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.
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.
- A
- Normal chromosomes pair 5
- B
- Deletion 5p
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.
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.
- A
- Normal chromosome pair
- B
- Ring formation
A ring can form if there has been a loss of the chromosome ends.
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.
- A
- Normal chromosome pair
- B
- Chromosome with one duplication
A chromosome segment is doubled and is built in again into the same chromosome.
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:
- 6 - 40 repetitions = weak form
- 41 - 60 repetitions = middle form
- 61 - 200 repetitions = strong form
- >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.
- A
- Normal chromosome
- B
- Chromosome with paracentric inversion
- A
- Normal chromosome
- B
- Chromosome with pericentric inversion
The piece that was cut out has been reinserted upside down.
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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.
- A
- 2 normal chromosome pairs
- B
- Insertion of a broken off piece on another chromosome