isochromosomer

An isochromosome is a chromosome that contains two identical arms, formed when the centromere divides in the wrong plane during cell division. As a result, the chromosome acquires two copies of one arm (either two short arms, i(p), or two long arms, i(q)) and loses the other arm. This creates a chromosome with an unbalanced dosage of genes from the affected arm.

Isochromosomes can occur for any chromosome and may be constitutional (present in all cells from birth) or mosaic (present in a subset of cells). The most well-described example in humans is an isochromosome of the long arm of the X chromosome, i(Xq), which can be found in Turner syndrome–related mosaicism. In autosomes, common variants include i(p) or i(q) for different chromosomes, each resulting in partial monosomy of one arm and partial trisomy of the other.

Clinical consequences depend on which chromosome is involved and the specific arm duplicated or deleted, as well as the degree of mosaicism. Gene dosage imbalance from an isochromosome can lead to developmental delay, growth abnormalities, congenital anomalies, or other phenotypic effects. Some individuals with isochromosomes may have milder or more variable features, especially if mosaic.

Diagnosis is typically made by routine karyotyping showing a structurally abnormal chromosome with two identical arms. Confirmation and characterization can involve fluorescence in situ hybridization (FISH), array comparative genomic hybridization (array-CGH), and other molecular cytogenetic methods. Genetic counseling is often recommended to discuss inheritance, recurrence risk, and implications for health and development.