Cytogenetic Tests

Question 1

How long is the cell cycle for actively dividing cells (excluding cells in G0)

Answer 1

Approximately 24 Hours

Question 2

Mitogen

Answer 2

Bioactive protein that can be used to stimulate cells in G0 to enter the cell cycle in vitro (Usually 24 hour lag before cells start cycling post exposure)

e.g. Phytohaemagglutinin A (PHA) - specific to T cells

Question 3

What is needed for cytogenetic analysis of cells

Answer 3

Fresh cells that are dividing in order to analyse their chromosomes

Question 4

Cell types usually used for prenatal and postnatal cytogenetic analyses

Answer 4

Postnatal - T cells exposed to mitogen PHA

Prenatal -
Amniotic fluid
Chorionic Villus

Question 5

Cell types usually used for leukaemia and lymphoma

Answer 5

Leukaemia - Bone marrow
Sometimes blood

Lymphoma - Lymph node biopsy
Bone marrow if BM involved

Question 6

3 Basic shapes of human metaphase chromosomes

Answer 6

Question 7

Which chromosome is the smallest

Answer 7

21

It should be 22 but the numbering was done incorrectly at first so it is maintained as per convention

Question 8

P arm and q arm of chromosome

Answer 8

p arm is short arm (think petite)

q arm is long arm

Question 9

How is each chromosome subdivided

Answer 9

Into G-bands numbered from the centromere outwards

e.g. q213

Question 10

Compare number of base pairs and gene loci on X and Y chromosomes

Answer 10

X:
153 Megabase pairs
195 known gene loci

Y:
50 Mb
approx 13 known gene loci (4 in common with X)

Question 11

X Inactivation

Answer 11

Women switch off one copy of their X chromosomes

Occurs at the 5,000 cell stage of the embryo

Occurs randomly; once established, all daughter cells retain same pattern of inactivation as progenitor

Classic example is tortoiseshell cats

Question 12

Isochromosome

Answer 12

Chromosome that contains two identical arms joined at the centromere (two p or two q) (abnormal)

Question 13

Down Syndrome Causes and Frequency

Answer 13

1/650 births

Trisomy 2
Non-Disjunction mostly at maternal meiosis I
(94%)

Robertsonian Translocation - Translocation of parts of chromosome 21
(4%)

Mosaic
Post-zygotic non-disjunction mitotic event

Question 14

Physical features of down syndrome

Answer 14

Flat facial profile (flattened nose)

Eyes slant upwards

Small ears

Flat back of the head

Protruding tongue (particularly large)

Bilateral single palmar crease

Shorter than average with poor muscle tone

Question 15

Clinical Features of Down Syndrome

Answer 15

Mild to Moderate Mental Retardation

Frequent ASD

Cardiac Defects

Increased Leukaemic risk

Question 16

F.I.S.H

Answer 16

Fluoerescence In Situ Hybridisation

Segment of single stranded DNA labelled with fluorescent tag

Hybridises to target DNA attached to a slide with its matching sequence

Question 17

Edwards Syndrome

Answer 17

3/10,000

Trisomy 18

Growth retardation, prominent occiput (back of head)

Small mouth, clenched hands, overlapping fingers

Prominent heels

Most have congenital heart disease and/or sometimes renal abnormalities

50% die by 2 months

Question 18

Patau Syndrome

Answer 18

2/10,000

Trisomy 13

Most have scalp defects

Hypotelorism (narrow eye distance)

Most have polydactyly (extra digits)

Cleft lip and palate

Brain malformation (70%)

Congenital heart disease, renal abnormalities, undescended testes

69% die by 6 months

Question 19

Turner syndrome

Answer 19

Monosomy X

1/10,000 Females

Short stature, webbed neck

Lymphoedema of hands and feet, low posterior hairline, wide carrying angle at elbows, small steep nails

60% have renal abnormalities (Often a single horseshoe shaped liver)

Coarctation of aorta

Gonadal dysgenesis and no secondary sexual development

Question 20

Wolf Hirschhorn Syndrome

Answer 20

Structural Chromosomal Abnormality

Question 21

Recurrent Micro-deletion/duplication syndromes

Answer 21

There are a number of such syndromes that arise as de novo events over and over again due to non-allelic homologous recombination (NAHR) (short regions of repeat sequences) events which may ‘confuse’ the DNA systems that lead to commonly seen micro-deletion/duplication syndromes

Question 22

22q11 Deletion Syndromes

Answer 22

Many names, e.g. Digeorge syndrome

Common deletion but highly variable phenotype

Clefting, Commonly Congenital Heart Disease, Learning difficulties, renal/skeletal abnormalities, skeletal illness, occasionally psychiatric illness without learning difficulties

Characterised by prominent nasal bridge, small mouth

1/4000-5000 births
>15% familial

Question 23

Prader-willi syndrome

Answer 23

Paternal deletion of proximal long arm of chromosome 15 or if both copies came from mother and no paternal copy

Extremely floppy in early infancy (hypotonia)

Develop marked obesity through over-eating; don’t have the feeling of fullness

Mild-Moderate learning difficulties

Question 24

Angelman Syndrome

Answer 24

Maternal deletion of proximal long arm of chromosome 15 or two paternal copies with no maternal (UPD) (reverse of prader-willi)

Inappropriate laughter, convulsions, poor coordination (ataxia), Learning difficulties

Question 25

UPD

Answer 25

Uniparental Disomy

Two chromosomal copies from one parent, none from the other

Question 26

How can the body differentiate between prader-willi or angelman syndrome

Answer 26

The deleted region on chromosome 15 has certain parts that are only expressed when originating from the mother and vice versa

Question 27

How does maternal or paternal UHD

Answer 27

Child would have probably had trisomy as an embryo

Mitotic non-disjunction event takes place and leads to only two copies in the embryo, but if the wrong chromosome is removed then UHD occurs

Question 28

Reciprocal Translocation

Answer 28

Parts of chromosomes translocate around the nucleus to other chromosomes; occur randomly

No clinical consequences; only issue is with reproduction as gametes will be unbalanced

Question 29

Unbalanced Translocation

Answer 29

Partial monosomy in one chromosome and partial trisomy in another due to faults in crossing over during meiosis

Question 30

Examples for cytogenetic referral

Answer 30

Learning difficulties/congenital abnormalities
Recurrent miscarriage (e.g. for unbalanced translocation)
Infertility (e.g. sex chromosome abnormality)
Prenatal Diagnosis (e.g. increased risk of trisomy - age)

Question 31

Klinefelter syndrome

Answer 31

XXY
Infertile
Usually a bit taller