chromosomal abnormalities

Question 1

• What is meant by karyotype?

Answer 1

An individual’s set of chromosomes in terms of number and structure of chromosomes

Question 2

• How do you prepare a karyotype?

Answer 2

1. Collect ~5ml heparinised venous blood
   Can use amniotic cells, CVS
   2. Isolate white cells
   3. Culture in presence of phytohaemagglutin
      Stimulates T-lymphocyte growth/differentiation
   4. After 48 hours add colchicine
      Causes mitotic arrest - metaphase
   5. Place in hypotonic saline
   6. Place on slide
   7. Fix and stain

Question 3

• What is meant by chromatin?

- What stain is used to make a karytype?

Answer 3

A substance within a chromosome consisting of DNA and protein

Giemsa dye

Question 4

• What are the 5 common structures, identified by the G-Banded architecture of a chromosome?

Answer 4

Telomere on either end of the chromosome

p-arm - short arm 

q-arm- long arm

G-dark and G-light bands

Centromere

Question 5

• How would you refer to the small aberrations within chromosomes?

Answer 5

State whether they are on the p or q arm

State their band number (numbers go before letters)  (based on position from centromere)

State their sub-band number

If possible state Sub-sub-band feature

Question 6

• What does bphs stand for?
• What do the darker regions of a chromosome karyotype mean?
• What do the lighter regions of a chromosome karyotype mean?

Answer 6

Bands per haploid set

Dark = heterochromatin - more compact fewer genes

Light = euchromatin - more open more genes

Question 7

• Why are karyotypes performed more in Prophase rather than Metaphase?
• What is meant by Aneuploidy?

Answer 7

Because the chromosomes are less compact and therefore you get more detail from the karyotype

An abnormal number of chromosomes

Question 8

• What is the purpose of meiosis?

- What method of genetic variation does meiosis enable?

Answer 8

To achieve reduction from diploid (2n = 46) to haploid (n = 23)
To ensure genetic variation in the gametes

Random assortment of homologues
Recombination

Question 9

• What is non-disjunction and what does it result in?
• If non-disjuncture occurs in meiosis I are all daughter cells affected?
• If non-disjuncture occurs in meiosis II are all daughter cells affected?

Answer 9

When chromosomes are not split properly between daughter cells
Always results in either 1+ (trisomy) or 1- (monosomy) chromosomes

Yes

No, half are affected

Question 10

• What is the most common form of chromosomal abnormality?

- Why is sex imbalance tolerated?

Answer 10

Sex chromosome aneuploidy

X-inactivation of excess X chromosomes
Only 1 X chromosome activity
Low gene content of Y chromosome- so not much impact on phenotype

Question 11

• Why, if inactivated, does abnormal number X have effect on phenotype?

Answer 11

Both X and Y chromosome have PAR (pseudo-autosomal region)

PAR still produces genes in this region

So when x chromosome is inactivated

genes in PAR region still active and can be transcribed (but autosomal function rather than sex chromosome)

Question 12

• What is the second most common chromosomal abnormality in live births?
• What is this abnormality due to?

Answer 12

Trisomy 21 (Down Syndrome)

3 copies of chromosome 21
Non-disjunction during maternal meiosis (85-90%)

Question 13

• What is correlation between maternal age and cases of trisomy per 1000 births?
• Why is it then, that 75% of children with Down syndrome are born to mothers under 35 (younger)?

Answer 13

As maternal age increases, the number of cases rises exponentially

Because 90% of children are born to mothers at this age

Question 14

• Why does the increased maternal age and increased risk of trisomy correlation exist?

Answer 14

Older the mother, the longer the oocyte has been in the state of meiosis

So chromosomes do not line up as neatly during metaphase

Likely due to degradation of factors which hold homologous chromatids together (cohesin)

premature separation of homologous chromosomes into separate sister chromatids

Question 15

• Why does non-disjunction occur in the spermatogenesis?

Answer 15

Primary spermatocyte undergoes ~ 23 mitotic divisions per year and potentially accumulate defects

Question 16

• What single gene disorders are parental age a risk factor of?
• What is the risk factor increasing chances of paternal involvement in aneuploidy?

Answer 16

Apert syndrome
Crouzon syndrome
Pfeiffer syndrome

Smoking

Question 17

• What do the terms qter and pter refer to?

- What is ‘der’ an abbreviation of?

Answer 17

Tip of long arm

Tip of short arm

Derivative chromosome (contains extra material)

Question 18

• What are ‘ins’ and ‘inv’ abbreviations for?

- What is the abbreviation for translocation?

Answer 18

Insertion

Inversion

t

Question 19

• What does a +/- before a chromosome number, tell us about it?
• What does a +/- after chromosome number, tell us about it?

Answer 19

Gain/loss whole chromosome

Gain/Loss of part of a chromosome

Question 20

• In what phase does the crossing over of chromosomes occur?
• What is the purpose of Crossing Over?
• How does Crossing Over occur?

