Translocation
Robertsonian
Translocation between acrocentric chromosomes (13,14,15,21,22) - commonly 13 and 14
Individual has 45 chromosomes as 2 have fused
Carriers clinically normal
Can have implications for Offspring - e.g. greater risk of Down's Syndrome
21:21 fusion will never give normal offspring - either trisomy 21 (Down's) or monosomy 21 (lethal)

Reciprocal

When breaks occur in two different chromosomes and material is mutually exchanged

The resulting chromosomes are called derivative chromosomes

Carriers are unaffected because they still have the complete complement of genetic material (a reciprocal balanced translocation)

Carrier’s offspring can be normal, carry the translocation or can have deletions or duplications of genetic material (unbalanced translocation)

This will be either partial trisomy or partial monosomy resulting in an abnormal phenotype

How does this happen? Either 2:2 segregation can occur which can be alternate or adjacent or 3:1 segregation can occur at meiosis.

See right

2:2 Segregation

Alternate – A and D will go into one gamete and C and B into another. This is balanced.

Adjacent – A and C will go into one gamete and B and D into another. This is unbalanced.

3:1 Segregation

If A C and D segregate together this is unbalanced. Resulting in trisomy for material on C in one gamete and monosomy for material on B in another gamete

Inversion

2 breaks occur in the chromosome

The reinsertion of the fragment is then in inverted order

They are a balanced structural rearrangement so don’t alter phenotype in the carrier

Pericentric – involve the centromere

Paracentric – don’t involve the centromere (only on one arm)

Inversions can interfere with meiosis, producing abnormalities in offspring due to deletions or duplications if crossing over occurs within the inversion

If it is a pericentric inversion this can result in an acentric (no centromere) or dicentric (two centromeres) chromatid

Ring

When deletions occur at both tips of the chromosome

Ends then fuse, forming a ring

If they contain a centromere, it can proceed through cell division

However they are often lost resulting in monosomy for the chromosome

When found in humans they are normally mosaic with rings in some cells and monosomy in others

e.g. ring chromosome 20 syndrome - epilepsy

Insertions

Segment of one chromosome inserted into the other

Can result in a frame shift

Also can disrupt gene exons

If the chromosome with the insertion is coupled with the chromosome with the deletion in the gamete then this is balanced. If not gametes will be abnormal (50%)

Deletion

Very rare

Can be ‘de novo’ mutation or inherited as a result of translocation or inversion in the parent

Interstitial – loss of middle segment of chromosome

e.g. Prader-Willi syndrome – a small deletion of seven genes

Terminal – loss of end of chromosome

Results in partial monosomy

Serious clinical effect

e.g. cri-du-chat syndrome (5p15): child has distinctive cry. Caused by a deletion of the distal short arm of chromosome 5. Results in mental retardation, small head, many survive to adulthood

	Created by Ellie Quinn about 10 years ago

Note on Chromosome Abnormalities, created by Ellie Quinn on 13/02/2014.

Chromosome Abnormalities