Origin of the deviating chromosome structure

Introduction

Structural aberrations are the result of chromosomal breaks that occur during meiosis. Here deletion, duplication and isochromosome formation lead to an abnormal phenotype, while insertion, inversion as well as translocation can be balanced. This means that the carrier of this structural chromosome aberration can escape notice phenotypically, because the entire genetic material is present.

Reminder
The various regions of a chromosome

Deletion

A deletion can happen in every chromosome and exhibit every size. The consequences of a deletion depends on the size of the missing segment and which genes are found on it.
A partial deletion on the short arm (p) of chromosome 5, for example, is responsible for the "cri du chat" syndrome.

Fig. 7 - Deletion
media/module11/k2c_deletion.gif

A
Normal chromosomes pair 5
B
Deletion 5p

Legend
Fig. 7

A DNA piece has been taken out of the chromosome 5. Depending on where the piece has been removed and how long it is, this leads to the loss of several to many genes.

 
Animation

Animation

 

More info

The "cri du chat" syndrome manifests itself through cat-like crying of the newborn. This disorder is accompanied by microcephaly, severe psychosomatic and mental retardation and cardiac defects.

Ends (telomeres) of chromosomes sometimes break off and are lost. In this case the formation of chromosome rings can take place in that the two ends bind to one another. If no genetic information is lost, such structural changes are balanced and the phenotype is inconspicuous. Problems only arise in gamete formation.

Fig. 8 - Ring formation
media/module11/k2d_ring.gif

A
Normal chromosome pair
B
Ring formation

Legend
Fig. 8

A ring can form if there has been a loss of the chromosome ends.

 
Animation

Ring formation

 
Image

Karyogram

Duplication

A chromosome duplication is the doubling of a chromosome piece. A duplication is sometimes termed a "partial trisomy". If, therefore, a duplication is present, the person is equipped with 3 copies of the genes in the associated chromosome segment.
This means that extra directions (genes) are present, leading to congenital abnormalities or developmental problems.

Fig. 9 - Duplication
media/module11/k2e_duplikation.gif

A
Normal chromosome pair
B
Chromosome with one duplication

Legend
Fig. 9

A chromosome segment is doubled and is built in again into the same chromosome.

 
Animation

Duplication

More info

The fragile X syndrome results from multiple duplications of a CGG segment (trinucleotide) in the 5' untranslated region of the FMR1 gene on the X chromosome (Xq27). Four forms of this syndrome are recognized:

  1. 6 - 40 repetitions = weak form
  2. 41 - 60 repetitions = middle form
  3. 61 - 200 repetitions = strong form
  4. >200 repetitions = full mutation, influencing the phenotype

Since it is an X linked mutation men are more affected than women. The fragile X syndrome leads to one of the most frequent forms of inherited mental retardation. In addition, the syndrome causes a prognathism, the face is long and small and the patient has large ears.

Inversion

If chromosome pieces that have been broken out become inserted again, but reversed, an inversion has occurred.
The phenotype of this disorder is usually unobtrusive, since the entire chromosomal information is still present. When the interchanged region includes the centromere, one refers to it as a pericentric inversion, otherwise to a paracentric inversion.

Fig. 10 - Paracentric inversion
media/module11/k2f_inversionPara.gif

A
Normal chromosome
B
Chromosome with paracentric inversion

Fig. 11 - Pericentric inversion
media/module11/k2f1_inversionperi.gif

A
Normal chromosome
B
Chromosome with pericentric inversion

Legend
Fig. 10

The piece that was cut out has been reinserted upside down.

 
Animation

Paracentric inversion

Fig. 11
 
Animation

Pericentric inversion

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Insertion

If chromosome pieces are reinserted somewhere else, this is referred to as an insertion. Carriers of such insertions can be phenotypically inconspicuous because no information has been lost.

Fig. 12 - Insertion
media/module11/k2g_insertion.gif

A
2 normal chromosome pairs
B
Insertion of a broken off piece on another chromosome

Legend
Fig. 12

Insertion of a broken off piece into another chromosome.

 
Animation

Insertion