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CASE 2 Flashcards

1
Q

what is genetic counselling?

A

the process of helping people understand and adapt to the medical, psychological and familial implications of genetic contributions to disease

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2
Q

what does genetic counselling involve?

A
  1. interpretation of family and medical histories to asses the chance of disease occurrence or recurrence
  2. education about inheritance, testing, management, prevention, resources and research
  3. counselling to promote informed choice and adaptation to the risk or condition
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3
Q

what is a recurrent miscarriage?

A

3 < miscarriages

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4
Q

risk factors of miscarriage

A

age, smoking, alcohol, drug use, obesity

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5
Q

what is a karyotype?

A

= a picture of someone’s chromosomes
= a complete set of metaphase chromosomes sorted by length. stain applied.

the number and visual appearance of the chromosomes in the cell nuclei of an organism or species

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6
Q

what are the 2 common chromosomal causes of a miscarriage?

A
  1. numerical abnormalities

2. structural abnormalities

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7
Q

what is non-disjunction?

A

failure of separation of chromosomes in meiosis 1

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8
Q

what is aneuploidy and where do they normally arise?

A

= having missing or extra chromosomes — numerical abnormality
- normally arise from errors in meiosis 1
= most common genetic cause of miscarriage and congenital birth defects

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9
Q

trisomy vs monosomy

A
  • trisomy = 3 copies of a chromosome. cause 35% of miscarriages before 20 weeks pregnancy
  • monosomy = 1 copy of a chromosome
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10
Q

what is a risk factor for trisomies?

A

maternal age

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11
Q

give examples of viable trisomies and a viable monosomy

A

trisomies = Patau’s Syndrome (trisomy 13), Edward’s Syndrome (trisomy 18), Down’s Syndrome (trisomy 21)

monosomy = Turner Syndrome (monosomy X)

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12
Q

structural abnormalities - what do they result from and key characteristics

A
  • result from chromosome breakage
  • usually involve 1 or 2 chromosomes
  • can be spontaneous
  • mutagenic agents increase rate
  • rate also increased in certain inherited conditions with defects of DNA replication and repair
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13
Q

when does an unbalanced translocation occur?

A

when a fetus inherits a chromosome with missing or extra genetic material from a parent with a balanced translocation

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14
Q

what is a balanced translocation and are they common?

A

chromosomes not arranged in a normal pattern, common: roughly 1/500 people

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15
Q

when is a balanced translocation identified?

A
  1. subfertility = the failure to establish a clinical pregnancy after 12 months of regular, unprotected sexual intercourse
  2. raised risk from a combined test
  3. parental testing carried out after child is found to have unbalanced chromosomal pattern
  4. incidental finding antenatally from an amniocentesis
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16
Q

types of chromosomal translocation

A
  1. reciprocal translocation (segments from 2 different chromosomes are exchanged)
  2. robertsonian translocation (an entire chromosome attaches to another)
  3. inversion (a portion of chromosomes in opposite orientation)
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17
Q

why does a translocation increase the chance of a miscarriage?

A

when a carrier of a balanced translocation has children, the consequence is the production of eggs or sperm with incomplete or partially duplicated sets of chromosomes — results in monosomic or trisomic zygotes

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18
Q

what is non-invasive prenatal screening (NIPT)?

A

a maternal blood test (at roughly 9 weeks) to refine T21/T18/T13 risk, reducing need for invasive testing

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19
Q

if a fetus has Down Syndrome, what is found slightly more in maternal circulation?

A

chromosome-21 specific DNA

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20
Q

what are the 2 types of invasive antenatal testing?

A
  1. chorionic villus sampling (CVS)

2. amniocentesis

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21
Q

what is involved in CVS? when is it performed? complications?

A
  • placental tissue (chorionic villi) is aspirated through the cervix (using a tube) or abdominal wall (using a needle) under ultrasound visualisation
  • performed after 10 weeks gestation
  • fetal cells obtained are karyotyped
  • complications = heavy bleeding and miscarriage (1 in 100)
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22
Q

what is involved in amniocentesis? complications?

A
  • amniotic fluid is removed and the foetal cells that it contains are analysed
  • the needle inserted (through abdomen or vaginal canal) to obtain a fluid sample is guided into position using ultrasound visualisation
  • only performed when there is a known risk because of potential threat to foetus/mother
  • complications/drawbacks = large volume needed but such volumes may harm foetus. usually performed in weeks 14-15 but results may take several weeks to come back by which time therapeutic abortion may not be available. chance of miscarriage = 1/100
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23
Q

what is the one way XYY can occur?

