Chromosome Rearrangement

Question 1

Rearrangements can be balanced (no loss or gain of genetic material) or unbalanced (gain or loss present). What are three types of balanced rearrangements?

Answer 1

Translocation
Inversion
Insertion

Question 2

Translocation is a type of balanced chromosome rearrangement. There can be Reciprocal or Robertsonian translocations. Describe the following about a Reciprocal translocation:

How common is it?
What does it involve?
What are the translocated segments called?
What are the remaining segments called?
What is the rearranged chromosome called?

Answer 2

A reciprocal translocation is quite common at 1/500
It involved 2 breaks then exchange
The translocating segments are called translocated segments
The remainder are called centric segments
The rearranged chromosome is called a derivative chromosome

Question 3

Reciprocal translocations can be random but which two recurrent reciprocations in humans are there?

Answer 3

7+9

Denoted t(7;9) for the swap with the derivative chromosomes being der(7) and der(9)

Question 4

What are the phenotypic effects of a reciprocal translocation?

Answer 4

Usually no effect (5-10% risk) due to the fact mostly happens in junk DNA- one break gives a 3% chance of breaking a gene

Those who do have a phenotype will be due to the fact that the position has altered the function of the genetic material

Sometimes there’s an increased reproductive risk with higher chance of miscarriage or abnormal offspring due to malsegregation at meiosis

Question 5

During meiotic pairing (of translocated chromosomes) tries to form the most intimate pair which means many different alternatives can be inherited. A pachytene cross can be formed to show these. What is the chance of inheriting an imabalance? What happens to those which do?

Answer 5

50% chance

High risk of miscarriage

Question 6

Another type of translocation is the Robertsonian translocation.

What is this?
How common is it?
What sort of phenotypic effects does it have?

Answer 6

When there are two breaks and there is fusion of q arms of acrocentrics (chromosomes with very very small p arms)
1/1000
Normal phenotype except reproductive risk

Question 7

What is the likely outcome at meiotic segregation if the diploid cell has a robertsonian translocation?

Answer 7

Likely to be unbalanced or have trisomic/monosomic gametes

Question 8

Are the outcomes of meiosis in a cell with a robertsonian translocations lethal or not?

Answer 8

There can be normal, balanced (as p-arms don’t encode genes in these so no info is actually lost) or unbalanced (down syndrome) which are not lethal. Other unbalanced gametes are lethal

Question 9

Inversion is another balanced rearrangement of chromosomes.

What is it?
What’s the different between pericentric inversions and paracentric inversions?
Is it common?
Is there a phenotypic risk?

Answer 9

2 breaks, rotation then rejoining

Pericentric inversion: break in q-arm, break in p-arm, flips 180 degrees
Paracentric inversion: two breaks in single arm then this bit rotates
1/1000
low phenotypic risk
reproductive risk

Question 10

Insertions are the final balanced rearrangements.

What are they?

Answer 10

3 breaks followed by movement of interstitial segment. (2 breaks in one chromosome then this sections fits into the other chromosome at the 1 break)

Question 11

Unbalanced rearrangements are those which have overall loss or gain of info. Deletions and duplications can cause genomic disorders.
Is this common?
Are they sporadic (inherited or not)?

Answer 11

1/2000- quite common
most common= 4Mb deletions and duplications
Mostly sporadic
phenotypic effects very varied

Question 12

Deletions can cause contiguous gene syndromes/genomic disorders. What does it mean if the segment lost is:
Interstitial?
Terminal?
Microdeletions?
Macrodeletions?

Answer 12

Interstitial= 2 breaks in a single chromosome
Terminal= 1 break at end
Microdeletion= 1 (can see down microscope)

Question 13

Prader Willi syndrome is a microdeletion syndrome. How common is it?
Where is the deletion?
How do you detect it?

Answer 13

1/10000
70% deletion 15q11-13 (paternal) (always on paternal chromosome 15)
FISH

Question 14

What are the phenotypic features of Prader Willi syndrome?

Answer 14

Short stature
Small hands/feet
Floppy newborn
Hypogonadism
Almond-shaped eyes
Obesity- especially early childhood
Babies difficult to feed

Question 15

Angelman’s syndrome is another microdeletion syndrome. How common is it?
What is it?
Who is it inherited from?
What detects it?

Answer 15

1/20000
Deletion of 15q11-13 (maternal)
-many unkown causes as well
FISH

Question 16

What are the phenotypic features of Angelman’s syndrome?

Answer 16

Floppy
Poor control of muscles
Seizures
Jerky movements
Happy appearance
Prominent jaw

Question 17

Duplications are also unbalanced rearrangements. Why are they called partial trisomy?
What is it called when the orientation is maintained?
What is it called when the orientation is reversed?
How do they form?
Are they more or less common than deletions?
Are they more or less detrimental?

Answer 17

Partial trisomy because there is an additional copy of part of a chromosome region
Direct duplication is when orientation is maintained
Inverted duplication is when orientation is reversed
Formation can be due to malsegregation of parental rearrangement or unequal crossing over at meiosis
More common than deletions and less detrimental

Question 18

An example of a microduplication is 16p11.2. Is this common? What are the phenotypic features of this?

Answer 18

3/10000

Varied:
Delayed speech and development
Minor facial or physical features
Delay in sitting and moving
Difficult behaviour
Increased susceptibility to autism/ mental health disorders
Possible overweight
Possible seizures
Birth defects 

Parents sometimes have same genotype but no phenotype or only a few traits

Question 19

Ring chromosomes are unbalanced rearrangements.
Are they common?
What are they?
What phenotype do they give?

Answer 19

Very rare
Breaks in two positions, one in p-arm and one in q-arm
Terminal ends lost- ‘sticky’ ends reuniteand form a ring/loop
Phenotype depends on origin and size of deletion
Centromere presence gives stability

Question 20

What is a marker chromosome?

How are they formed?

Answer 20

A marker chromosome (mar) is a structurally abnormal chromosome in which no part can be identified. The significance of a marker is very variable as it depends on what material is contained within the marker. It is essentially a partial trisomy

Formed by chromatid exchange of malsegregation of a parental translocation (e.g t(11;22))

Question 21

What is an isochromosome?

How are they formed?

Answer 21

An isochromosome is an unbalanced structural abnormality in which the arms of the chromosome are mirror images of each other

Formed by U-loop formation of replicating DNA strands followed by endoreplication (e.g i(12p), i(18p), i(Xp&q)
(breaks at centromere in a transnverse way)

Question 22

What are dicentrics?

What drives these?

Answer 22

Similar to reciprocal translocation but two whole chromosomes join together- breakage in 2 chromosomes, centric segments rejoin, segments distal to breaks lost- big structural and deleterious changes

Hallucinogenic drugs, radiation exposure drive these (used to be used to measure radiation exposure)