Structural Biochemistry/Structural Variation

A structural variation (SV) is a alteration of the genome in an organism's chromosome. These variations may be deletions, duplications, copy-number variants, inserts, inversions or translocation. Regions of alteration can range from as small as 1Kb to 3Mb in size.

Duplication

In chromosomal duplication, extra copies of a region in the chromosome are formed. If these duplications are side by side, this is known as tandem duplication. If they are separated, however, they are said to be displaced.

If an organism has several copies of the same gene, its phenotype may be altered. As the gene segments are multiplied, so are the chances of that gene being transcripted, translated and expressed. As a result, people may produce more protein than others. This may play either a detrimental or beneficial role in the organism's physiology.

Organisms may also benefit from this anomaly in that it gives them extra genes that may mutate into other vital genes necessary for evolutionary fitness.

Deletion

Deletion is a mutation in the chromosome that results in a loss of DNA sequences. Depending on the size of the deletion, the organism may experience various phenotypes. Similar to duplication, deletion may alter gene dosage.

Deletion can be extremely harmful to an organism, depending on the size and location. Some phenotypes require two genes to be present for normal expression. Haploinsufficiency is the term used to describe a mutant phenotype that only has one of the two genes.

Sources

Amos-Landgraf, J. M., et al. Chromosome breakage in the Prader-Willi and Angelman syndromes involves recombination between large, transcribed repeats at proximal and distal breakpoints. American Journal of Human Genetics 65, 370–386 (1999)

Christian, S. L., et al. Large genomic duplicons map to sites of instability in Prader-Willi/Angelman syndrome chromosome region (15q11-q13). Human Molecular Genetics 8, 1025–1037 (1999)