Chromasomal Abnormalities Flashcards
What is a karyotype?
Collection of someone’s chromosomes
How do you prepare a karyotype?
Collect approx 5ml herpanised venous blood —> can use amniotic cells, CVS
Isolate white cells
Culture in presence of phytohaemagglutinin —> stimulates T-lymphocyte growth/differentiation
After 48 hours add colchicine
Place in hypotonic saline
Place on slide
Fix and stain
What do band numbers mean?
Bands caused by different staining
Dark (hetrochromatin) —> more compact fewer genes
Light (euchromatin) —> more open more genes
What is aneuploidy?
Abnormal no. of chromosomes
How does non-disjuncture result in uneven no. of chromosomes in daughter cells?
Chromosomes don’t split properly in meiosis
Meiosis 1 —> all daughter cells affected
Meiosis 2 —> half of daughter cells affected
Result in either +1or -1 chromosome
Why is sex chromosome imbalance tolerated?
X-inactivation of excess X chromosome
Low gene content of Y chromosome
Why is there a maternal age effect?
Vulnerability of oogenesis
—> pauses in utero in prophase I until puberty
—> secondary oocyte arrests in metaphase II
—> only competes if fertilised
—> one primary oocyte yields only one ovum
—> finite number of primary oocytes
What is most aneuploidy caused by?
Non-disjunction which arises in oogenesis —> likely due to degradation of factors which hyoid homologous chromatids together
Why is paternal age not a risk in aneuploidy?
Primary spermatocytes undergo approx 23 mitotic divisions per year —> potentially accumulate defects
What is mosaicism?
Presence of two or more populations of cells with different genotypes
What results in mosaic expression?
X-inactivation
What two mechanisms does mosaicism arise form?
Non disjuncture during early development
Loss of extravasation chromosome in early development
What is cross over chromosomes?
Occurs in prophase 1
Increases genetic diversity
Pairs of chromosomes align —> chiasma forms and cross over occurs
1-3 times per chromosome per meiosis
What can go wrong in cross over chromosomes?
Do not align properly —> extra + lost material
If balanced has no effect
What causes Williams syndrome?
Imbalance of genes —> deletion from one chromosome
—> unrelated as part form their location
