Genetic Screening

Question 1

What are the uses of genetic screening?

Answer 1

To confirm a diagnosis:
Genetic testing can confirm an initial diagnosis based on
sweat testing or the trypsinogen test. This is because initial
tests can result in false positives (test shows that the
patient has the disease where in fact she does not) or false
negatives (having the disease but not showing up on the
test).

To identify carriers:
Genetic testing can identify carriers and this can be used to
assessing the probability of producing offspring with the
disease. Counselling can be offered before and after
testing.

Question 2

What are the disadvantages of Amniocentesis?

Answer 2

 Done in late pregnancy, so if abortion is decided
is more traumatic for the mother
 Results are available in 2-3 weeks
 It carries about a 1% risk of spontaneous abortion
(or miscarriage)

Question 3

What is amniocentesis?

Answer 3

• Can be done about the 16th
  week of pregnancy
• Remove amniotic fluid which
  surrounds the fetus with a
  needle
• Fetal cells and blood cells can
  be recovered from the fluid
• Cells need to be culture of 2-3
  weeks
• Identifies a number of genetic
  diseases and the sex of the
  baby
• Analyse DNA

Question 4

What is Chorionic villus sampling?

Answer 4

• Can be done about the 8th
  week of pregnancy
• A small embryonic tissue
  is taken from the
  placenta.
• Analyse DNA
• Cells can be tested for a
  wide range of genetic
  diseases
• Results ready in short
  period of time

Question 5

What is Non-invasive prenatal diagnosis (NIPD)?

Answer 5

 This is a non-invasive technique.
 Blood plasma from the mother is collected and
the free DNA fragments found in it are used for
DNA analysis.
 About 20% of the fragments come from the
embryo.
 Cell-free fetal DNA (cffDNA) becomes detectable
at about 4-5 weeks of pregnancy, but DNA levels
are too low to be tested.
 Samples are collected after 7-9 weeks of
pregnancy for more accurate testing

Question 5

What are the advantages and dissadvantages of CVS?

Answer 5

Advantage:
 The test is done earlier (8-12th week vs. 15th
week) than amniocentesis so if pregnancy
termination is selected it is less traumatic.

Disadvantages:
 It carries a higher risk of spontaneous
miscarriage.
 All paternal X chromosomes are inactivated in
fetal placental cells so any problems in the
genes on that chromosome cannot be detected
by this technique.

Question 6

What is Preimplantation genetic diagnosis?

Answer 6

 In vitro fertilisation (IVF) can be used to create
embryos that are tested prior to uterus transfer.
 After about a few cell divisions, a single cell can
be removed from the embryo without damaging it.
 The DNA of the cell is then analysed to avoid
implantation of embryos with problem alleles.
 Preimplantation tests the embryo before it is
inserted into the mother.
 IVF is expensive and often stressful.

Question 7

What are the Benefits and risks of prenatal
testing?

Answer 7

Benefits
 Gives information about abnormalities in fetus
 Opportunity for choice / consider termination/ time for
preparation

Risks
 Possibility of miscarriage due to the procedure
 Potentially healthy baby could be lost due to the above
effect
 False positive or false negative testing
 Wrong decision could be made due to the above result
 Damage to the fetus due to the procedure and lead to miscarriage

Question 8

What is Genetic counselling?

Answer 8

 If people might have a child with a genetic
disorder, genetic counsellors can provide
advice.
 Genetic counsellors give information on how
the disease is inherited, what the
probabilities are, and what options are
available.
 The counselling should help the couple
decide what course of action to take