Test Details: FISH helps in detection of numerical chromosomal abnormalities. Autosomal monosomies are usually not compatible with life. Autosomal trisomy of chromosomes 13 and 21 are most common trisomy detected in live-births. Trisomy 21 is associated with Down Syndrome and is the single most common known cause of mental retardation. Trisomy-13 is associated with Patau Syndrome. Individuals with trisomy 13 rarely live beyond the age of 1 year.