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
Effect of aneuploidy on pregnancy
Causes 5% of still birth and 50% abortion
Most trisomies aren’t compatible with life
Monosomy poorly tolerated
Purpose of chromosome crossing over
Increase genetic diversity
How does crossing over occurred
Pairs of chromosomes align
Chiasma firms
Crossover occurs
Unequal crossing over
Chromosomes don’t align at centromeres
Unequal lengths of chromatids are exchanged leading to deletion of genes in on chromosome and duplication of genes in another
Single chromosome abnormalities
Deletion-loss of genes due to unequal cross over or breaks in chromosome which can occur at ends or middle
Duplication-unequal crossover where material from one chromosome transferred to another
2 types of inversion in single abnormalities
Paracentric- breaks in chromosome occur in middle of chromosome arm and that section of chromosome is inverted creating new piece of dna
Perricentric- break occur around centromere and that section of chromosome is inverted
Why does material exchange cause issues in offspring
Many chromosomal abnormalities are de novo- occur in individual gametes during oogenesis/spermatogenesis and parents not affected by conditions
Two types of chromosomal deletion
Microscopic-eg cri du chat syndrome means loss of whole short arm of chr 5.46,XY,del(5p)
Micro deletion- needs high resolution banding or molecular genetics to see defects large. Large numbers of genes deleted eg velocardiofacial syndrome/digeorge syndrome 22q11.2del
Williams syndrome
Caused by 7q11.23 deletion
Long philtrum,upturned nose,lack social anxiety,arched brows,supravalvular aortic stenosis
Lack of elastin on chromosome
How is William syndrome detected
FISH fluorescent in situ hybridization
Duplication of 7q11.23
Caused delayed speech,autistic behavior,affects social interactions,flat brows,dilatation of aorta,broad nose,shirt philtrum
3 broad structural classes of chromosomes
Metacentric-shirt and long arms are equal and centromere in middle
Submetacentric- short arm is shorter than long arm
Acrocentric- short arm has been shortened down to residual stump
Robertsonian translocation
Occurs in Acrocentric chromosomes
Can be homologous or non homologous
Commonly between 13-14,14-15,14-21
Most people are unaffected but can cause problem in offspring
How can Robertsonian cause issues in offspring
During gamete formation there is unbalanced segregation of chromosomes
Trisomy 14 is lethal
Mosaicism
Presence of 2 or more populations of cells with different genotypes
Eg females are often examples due to X inactivation where 1 chromosome randomly turned off not true mosaicism as each cell has same genotype
What two mechanisms result in true mosaicism
- Non-disjuncture during early development
- Loss of extra chromosome in early development if embryo starts of as aneuploid
Most common mosaic is 46,XX/45,X and mosaic 46,XY/45,X → where 1 of sex chromosomes has been lost in development
