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Chromosome Disorders
Structural abnormalities
Structural chromosome abnormalities arise when there are breakages in chromosomes, leading to a net loss, gain or abnormal rearrangement of one or more chromosomes. Structural abnormalities include deletions and duplications, including those too small to be seen down the light microscope (microdeletions and microduplications) those near the end of the chromosome (subtelomeric deletions and duplications), ring chromosomes, reciprocal translocations (balanced or unbalanced), Robertsonian translocations (balanced or unbalanced), insertional translocations (balanced or unbalanced) and inversions (pericentric or paracentric). As analytical techniques improve, smaller and more subtle structural abnormalities can often be detected where previously a person's karyotype was thought to be normal.
Balanced and unbalanced reciprocal translocations
Balanced reciprocal translocations arise when breakages occur in two or more different (non-homologous) chromosomes with the resulting detached segments swapping places with each other. No chromosomal material has been lost or gained, just rearranged. Usually, carriers of a balanced reciprocal rearrangement will have no symptoms themselves. However, they run the risk of having problems at reproduction, e.g reduced fertility, miscarriage and/or the birth of a child with an unbalanced reciprocal translocation (in which there is both a net loss and gain of chromosomal material) and therefore symptoms. A small proportion of babies born with an apparently balanced reciprocal translocation will have symptoms. One of the reasons may be that the break has caused damage to a gene. Another may be that there is a microdeletion or microduplication that has caused the symptoms.
Robertsonian translocations
Balanced Robertsonian translocations arise when the short arms of two of the acrocentric chromosomes (chromosomes 13, 14, 15, 21 or 22) are lost and the remaining parts of the centromeres and the long arms fuse together. Although carriers of such balanced Robertsonian translocations would not be expected to have symptoms themselves, they run the risk of reduced fertility, miscarriage and/or producing babies with unbalanced chromosomes and therefore symptoms.
Balanced and unbalanced insertional rearrangements
Balanced insertions occur when a segment of one chromosome is inserted into a break in the arm of another chromosome. No chromosomal material has been lost or gained, just rearranged. Carriers of such balanced insertions should not have symptoms but they are at risk of fertility problems, miscarriage and/or the birth of a child with an extra or missing copy of the inserted chromosomal segment and therefore symptoms.
Deletions
A deletion (sometimes called a partial monosomy) involves loss of a segment of a chromosome. Deletions can occur near to the centromere (proximal deletion), nearer to the end of the chromosome (distal deletion), in the middle of a chromosome arm (interstitial deletion), at the end of the chromosome (terminal deletion) or as a tiny segment missing very near to the telomere (subtelomeric deletion). Deletions can cause symptoms due to loss of any genes contained within the deleted chromosomal segment.
Ring chromosomes
Ring chromosomes arise when the ends of both arms of a chromosome are lost and the remaining broken ends join together to form a ring shape. Thus a ring chromosome is essentially a double deletion. However, if a ring chromosome exists as an extra, supernumerary chromosome, then it is the material contained within the ring that is in effect duplicated.
Duplications/triplications
A duplication (sometimes called a partial trisomy) occurs when an extra copy of a segment of a chromosome is present. If a person has two extra copies of a chromosomal segment, this is known as a triplication (or sometimes as a Partial tetrasomy). Duplications and triplications can cause symptoms due to gain of extra copies of any genes contained within the additional chromosomal segment.
Inversions
Inversions arise when a chromosome breaks in two places, the intervening segment flips around and is then re-inserted into the 'gap.' Inversions are known either as paracentric, if the two breaks are in the same chromosomal arm, or as pericentric if the breaks occur in different arms of the same chromosome. Inversions would not usually cause symptoms in the carrier but can lead to fertility problems, or miscarriage. In the case of pericentric inversions there is also a risk for the birth of an affected baby with duplications and deletions of the two end segments of the chromosome involved. Carriers of paracentric inversions very rarely give birth to children with symptoms.
Isochromosomes and extra structurally abnormal chromosomes
Sometimes people carry an extra or supernumerary chromosome made up of part of one or more chromosomes. They will effectively carry a duplication of the material forming this extra chromosome. If the origin of the extra chromosome is unknown, it is referred to as an extra structurally abnormal chromosome or an extra marker chromosome. If the extra chromosome is made up of two copies of the same segment of a chromosome, this is called an isochromosome. When these extra chromosomes carry two copies of the same centromere, they are called isodicentric chromosomes.
Uniparental disomy
Normally one member of each pair of chromosomes is inherited from the father and the other from the mother. In rare cases, a trisomy in an early embryo can 'correct' itself by losing one of the extra chromosomes. If the remaining two chromosomes are inherited from the same parent, then this is called uniparental disomy (mUPD for inheritance from the mother and pUPD for inheritance from the father). UPD will only cause problems when the chromosome involved is susceptible to imprinting (i.e. the expression of the genes is parent-specific).
Numerical abnormalities
| Inheritance patterns and prenatal diagnosis 
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