Genetic Testing Flashcards
Why might a Normal Nuchal scan (ultrasound) be taken and when are they taken
To read thickness of fluid at the back of the neck at 12 weeks
Aim of 12 week scan
To date the pregnancy accurately
To diagnose multiple pregnancies
To diagnose major foetal abnormalities
To diagnose early miscarriage
To assess the risk of Down Syndrome and other chromosomal abnormalities
Nuchal translucency
- Done at 10-14 weeks
- It is thickness of fluid at back of foetal neck
- Is a screening test and NOT diagnostic
- Can indicate a range of genetic syndromes
Increased >3mm indicated chromosome abnormalities eg diwns syndrome,Edward’s,turners,patau
When is prenatal testing arranged
- If previous pregnancy was affected with a condition e.g. DS, CF
- If parent(s) is a carrier of chromosome rearrangement or genetic condition e.g. t(13;14), DMD, Huntingtons
- Family history of genetic condition
- Following abnormal findings at nuchal scan or mid-trimester scan
- Following results of combined test which given an increased risk of Down syndrome
Non invasive prenatal diagnosis
Fetal dna found in mothers plasma at around 9 weeks
NIPD for gender
SRY gene in Y chromosome
When would there be an increase in the amount of cfDNA for chromosome 21?
In trisomy 21
- For Cystic Fibrosis, what test is used to detect both maternal and paternal mutation in the fetal DNA?
-
NIPD offered privately via nhs
Aneuploidy testing
Offered privately or via research studies
Non invasive prenatal test is used which isn’t diagnostic
Limitations of NIPD and NIPT
not possible in multiple pregnancies
Benefits of NIPD and NIPT
- The number of invasive tests carried out is likely to reduce as a result
- There is no increased risk of miscarriage- invasive do
- Less expertise is required to perform a blood test than an invasive test
- In many cases we can offer NIPD/NIPT earlier than traditional invasive testing, thereby getting a result much earlier
Invasive testing
Only given if known risk
CVS
A sample of chorionic villi which is a part of the placenta which has the same dna as the fetus is taken trans abdominal or transvaginally
Carried out at 11-14 weeks
Amniocentesis
At 16 weeks
Sample of amniotic fluid which contains fetal cells is taken
Risk of miscarriage infection and Rh sensitisation
CGH array
If there are concerns on 20 week scan, the gold standard is to offer CGH array
Look for small/large imbalances in chromosomes (microdeletions and duplications)
What happens if something is found on CGH array
Standard test parents to see if either is a carrier
What are the 4 steps in carrying out a CGH array?
Extract and label dna
Hybridize and wash
Scan
Analyze data
Trio exome
Allows us to see exome of baby to find anomalies
Exome is the coding region
Why are there fewer egg and sperm donors
As donations are no longer anonymous
PGD
uses IVF to genetically test embryo before implantation
- Stimulation of ovaries
- Egg collection
- Insemination - intracytoplasmic sperm injection (ICSI) of only a single sperm to reduce contamination from more sperm cells
- Fertilisatiton
- Embryo biopsy
- Embryo testing - at blastocyst stage
- Embryo transfer - of those that did not inherit the genetic condition into the womb
- Pregnancy test
What disorders do PGD usually test for
Translocation carriers
HD
DMD
CF
Timings of scans
Nuchal translucency (12 weeks)
NIPD cell free foetal dna (9weeks)
CVS (12 weeks)
Amniocentesis (15 weeks)
Mid trimester scan (20 weeks)
CGH array
Prenatal exome
Prenatal exome
Done if there are major fetal abnormalities
Exome is the coding region take dna from parents and children
