Chromosome analysis The Genetics Center, Inc.

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Genetics and Chromosome Analysis:

Some straightforward answers to
common questions

Chromosome analysis provides a "birds eye view" of an individual's genetic information. Missing, extra, or rearranged chromosome material may be responsible for such diverse problems as birth defects, mental retardation, infertility, and repeated miscarriages.

What are chromosomes?

Chromosomes are microscopic structures containing all of an individual's genetic information. The chromosomes are composed of DNA and various proteins. The DNA contains the actual genetic code of an individual, and the proteins protect the DNA and allow the DNA to duplicate properly when the cell divides.
Each cell in our body, except for red blood cells, has a nucleus. The chromosomes are contained in this nucleus. Since the chromosomes contain an individual's entire genetic code, every cell contains all the genetic information of an individual. Through a process known as gene expression, only selected portions of the DNA are expressed. Thus, only the DNA necessary for a brain cell is used in brain cells, although all of the genetic material needed for a liver cell or any other type of cell is contained within that brain cell nucleus.
Most normal individuals have 46 chromosomes, half of which came from the mother and half from the father. Chromosomes are classified by their size and appearance under the microscope, and normal individuals typically have twenty-two chromosome pairs and a pair of sex chromosomes. The non-sex or autosomal chromosome pairs are numbered 1 through 22, and there are two sex chromosomes, designated X and Y. Individuals with two X chromosomes are female, and individuals with an X and a Y chromosome are male.
When a cell divides, each of the 46 chromosomes is duplicated, and each of the two new cells contains an exact duplicate of the genetic information in the original cell. An egg or sperm cell divides one more time, so that only one chromosome from each pair remains. When a sperm fertilizes an egg, the fertilized egg should have 23 chromosomes from each parent, making a total of 46 chromosomes.

What can be abnormal?
Rearranged chromosome material:

Sometimes, portions of one chromosome are exchanged with another chromosome. If there is no damage to the genetic information when this exchange occurs, or if the exchange occurred many generations ago and was inherited, the only problems relate to reproduction.

When there is a chromosome rearrangement, it is more difficult for the cell to divide the chromosome material equally, i.e. so that each egg or sperm cell has exactly half the genetic material. This may result in a conception in which there is too much or too little genetic material, causing the fetus to develop abnormally. Often when this happens, a miscarriage occurs, and the fetus does not appear to have developed at all. This is sometimes called a blighted ovum. If a miscarriage occurs within a week or two of conception, the pregnancy may not be noticed, because no menstrual period was missed.

Extra or missing chromosome material:

Whenever an individual has too much or too little chromosome material, serious problems are likely to be present. These include birth defects, growth problems, mental retardation, or any combination of these. Down syndrome, for instance, is usually caused by an entire extra chromosome 21, although it can be caused by chromosome 21 material attached to another chromosome. Abnormalities of the X or Y chromosomes tend to be more subtle. An extra Y chromosome in a male is usually of no clinical significance, and women who are missing part of an X chromosome usually have few medical problems.
Extra or missing chromosome material is often found in miscarried fetuses. In many cases, by doing chromosome analysis on the miscarried products of conception, it is possible to determine why a miscarriage occurred and whether there is a risk that this may happen again in the future.
Chromosomes rearrangements are common in tumor cells or in leukemia, and chromosome analysis on the tumor cells or on blood can help in making a correct diagnosis and deciding on appropriate treatment for the condition.

Chromosomes are invisible unless the cell is actually dividing. Therefore, in order to do chromosome analysis, cells must be "persuaded" to divide. For blood samples, a substance is added to stimulate the white blood cells to divide. Amniotic fluid contains fetal cells that can be grown in tissue culture. Cells from almost any tissue of the body can be cultured if the laboratory receives the sample promptly.

What can be used for chromosome analysis?
A blood sample:

Although red blood cells do not contain chromosomes, white cells do. Chromosomes can be analyzed from less than one teaspoon (five milliliters) of blood, collected into a tube containing a chemical that prevents clotting. In the case of a small baby, chromosome studies can be obtained on as little as one quarter teaspoon of blood, or even on blood obtained from a fingerstick.

Amniotic fluid:

Amniotic fluid contains cells from the fetus, and these cells can be grown in tissue culture so that sufficient living, dividing cells are available for chromosome analysis within 5-10 days.

Bone marrow:

This is useful for diagnosis of leukemia and other bone marrow cancers, where the chromosome abnormalities are only present in the leukemia or tumor cells. Bone marrow cells can be used to provide extremely rapid chromosome diagnosis in a seriously ill infant.

Tissue:

Sometimes a skin biopsy or tissue biopsy is used to provide material for chromosome analysis. Fetal tissue obtained after a miscarriage can also be used for chromosome analysis.

How accurate is chromosome analysis?

Chromosome analysis is highly accurate. Not just one cell, but at least 15-20 cells are examined whenever a chromosome analysis is done. This is to determine whether all cells, or just some cells, have normal chromosomes. Detailed analysis at the microscope and the computer-based image analysis system are performed on at least two cells. The analysts are highly skilled cytogenetic technologists with many years of experience. Laboratory directors (The Genetics Center has two) are licensed by New York State, and the laboratories must pass periodic inspections and proficiency testing.
Very small chromosome abnormalities may not be detected by routine chromosome analysis. High resolution chromosome analysis is available, as are special staining techniques and techniques based on molecular genetic technology. Since genetic conditions may be from changes that are too small to be seen under a microscope, normal results of chromosome analysis do not guarantee that there are no genetic problems. The geneticist can discuss these issues more thoroughly, on an individual basis, after obtaining a detailed family history and reviewing any relevant medical information.

What if a chromosome result is abnormal?

Accurate interpretation is as important as an accurate result. Interpretation of abnormal chromosome results takes a great deal of skill; only a qualified clinical cytogeneticist or clinical geneticist, with a PhD or MD, should interpret the significance of chromosome results. Clinical geneticists and cytogeneticists have experience in dealing with chromosome abnormalities and can explain the clinical implications of a chromosome result.
Genetic counseling, with either a geneticist or a genetic counselor, will often be recommended when a chromosome abnormality is discovered. Genetic counseling includes explaining what results of genetic and chromosome testing mean for an individual, his or her family, and future family members.