: the presence of an additional chromosome 21.
What is Patau syndrome?
Trisomy 13, short survival if live birth.
What is Edwards syndrome?
Trisomy 18, short survival if live birth,
What is Turner syndrome? Who does it appear in?
Monosomy X.
Only appears in females.
What is polyploidy?
The gain of a full haploid set of chromosomes.
What causes polyploidy?
Polyspermy: fertilisation of an egg by more than 1 sperm.
What is aneuploidy?
An irregular number of chromosomes.
What causes aneuploidy?
Non-disjunction at one of the meiosis cell divisions: forms gametes with one extra or one less chromosome.
What is anaphase lag?
Delayed movement of a chromosome in anaphase which leads to it not being incorporated into either daughter cell. Produces a monosomic daughter cell and a normal daughter cell.
What are the characteristics of Down’s syndrome?
Hypotonia, characteristic facial features, intellectual disability, heart defects, increased prevalence of leukaemia and increased incidence of early Alzheimer’s disease.
What are the characteristics of Turner syndrome?
Puffy feet, redundant skin on the back of the neck, short stature, heart defects, mild learning difficulties, infertility.
What is the function of X chromosome inactivation?
To ensure that individuals only have the same X chromosome compliment that is active.
What is mosaicism?
Presence of 2 or more cell lines in an individual.
What causes mosaicism?
Mitotic non-disjunction.
What is uniparental disomy?
The presence of homologous chromosomes from one parent.
How is DNA activated/inactivated?
By making changes to the DNA and histones without changing the DNA sequence.
What are reciprocal translocations?
Two break rearrangements. Cause carriers to produce balanced and unbalanced gametes.
What is a Robertsonian translocation?
Two acrocentric chromosomes fused together so chromosome count is 45
What causes chromosome deletions?
Uneven pairing and recombination in meiosis.
What should be done if a genetic abnormality is found?
Request parental/family samples of required, refer the patient to clinical genetics for counselling, relate the disorder to clinical problems that may arise and provide the patient with appropriate literature if available.
What are the types of FISH probe and what are they used for?
Locus/gene specific probes: to identify micro deletion syndromes.
Centromere probes: to identify chromosome of origin.
Telomere probes: to identify micro deletion syndromes.
Whole chromosome paints: to identify a chromosome in a rearrangement.
What are the advantages of aCGH?
Examines the entire genome at high resolution.
1 array is equivalent to 1000s of FISH.
Can be automated.
Peter phenotype/genotype correlation.
What are the disadvantages of aCGH?
More expensive than karyotyping.
Will not detect balanced arrangements.
Mosaicism may be missed.
