An isodicentric chromosome 15, or idic(15), is an extra chromosome that is made from two pieces of chromosome 15 that are stuck together end-to-end by their q arms. They typically have two p arms, one on each end, and the amount of DNA that is included from the q arm can vary. When an idic(15) is described on a genetic report, the band numbers are used to give approximate coordinates of the two ends that fused. Essentially all idic(15) chromosomes that we are aware of are maternally inherited.
This shows two different forms of idic(15). On the left, the idic(15) is carrying two extra copies of part of chromosome 15, starting at the p - arm and extending down the long arm to band 15q13.2. When this idic(15) is found in a cell that already has 2 normal copies of chromosome 15, there are 4 total copies of the genes included on the idic(15). This would be 2 extra “Core” and “E” pieces. The idic(15) on the right is the most common type and is asymmetric. On one end, the idic(15) contains part of the q arm down though 15q13.3, but on the other end, it only goes to 15q13.2. This results in 2 extra “Core” and “E” regions but only 1 extra “e” region.
Some children and adults with idic(15) are said to have "mosaicism", meaning that their idic(15) is not present in all of the cells in their bodies. Mosaicism happens by chance in this and many other chromosomal disorders.
Generally, people with idic(15) do not have other family members with the chromosomal abnormality. The idic(15) usually forms by chance in one person in the family. Children with idic(15) are born to parents of every socioeconomic, racial, and ethnic background. There is no known link between idic(15) and environmental or lifestyle factors. In other words, there is nothing that parents did before or during pregnancy to cause their child to be born with idic(15).
This is general information. Families should always seek the advice of a genetics specialist to discuss their specific situation.