L20 Genetic Testing 1 Flashcards
Genetic Testing - categories
- before implantation
- prenatally - before birth
- neonatally - new born
- child
- adult
Prenatal
- ultrasound
- maternal blood triple test
- maternal blood: fetal cell sorting, cell free DNA
- fetal blood sampling
- chorionic villus sampling
- amniocentesis
Why test?
Has two associated slides
chromosomal aneuploidies
- extra chromosomes generally inviable
- loss of an autosome inviable
- extra sex chromosome can be viable
- loss of sex chromosome viable but must have an x
aneuploidy frequency increases with maternal age
women of advanced maternal age are at increased risk of producing an embryo affected with a genetic disease
Trisomy 21
see onenote slides
Changes in chromosome number
see onenote
Non-disjunction
see onenote
Non-disjunction at anaphase 2 would produce half normal gametes
Non-disjunction at anaphase 1 - no normal gametes produced
If not 21, 18 or 13 trisomy, would most likely lead to a miscarriage
Aneuploidy
see onenote slides
Older effects - problems
see onenote
more likely to have abnormally functioning spindles - causes an increased rate of chromosomal problems in the mature eggs
Non-disjunction in mitosis - mosaic
Mosaic
- Normal zygote, in one of the cells you get non-disjunction, all cells derived from that will have trisomy
- The other cells dividing normally => mosaic
- Can happen for trisomy 13 and 18, often these children don’t have such severe phenotypes if they are mosaics, has some normal cell lines
Translocation
see onenote
Translocation carrier
- Don’t have phenotypic evidence of being a carrier
- Have a balanced genome
In karyotype we can see in chromosome 13, 18, 21:
- Achrocentric - centromeres are near the end of the chromosome
- Nucleolar organiser region - dna sequence that code for ribosome RNA, these are repeated and so we can lose it without any phenotypic outcome
- Lose nucleolar organiser in these translocation carrier but don’t see any effect as they are repeated
14/21 21
- When fertilised, will get 3 copies of chromosome 21
From translocation carrier
- Can have normal offspring but some of their gametes might not be viable at all => fertility problems
- But can also lead to trisomy 21
Human reproduction - rate
see onenote slides
birth rate is increasing but so is age of parents => genetic testing becomes more relevant
Why test? - other reasons
see onenote
Australian Genetic Testing
see onenote
Preimplantation testing
Preimplantation
- Embryo that develops till 8 cell stage
- Take out single cell
- Has to be done along side IVF, test it before you implant it
PGD requires IVF
see onenote
Preimplantation Genetic Diagnosis
Polar body biopsy
Polar body biopsy
- Sometimes can take a human oocyte (the gamete itself) and do a polar body biopsy
- before you even put the egg and sperm together
- If we know female is het for haemophelia, we know in one cell will have the normal factor 8 gene and the other will have the mutation 8 gene
- Look at polar body to see if they have the mutant or normal factor 8 gene, the oocyte will have the other gene
After biopsy
see onenote
FISH (Fluorescence in situ hybridisation) analysis - sex determination for x-linked diseases
PCR - to amplify region that may contain mutation
To whom is PGD advised?
see onenote
- couples with repeated pregnancy loss due to genetic disorders
- couples who have a child with a genetic disease and are at high risk of having another
- couples who wish to identify a tissue match for a sick sibling who can be cured with transplanted cells - preimplantation tissue typing (PTT)
Intra cytoplasmic sperm injection (ICSI)
for couples with male infertility or to reduce risk of contamination
- used when there is abnormal semen or PCR analysis is required
- to reduce risk of contamination with sperm DNA during PCR
Ultrasound
Ultrasound
- Done 12-20 weeks of pregnancy
- Looking for nuchal (neck translucency) = gap at base of skull, in children with trisomy 21 it’s wider than it should be
- Neural tube defects e.g. spina bifida, anencephaly (Foetus usually dies within minutes of being born)
- Screen, looking for parameters that may indicate the something’s wrong then woman could undergo further testing
Maternal blood triple test
see onenote slides
- screening
three specific substances:
- AFP - protein produced by foetus, high = indicates neural tube defect
- hCG - hormone produced by placenta
- Oestriol - estrogen produced y both fetus and placenta
Estimating - triple test of maternal blood
Age of mother
Ultrasound
Blood test:
- AFP (low in trisomy 21, high with neural tube defects)
- hCG (low in trisomy 21)
- oestriol (high in trisomy 21, trisomy 18 low in all)
Maternal blood - fetal cell sorting
see onenote
70% of pregnancies have fetal cells in maternal circulation
can use Y specific probe
XX fetal cells use fluorescent activated cell sorting - use cell surface differences between mother and fetus
cells from previous pregnancy may remain in circulation - unreliable
NIPD in the UK
Non-invasive pre-natal diagnosis
CVS and aminocentesis carry 1% risk of miscarriae
