Probability & Genetic Testing

Genetic Testing Overview

Genetic testing finds flaws in chromosomes and genes. Some genetic tests can determine definitely whether a disease is present. For instance, amniocentesis, a test of the amniotic fluid, can determine whether a fetus has Down Syndrome. However, prenatal genetic screening exams, such as the first trimester screening exam, give a probability that the fetus has a genetic condition. For example, the test may state that the fetus has a 1 in 100 chance of having Down Syndrome. In these cases, amniocentesis would be offered if the chance of a genetic disease is greater than the chance of miscarriage from amniocentesis.


Prior to any genetic testing, the sample must be sent to a cytogenetics laboratory. The samples can be either blood, bone marrow or tissue cells. The laboratory extracts the DNA from these samples to karyotype and analyze. Karyotyping is the process of sorting the 46 chromosomes and seeing if there are any deletions, additions or translocations. The laboratory technicians use special techniques to stop cell division at metaphase, when the chromosomes are most visible, and stain them onto a slide. These laboratory techniques isolate the chromosomes and allow the technician to count them under a microscope.

Prenatal Genetic Testing

Prenatal genetic testing tests the fetus for certain genetic diseases. Women are only offered genetic testing if they are over 35 years old, have had several miscarriages or have a history of genetic diseases in their family. Women are also offered specific testing for diseases that are prevalent in a certain ethnic group. For instance, Ashkenazi Jewish women are offered testing for Tay-Sachs disease. Prenatal testing is divided into screening and invasive testing. Screening is a blood test and gives a probability of having certain very common genetic diseases, such as Down Syndrome and Trisomy 18. The screening takes into account these blood test results and the sonogram to give a chance for a fetus to have these genetic diseases. If the chances are high, invasive procedures, such as Chronic VIlli Sampling (CVS) and amniocentesis are offered.

FISH Testing

FISH, or Fluorescence In Situ Hybridization, is a genetic test that is offered if a chromosomal analysis is not sufficient. In many instances, there are mutations in a specific gene, rather than a deletion or addition of an entire chromosome. In these cases, FISH would be done to focus on that specific gene. During this test, technicians generate a DNA strand that is complementary to a gene in a chromosome that the researchers are looking for. These generated strands have a fluorescent dye attached to them which makes them easily visible under a microscope. In this way, researchers can see if there is a mutation in a gene that is responsible for a genetic disease. FISH gives a definite answer of the presence of a genetic disease, rather than a probability.

Biochemical Testing

Many genetic diseases occur due to a mutation in a protein. Proteins are made from the genes in DNA. If there are strange levels of a certain protein, this can be attributed to a mutation in the gene that codes for that protein. For instance, phenylketonuria (PKU) is a disease where a person cannot break down phenylalanine because of a lack of a protein that breaks down this amino acid. If foods with this amino acid are consumed, the person can have severe adverse reactions. Thus, biochemical testing would inform the individual of the presence of this condition and allow them to lead a normal life by following a strict diet. Like most genetic tests, biochemical tests give a definite answer of the presence of a certain genetic condition. Genetic testing is increasingly becoming a vital aspect of an individual's health care. Genetic tests look for errors in the genes or chromosomes that may lead to certain genetic conditions. Most genetic tests only educate the patient about their genetic health, which allows them to make proper decisions about their own health management. However, other tests reveal conditions that can be treated or even eliminated.