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Human and Medical Genetics > L20 Genetic Testing 1 > Flashcards

L20 Genetic Testing 1 Flashcards

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USMLE® Step 1 flashcards Biology 101 flashcards
1
Q

Genetic Testing - categories

A
  1. before implantation
  2. prenatally - before birth
  3. neonatally - new born
  4. child
  5. adult
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2
Q

Prenatal

A
  • ultrasound
  • maternal blood triple test
  • maternal blood: fetal cell sorting, cell free DNA
  • fetal blood sampling
  • chorionic villus sampling
  • amniocentesis
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3
Q

Why test?

Has two associated slides

A

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

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4
Q

Trisomy 21

A

see onenote slides

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5
Q

Changes in chromosome number

A

see onenote

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6
Q

Non-disjunction

A

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

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7
Q

Aneuploidy

A

see onenote slides

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8
Q

Older effects - problems

A

see onenote

more likely to have abnormally functioning spindles - causes an increased rate of chromosomal problems in the mature eggs

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9
Q

Non-disjunction in mitosis - mosaic

A

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
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10
Q

Translocation

A

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
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11
Q

Human reproduction - rate

A

see onenote slides

birth rate is increasing but so is age of parents => genetic testing becomes more relevant

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12
Q

Why test? - other reasons

A

see onenote

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13
Q

Australian Genetic Testing

A

see onenote

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14
Q

Preimplantation testing

A

Preimplantation

  1. Embryo that develops till 8 cell stage
  2. Take out single cell
  3. Has to be done along side IVF, test it before you implant it
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15
Q

PGD requires IVF

A

see onenote

Preimplantation Genetic Diagnosis

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16
Q

Polar body biopsy

A

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
17
Q

After biopsy

A

see onenote

FISH (Fluorescence in situ hybridisation) analysis - sex determination for x-linked diseases

PCR - to amplify region that may contain mutation

18
Q

To whom is PGD advised?

A

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)
19
Q

Intra cytoplasmic sperm injection (ICSI)

A

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
20
Q

Ultrasound

A

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
21
Q

Maternal blood triple test

A

see onenote slides
- screening

three specific substances:

  1. AFP - protein produced by foetus, high = indicates neural tube defect
  2. hCG - hormone produced by placenta
  3. Oestriol - estrogen produced y both fetus and placenta
22
Q

Estimating - triple test of maternal blood

A

Age of mother

Ultrasound

Blood test:

  1. AFP (low in trisomy 21, high with neural tube defects)
  2. hCG (low in trisomy 21)
  3. oestriol (high in trisomy 21, trisomy 18 low in all)
23
Q

Maternal blood - fetal cell sorting

A

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

24
Q

NIPD in the UK

A

Non-invasive pre-natal diagnosis

CVS and aminocentesis carry 1% risk of miscarriae

25
Q

Maternal blood - cell free DNA

A

Cell-free fetal DNA (cffDNA) is fetal DNA which circulates freely in the maternal blood. Maternal blood is sampled by venipuncture. Analysis of cffDNA is a method of non-invasive prenatal diagnosis frequently ordered for pregnant women of advanced maternal age.

Cell-free DNA screening is a test that can determine if a woman has a higher chance of having a fetus with Down syndrome (trisomy 21), trisomy 18, trisomy 13 or an abnormality in the sex chromosomes (X and Y chromosomes). With this test, a sample of the woman’s blood is taken after 10 weeks of pregnancy.

26
Q

Maternal blood: NIPD

A

see onenote

analyse cell free DNA of fetus in maternal circulation

can test sites to determine aneuploidies and mutations in fetus

27
Q

NIPT

A

see onenote

non-invasive prenatal testing

very accurate for detection of down syndrome and edward syndrome

28
Q

Fetal blood sampling

A

sample from umbilical cord or fetus

risky

ultrasound may indicate signs of severe anaemia

maybe if fetus if Rh+ve and mother Rh=ve and sensitised check for fetal anaemia

29
Q

Indications for pre-natal diagnosis in Victoria

A

see onenote

  1. abnormal first trimester
  2. advanced maternal age
  3. abnormal routine ultrasound
  4. second trimester serum screen
30
Q

Chorionic villus sampling

A

see onenote

  • invasive
  • performed from 10-13 weeks
  • increases spontaneous abortion rate by 0.8%
  • 10-30 mg chorion required
  • separate maternal and fetal tissue (contamination problem(
  • mosaicism
31
Q

Amniocentesis

A

see onenote slies

  • from 15-20 weeks
  • 20ml amniotic fluid
  • spontaneous abortion rate increases by 0.3%

Human and Medical Genetics (32 decks)

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