In a karyotype, the chromosomes can look bent or twisted. This is normal and is a result of how they were sitting on the slide when the photograph was taken. Chromosomes are flexible structures that condense and elongate during different stages of cell division. If you unraveled all of the DNA that makes up the 46 chromosomes, you would find over 7 feet of DNA from one single cell.

For example, if a baby is suspected of having trisomy 21 Down syndrome and an amniocentesis is done on the pregnancy, a FISH study can be performed on the cells found in the amniotic fluid. A probe made for chromosome #21 can determine how many copies of the #21 chromosome the baby has. Under a special microscope, the cells from a baby with trisomy 21 would contain three 'signals' or three brightly colored areas, where the probe matched up with the three #21 chromosomes. A FISH study does not replace a chromosome study, but is done in addition to a standard chromosome study, depending on the birth defect in question.

FISH can be used to detect structural chromosome abnormalities (such as submicroscopic deletions) that are beyond the resolution of extended banding chromosome studies.

A sample of DNA from the individual to be tested and a control DNA sample are arranged in a particular order (array) on a glass slide. Fluorescent dyes are attached to the DNA samples. These slides are then placed in a special scanner which measures the brightness of each fluorescent area.

This process looks for identification of a change in DNA copy number. These changes in DNA copy number may represent changes seen in the general population which do not cause genetic diseases. However, some changes in copy number may indicate a chromosomal abnormality such as a chromosomal imbalance, loss, or gain. Types of chromosomal abnormalities may include small chromosomal rearrangements, small duplicates of chromosomal material (trisomy), or small deletion of chromosomal material (monosomy).