Karyotyping

Karyotyping examines cells under a microscope to study the size, shape, and number of chromosomes, which are the thin strands in the center (nucleus) of most of the cells in the body. Every person normally has 46 chromosomes. The chromosomes carry genetic information that determines a person's inherited features, such as sex, eye color, and blood type. Each chromosome is made up of many sections called genes. Extra or missing pieces of a chromosome can cause problems with a person's development and growth and with any of the body's functions. For karyotyping, the chromosomes in a cell are removed and stained with a special dye to make the bands of each chromosome visible. Banding helps show differences in structure among the chromosomes and helps arrange them into pairs. The chromosomes are then photographed through the microscope. The information also can be entered into a computer that displays a picture (karyotype) of the chromosomes arranged in a specific order from the largest to the smallest. The picture of all of the chromosomes is enlarged and studied for specific chromosome abnormalities. Normally there are 23 pairs of human chromosomes. In each pair, one chromosome comes from the mother and one comes from the father. In 22 of the pairs, both chromosomes of a matched pair should look the same. The 23rd pair stipulates a person's sex. In a female, the two chromosomes of the 23rd pair look the same (both of them are X chromosomes). In a male, the 23rd pair contains two different chromosomes (one X chromosome and one Y chromosome). Most cells in the body have identical chromosome pairs. However, egg and sperm cells have only 23 chromosomes, and mature red blood cells do not contain any chromosomes at all. Karyotyping can be done on most other cells in the body, but usually it is done on white blood cells (lymphocytes). A karyotyping test most frequently is done on a blood sample taken from a vein. It also can be done on cells taken from a tissue sample (such as skin or bone marrow), from amniotic fluid, or from chorionic villus sampling (CVS).  Karyotyping Results Normal: There are 46 chromosomes that can be grouped as 22 matching pairs and 1 pair of sex chromosomes (XX for a female and XY for a male). The size, shape, structure, and location of each chromosome are normal. Abnormal: There are more than or fewer than 46 chromosomes. The shape or size of one or more chromosomes is abnormal. A chromosome pair may be broken or incorrectly separated. Down syndrome results from having an extra (number 21) chromosome. In Turner syndrome, a defect that results in abnormal female sexual development, only one X chromosome is present. In Klinefelter syndrome, a disorder that affects males, at least one extra X chromosome is present.