Isodicentric chromosome 15 syndrome: Year of the Zebra 2026

Isodicentric chromosome 15 syndrome, also known as idic(15) or inverted duplication 15 syndrome, is a rare genetic condition where an individual has extra genetic material from chromosome 15.

Chromosomes contain hundreds to thousands of genes that contain the genetic information needed to set up the normal structure and function of the human body. Each person has two sets of 23 chromosomes, for a total of 46. One full set of chromosomes is inherited from each parent.

Idic(15) is a supernumerary marker chromosome, meaning it’s an additional chromosome beyond the usual 46. However, this chromosome doesn’t contain any new genetic information like the others. Instead, it comes from the long arm, or q arm, of chromosome 15. During cell division, this region is duplicated twice, like making two identical photocopies of a page in a book. These two copies then join together, but just before they fuse, one of the copies flips upside down, creating a mirror-like arrangement.

Because there are two normal chromosome 15s, plus this extra marker chromosome, which itself contains two additional copies of the duplicated region, the cell ends up with four copies of those genes instead of the usual two.

Naturally, you’d think this would lead to four active copies of the affected genes, but here’s the twist. Many of the genes in the 15q region are imprinted, meaning only the maternal copy is active, while the paternal copy is usually silenced. In symptomatic idic(15), the extra chromosome typically comes from the maternal parent, so each cell has three active copies of maternally expressed genes rather than one. This increases the expression of these genes, like turning up the volume on their expression, ultimately affecting development.