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Medical Genetics: Chromosome Studies

What are chromosome studies?

Chromosomes are stick-shaped structures in the middle (nucleus) of each cell in the body. Each cell has 46 chromosomes grouped in 23 pairs. When a chromosome is abnormal, it can cause health problems in the body. Special tests called chromosome studies can look at chromosomes to see what type of problem a person has. Chromosome studies are usually done from a small sample of tissue from a person’s body. This may be a blood sample, skin biopsy, or other tissue. Chromosomes are analyzed by healthcare providers trained in cytogenetic technology and genetics. Cytogenetics is the study of chromosomes.

There are different types of studies, such as:

  • Karyotyping

  • Extended banding chromosome studies

  • Fluorescence in situ hybridization (FISH)

  • Chromosomal microarray analysis (CMA)


A karyotype is a single person’s set of chromosomes. Karyotyping is a way of looking at the set of chromosomes a person has. The study can look for abnormal amounts or shapes of chromosomes. The chromosomes are stained so that they can be seen with a microscope. The chromosomes look like strings with light and dark stripes called bands. This helps the healthcare provider see problems on the chromosomes.

Extended banding chromosome studies

These types of studies are also known as high resolution chromosome studies. These studies look at chromosomes in more detail. The chromosomes are prepared so that more bands can be seen. This lets the healthcare provider see smaller pieces of the chromosome more easily.

Fluorescence in situ hybridization (FISH)

This test is another way to look for changes in chromosomes. The cells in the sample are stained with fluorescent dyes that will only attach to certain parts of chromosomes. The cells are then viewed with a microscope using a special light. This test can find some chromosome changes that can't be seen with standard cytogenetic testing. For example, cells from a baby with Down syndrome would have 3 brightly colored areas. A FISH study may be done in addition to a standard chromosome study. FISH can be used to find chromosome abnormalities that may not show up in an extended banding chromosome study.

Chromosomal microarray analysis (CMA)

CMA can find chromosome problems with more detail than karyotyping or FISH. Fluorescent dye is added to a person’s DNA sample. The sample is then analyzed with special equipment to find out if any of the pieces of the chromosomes are extra or missing.