Answer 20

Prophase I

To increase genetic diversity

Pairs of chromosomes align
Chiasma form
Crossover occurs

Question 21

• How often does Crossing Over occur?

- Explain how Unequal Crossing Over occurs?

Answer 21

1-3 times per chromosome per meiosis

Chromosomes do not align correctly at centromeres
Unequal lengths of chromatids are exchanged
Leading to deletion of genes in one chromosome
And duplication of genes in the other chromosome

Question 22

• What are the reasons for deletion of genes in single chromosome abnormalities?
• What is the most common cause for a duplication abnormality?

Answer 22

Unequal crossing over
Breaks in chromosome
Can occur at end

Most often caused by unequal crossing over

Question 23

• What are the 2 types of inversion in single chromosome abnormalities?

Answer 23

Paracentric inversion - Breaks in chromosome occur in middle of chromosome arm & that section of chromosome is inverted, creating reinvented piece of DNA

Pericentric inversion - Breaks occur around centromere & that section of chromosome is inverted, creating reinvented piece of DNA

Question 24

• Are carriers of inversion abnormalities affected?

Answer 24

Often carriers are not affected (as the same genes are present) , but can cause reproductive problems as children can have deletions and insertions

Question 25

- Explain how the insertion occurs in 2-chromosome abnormalities - what does this means for the patient

Answer 25

Section of chromosome from one chromosome is inserted into another chromosome If translocation in balanced fashion, carrier is unaffected May cause problems in offspring if they inherit one normal and one translocated one- need to have either no translocated or both translocated to not be affected

Question 26

- What is the theoretical % chance that the child of a parent with translocation of chromosomes will be affected?

Answer 26

50% as there is a 2 in 4 chance that there will be an unbalance

Question 27

- What is meant by a Microscopic chromosomal deletion? | - What is meant by Chromosomal Microdeletion?

Answer 27

Deletion of chromosome can be detected easily in microscope Cri-du-chat syndrome 46, XY, del(5p) Deletion can be seen in high resolution banding Velocardiofacial/DiGeorge syndrome 22q11.2del but still 20+ genes deleted

Question 28

- What is meant by a de-novo chromosomal abnormality? | - What causes William's syndrome?

Answer 28

When the abnormality was not inherited from the parent of the patient and so it is new to the individual 7q11.23 deletion

Question 29

- What are the symptoms of William's syndrome?

Answer 29

Long philtrum Slightly upturned nose Lack social anxiety Arched eyebrows Supravalvular aortic stenosis

Question 30

- What are the phenotypes of William's syndrome caused by? | - What is one of the chromosomal features of William's syndrome?

Answer 30

Imbalance of genes which are unrelated apart from their genomic location Lack of elastin on affected chromosome

Question 31

- What are some of the symptoms of 7q11.23 duplication syndrome?

Answer 31

Delayed speech development Autistic behaviours (social anxiety) Dilation of aorta Flat eyebrows Broad nose and short philtrum

Question 32

- Compare the severity of the phenotypes of duplication and their corresponding deletion

Answer 32

Duplications usually have milder phenotype than the corresponding deletion

Question 33

- Describe the 3 classes of chromosomes

Answer 33

Metacentric - Short arm p and long arm q (but relatively similar in length) Submetacentric - short arm considerably shorter than long arm Acrocentric - short arm has been reduced to a short stump- non coding satellites

Question 34

- Which chromosomes are generally acrocentric in humans? - What type of translocation can only acrocentric chromosomes undergo? - Can this type of translocation be homologous or non-homologous?

Answer 34

13, 14, 15, 21, 22 Robertsonian translocation Both

Question 35

- Explain the mechanism behind Robertsonian translocation?

Answer 35

Both non coding satellites of the 2 different acrocentric chromosomes are removed The remaining long arms join together - translocating to form a derivative chromosome

Question 36

If this occurs during the development, most people are unaffected but why might their offspring still be affected?

Answer 36

no genetic information has been lost, just rearranged but offspring may not inherit it in a balanced manner

Question 37

- What are the 6 possible segregation products of the carrier that would be passed onto gamete for 21q14q?

Answer 37

14 & 21 - Normal Derivative - Balanced Derivative & 21 - Unbalanced (Down Syndrome) 21 - Unbalanced (lethal) 14 - Unbalanced (lethal) Derivative & 14 - Unbalanced (lethal)

Question 38

- What is meant by Mosaicism? | - By what 2 mechanisms can mosaicism occur?

Answer 38

Presence of 2 or more populations of cells with different genotypes Non-disjuncture during early development Loss of extra chromosome in early development

Question 39

- Are the results of the phenotypes of mosaicism generally mild or heavy? - If everyone is thought to be mosaic, then why are most people not affected by it? - What are the most common mosaics?

Answer 39

Mild This mosaicism is lost in early development It will only become evident if passed on in germ cells Where one of the sex chromosomes has been lost during development 46XY/45X or 46XX/45X