Karyotypes Flashcards
What is a karyotype
Collection of someone’s chromosomes
How to collect karyotype
1) Collect 5ml heparinised venous blood (peripheral)- can also use amniotic cells, CVS (chorionic villus sample)
2) Isolate white cells
3) Culture in presence of phytohaemagglutinin which stimulates T lymphocyte growth/differentiation
4) After 48 hours add colchicine which causes mitotic arrest at metaphase
5) Place in hypotonic saline which bursts cells
6) Place on slide
7) Fix and stain with Gisema
8) Cut out individual chromosomes and arrange into karyotype
Dna compaction
Dna is compacted around his tones and further condensed into chromatin
Proteins bound affect regulation
3D genome is important
What stain makes karyotype
Giemsa staining
Ideogram
Shows chromosomes based on g based architecture
P arm is short
Q arm is long
How are bands formed and numbered
Some darker as take up more stain
Bands measured in bands per haploid set
Darker and lighter regions in karyotype
Darker means heterochromatin more compact fewer genes
Lighter means euchromatun more open genes
Why are karyotype performed more in prophase
As chromosomes are less compact and thus you get more detail
Aneuploidy
Abnormal number of chromosomes
Non disjunction
Occurs when chromosomes don’t split properly between daughter cells which can occur during meiosis I or II
Unencrypted numbe if chromosomes
Trisomy or monosomy
If occurs in meiosis I all daughter cells are affected but in II half ae
Most common form of aneuploidy
Sex chromosome abnormality
Tolerated due to random X inactivation
Why do abnormal numbers of X and Y have an effect
Both X and Y have PAR which still produces gene and doesn’t get inactivated
Second most common form of aneuploidy
Trisomy of chromosome 21
3 copies of chr 21
Our reuksting from maternal non disjunction
Risk of Down syndrome increases as mothers age increases
Karyotype 47+21 or 47,XX+21
Why is there a maternal age effect
Due to vulnerability of oogenesis
Primary oocytes are made before birth and arrest in prophase until puberty
The older the mother is, the longer the oocyte has been paused in meiosis for which increases chance of non-disjunction likely due to degradation of factors which hold homologous chromatids together- older you are the more disorganised the chromatid arrangement
Is there a paternal age effect
Isn’t a risk factor for increased aneuploidy but does affect single gene disorder subset caused by mutations in FGFR2,FGFR3 and RET
enhanced by selfish spermatogonial selection
Smoking increases risk
