What is karyotyping?

The Cell and Chromosomes

Every cell in the human body—except for red blood cells—contains a nucleus with rod-shaped structures called "chromosomes." The chromosomes, in turn, contain the genes, which are the units that transmit heredity from parents to offspring.

The forty-six individual chromosomes in a human cell exist as twenty-three pairs of so-called homologues. Homologous chromosomes are similar in size and appearance. The first twenty-two pairs of homologues are referred to as "autosomes," while pair number twenty-three contains two dissimilar chromosomes known as the X and Y chromosomes, which determine sex. Sperm and egg cells each have twenty-three chromosomes, or half the usual number. When a sperm fertilizes an egg, the normal chromosome number (forty-six) is reestablished in the first cell of life, the zygote.

Cells reproduce by mitosis, a process of simple division, the result being two identical daughter cells, each containing forty-six chromosomes. Since chromosomes contain the genetic information, it follows that mitosis must proceed with precision every time a cell divides. During the earliest stages of embryonic development, however, mistakes sometimes occur and the cells wind up with more or fewer chromosomes. This malfunction of mitosis is called "nondisjunction," and the incorrect number of chromosomes is passed to all the cells in the developing embryo. This leads to a variety of abnormal conditions, all of which can be diagnosed with the procedure known as "karyotyping."

Procedure and Interpretation

A karyotype is an analysis of all the chromosomes in a single cell. The prefix “karyo-” refers to the nucleus, the part of the cell where chromosomes reside; the suffix “-type” means characterization. Thus, a karyotype is a characterization of a nucleus in terms of its chromosomes.

Karyotypes are performed on embryos to diagnose chromosomal abnormalities and on adults who suspect chromosomal aberrations that could be passed on to offspring. Although a karyotype can be constructed from almost any cell in the body that contains a nucleus, it is most often performed on white blood cells, which are easily harvested from a routine blood sample.

The procedure is simple. Once the blood is collected, the white cells are separated from the red. In the laboratory, the white blood cells are then stimulated to undergo mitosis. At the stage of mitosis when the chromosomes are most visible, the process is chemically halted. The chromosomes are then stained to make them more visible, after which they are photographed and the individual chromosomes cut out and rearranged as homologous pairs in descending order by size. Each pair of chromosomes is also given a number, the largest pair being designated number 1. Then, another photograph is taken of the chromosomes in the rearranged format. The result is the karyotype. The entire process, from collecting the blood sample to growing the cells to preparing the karyotype, takes from one to three weeks.

Once the karyotype has been created, it is ready to be interpreted by a cytogeneticist, an expert in the study of chromosomes. The most common disorders visible with karyotyping are Down syndrome, an extra copy of chromosome number 21; Klinefelter syndrome, a male with an extra X chromosome, resulting in sterility and the development of some feminine features; and Turner syndrome, a female missing an X chromosome, resulting in sterility and a masculine body build.

Perspective and Prospects

Karyotyping was first reported in the mid-1950s when chromosomes were examined in fetal cells collected from amniotic fluid. This was the beginning of the discipline of prenatal genetic diagnosis. At the time, there was no ultrasound to guide the needle through the amniotic membrane, which increased the risk of damaging the fetus. The karyotyping itself required four or five weeks of cell culture and was not always successful.

Today, karyotyping is commonly used to diagnose chromosomal abnormalities in both fetuses and live individuals. It is considered an absolutely safe procedure, the only risks being those inherent in penetrating the amniotic sac with a needle. Also, although the advent of ultrasound has greatly reduced the risk to the fetus, the possibility always exists of inadvertently collecting maternal cells when the mother’s tissues are penetrated.

Since the 1970s, dyes have been added to karyotypes to highlight the chromosomes for identification purposes. Today, when abnormalities are found on a karyotype, researchers can then examine the individual genes for deletions and duplications using molecular cytogenic procedures, such as fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH). For instance, in the multicolor FISH method called "spectral karyotyping (SKY)," fluorescent dyes highlight specific regions of the chromosomes. A device called an "interferometer" is used to detect slight color variations invisible to the human eye and then assign visible colors to the homologous chromosomes. The SKY method is superior to traditional karyotyping with chemical stains because it more clearly identifies chromosomes that are damaged or that contain fragments of other, nonhomologous chromosomes.

Thanks to advances in computer technology and the Human Genome Project, digital karyotyping was developed in 2002. Unlike traditional karyotyping, digital karyotyping maps representative DNA fragments, known as "tags," and computationally models these tags to determine whether abnormalities are present in the sample. This genomic method is now being used for cancer research, among other applications.

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