A

through non-disjunction in meiosis 2

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24
Q

Down’s Syndrome

A

= the most common of the chromosomal disorders

  • 95% result from non-disjunction (parents of these children have a normal karyotype) — trisomy 21 — chromosome count is 47
  • approx 75% with trisomy 21 die in embryonic or foetal life
  • 2% robertosonian translocation (50% familial, 50% de novo)
  • 1% other chromosomal arrangement involving 21q22
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25
structural chromosomes rearrangements
- can be spontaneous. rate increased by exposure to mutagenic agents - can be balanced or unbalanced - balanced: rearrangement is complete. no loss of genetic material. usually harmless. carriers often at risk of producing offspring with an unbalanced chromosomal complement - unbalanced: chromosome complement contains an incorrect amount of chromosome material. usually has serious effects
26
robertsonian translocation
- results from the breakage of two acentric chromosomes at or close to their centromeres, with subsequent fusion of their long arms - the short arms of each chromosome are lost but this is of no clinical importance - no loss or gain of important genetic material — functionally balanced arrangement - outcomes: normal, carrier of balanced translocation, complete trisomy of one of the involved chromosomes, complete monosomy of one of the involved chromosome = always lethal (can have a conceptus but not a child)
27
what is an acentric chromosome?
chromosome where the centromere is near the end of the chromosome
28
why is the fact that the short arms of each chromosome are lost in robertsonian translocation of no clinical importance?
contain genes for only rRNA, for which there are multiple copies on various other acentric chromosomes
29
robertsonian translocation in relation to downs
translocation is unbalanced and the baby has 3 copies of the long arm of chromosome 21 instead of 2
30
reciprocal translocations
- involves the breakage of at least 2 chromosomes with the exchange of fragments - usually the chromosome number = 46 - unique to a particular family - problems arise in meiosis as the chromosomes involved in translocation cannot pair normally to form bivalents — instead they cluster to form a pachytene quadrivalent - each chromosome aligns with homologous material in the quadrivalent - several ways the constituent chromosomes in the quadrivalent can separate
31
definition of genetic disease
any disease which results from ‘changes’ in the genetic material
32
what can genetic disease affect?
germ line cells = inherited condition | somatic cells = non-inherited condition
33
changes to single chromosomes (4)
1. deletion = loss of a part of the chromosome 2. inversion = inversion of a segment of the chromosome which may (pericentric inversion) or may not (paracentric inversion) involve the centromere 3. duplication = duplication of a chromosomal segment 4. isochromosome = duplication of one part of the chromosome, coupled with the loss of the other arm so that the chromosome consists of 2 identical arms
34
changes to two chromosomes (2)
1. insertion = breakage of material from one chromosome and insertion into another 2. translocation = exchange of genetic material between 2 chromosomes (reciprocal and robertsonian)
35
phenotypic consequences of chromosomal abnormalities: single gene
disrupt a single gene leading to loss of product/translocation it to a region of active chromatin domain so that it is inappropriately expressed/create chimaeric gene that expresses an altered protein
36
phenotypic consequences of chromosomal abnormalities: small group of genes
- where there is a deletion of a small group of genes, the resulting phenotype can be attributed to the lack of product of several genes - referred to as contiguous gene syndrome
37
phenotypic consequences of chromosomal abnormalities: large region of chromosomes with many genes or whole chromosome
- eg. trisomy 21 - results in severe brith defects, including mental and growth retardation, specific abnormalities - largely due to dosage imbalance of only a few genes on those chromosomes — for most genes, having an extra copy/lack of the gene, which results in 5-% increase/decrease in its product, is unlikely to be significant
38
what antenatal screening is offered in the 1st 10 weeks?
offered screening for sickle cell and thalassemia — blood conditions passed down through genes
39
what antenatal screening is offered in weeks 8-12?
midwife offers test for HIV, hepatitis-B and syphilis. | if woman is diabetic she will also be offered eye screening
40
what antenatal screening is offered in weeks 10-14?
offered combined test = USS + blood test
41
what does the combined test look for?
Downs/Edwards/Patau’s syndrome
42
what is offered up to 20 weeks if the combined test is not done?
quadruple test — looks for only Down’s Syndrome and is not as accurate as the combined test
43
what antenatal screening is offered in weeks 18-21?
another USS = ‘20 week scan’ — looks for 11 physical conditions in baby
44
what is offered towards the end of the pregnancy?
midwife discuses screening options for baby so she knows what to expect
45
what is offered within 3 days of the birth?
physical examination of baby: heart/eyes/hip conditions
46
what is offered up to 3 months after birth?
hearing test — either in hospital or at home
47
what is offered 5 days after birth?
midwife offers blood spot test — screens for 9 rare but serious conditions
48
the combined test is used to calculate the RISK of an unborn baby having Down syndrome. the results of which tests are used to calculate this risk?
ultrasound scan and blood test
49
what is lyonisation?
inactivation of an X chromosome to prevent females having twice as many gene products from the X chromosomes as males
50
in transcription, which enzyme is used to ‘unzip’ the DNA strands?
DNA helicase
51
what is the most common cause of spontaneous abortions during the first trimester?
chromosomal abnormalities
52
how likely is it that a child born to a 45 year old mother will have Down syndrome?
1 in 19
53
what is the function of the ductus venosus?
causes blood to bypass the liver
54
which technique would be most suitable to test a foetus for CONFIRMATION on the presence of trisomy 21 before 15 weeks gestation?
= CVS (at 10-12 weeks) | amniocentesis from 15 weeks
55
according to Piaget, what is a schema?
a pattern of thought made up of preconceived ideas representing an aspect of the world used to interpret new information
56
what are the most common features of Down’s syndrome?
stunted growth, low muscle tone, flattened facial features, abnormal fingerprints
57
satellites are present on which type of chromosome?
acrocentric
58
how does minepristone act?
by blocking the action of progesterone
59
what is the earliest test that can be performed for chromosomal abnormalities?
vaginal ultrasound
60
what is nuchal translucency? when is it measured? what is normal?
the sonographic appearance of a collection of fluid under the skin behind the fetal neck in the first-trimester of pregnancy. less than 3.5mm is normal. measured between weeks 11-14
61
what is the usual means of termination carried out at 6 weeks?
mifepristone followed by Prostaglandin
62
what is the usual means of termination from 6-14 weeks?
vacuum aspiration or suction termination
63
what is the usual means of termination for weeks 16-24?
prostaglandin injection into womb
64
what is the role of the ductus arteriosus?
directs transfer of blood from the right ventricle into the aorta
65
what is polyploidy?
multiples of N greater than 2N
66
what does the combined test involve?
serum screening and ultrasound screening (nuchal translucency)
67
what is Klinefelter syndrome?
a disorder of the sex chromosomes resulting in 47 XXY (boys born with an extra X chromosome)

PBL SEM 1 (9 decks)

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