Abnormalities of Chromosomal Structure

Four types of structural changes may occur in chromosomes: duplications, deletions, translocations, and inversions. All may result when there is breakage of the chromosomes and a rejoining or loss of chromosome fragments. If the same broken ends rejoin, the chromosome becomes intact once again. The resulting effects of such events depend on how large they are and where they occur on the chromosome. Rearrangements may occur in many forms and are less common than abnormalities of chromosome number.

The most common type of rearrangement is called a balanced translocation because the amount of genetic information within that cell is normal even though it is repositioned. Therefore the individual with a balanced translocation may appear normal. However, there will be a risk to the children of a carrier of a balanced translocation since that person is likely to produce unbalanced gametes (bearing too little or too much genetic information), and therefore the risk of having abnormal offspring is increased. Rearrangements such as aneuploidy may be found in all cells of an individual, or they may occur only in a percentage of an individual's cells. This latter condition is known as mosaicism. In general, mosaic individuals show a less severe expression of their syndrome than those with chromosome abnormalities in all their cells.

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