Chromosomes

Human Chromosomes

Each cell in our body contains 46 chromosomes: 22 pairs of identical chromosomes and either two X chromosomes (in a female) or one X and one Y chromosome (in a male). Each chromosome contains many genes which are the blueprints that control our development.

Chromosomes can be thought of as box cars in a long train. Though we can see the box cars, and by their shape and size guess what is probably in them, we cannot tell what their exact cargo is without looking inside. Like the box cars, the chromosomes contain normal or abnormal genes. But, unlike the box cars, at the present time medical science cannot look into the chromosomes and directly see the genes (though this may be possible in the future).

Genetic amniocentesis obtains fetal cells that can be cultured and analyzed to see if there are a normal number and structure of the fetal chromosomes. Many genetic diseases, therefore, cannot be diagnosed by chromosomal studies because an abnormality of one or a few genes will not cause an abnormality in the chromosome.

To study chromosomes, cells are grown in the laboratory. The cells are broken open so that the chromosomes spread out on the microscope slide. The chromosomes are then stained so that they can be viewed under the microscope. Above is a picture of a normal male chromosome pattern.

Chromosome studies can be done for many reasons. For example:

Your doctor may request a chromosome study if he or she suspects your child may have a chromosome disorder. You may be offered an amniocentesis to test the baby’s chromosomes during pregnancy. This testing may be offered if:

• you will be 35 years of age or older when your baby is born
• you have had a blood test that shows you are at higher risk to have a baby with a chromosomal problem
• you have a family history of a chromosome disorder or rearrangement
• you have had an ultrasound which found a difference in your baby which may be due to a chromosome disorder.

Couples who have two or more pregnancy losses (miscarriages or stillbirths) are at increased risk to carry a chromosome rearrangement. When balanced, chromosome rearrangements do not cause health problems. However, a person with a balanced chromosome rearrangement is at increased risk to pass an unbalanced rearrangement on to a pregnancy. An unbalanced chromosome rearrangement can lead to miscarriage or the birth of a baby with birth defects. If you have had several miscarriages, your doctor may recommend blood chromosome testing for you and your partner.

FISH is another type of test offered in some circumstances. FISH stands for Fluorescence In Situ Hybridization. In FISH, fluorescent molecules attach to specific areas of the chromosome. This allows the laboratory to study smaller areas of the chromosome than what can be seen in normal chromosome studies. It also allows the laboratory to count the number of certain chromosomes without having to wait for the cells to grow.

FISH may be offered if an amniocentesis is done and the number of chromosomes 13, 18, 21 and X or Y needs to be determined in a few days. This type of FISH testing is always followed by complete chromosome analysis. FISH testing for the number of chromosomes 13, 18, 21, X and Y is greater than 99.5% accurate. Full chromosome analysis is greater than 99.5% accurate. Full chromosome testing also allows for study of all the chromosomes.

Other types of FISH may also be requested if your physician is suspicious of a genetic disorder where a very small piece of chromosome material is missing—a piece so small that it wouldn’t be seen under the microscope in normal chromosome studies.