Prader-Willi syndrome chromosomes. Fluorescence in situ hybridisation (FISH) micrograph of Prader- Willi syndrome chromosomes. This is a genetic disorder caused by the deletion of a region on chromosome 15. At upper right, a normal chromosome 15 is seen, with the healthy areas marked in yellow. At lower left is a damaged chromosome 15 (marked del(15)), which has no yellow areas. The deletion of this part of the chromosome causes poor foetal muscle tone, poor genital development and severe mental retardation. Older infants may become obese, and fail to thrive. Support is often necessary throughout life. Magnification unknown.