Genetic testing or screening is a procedure for determining any changes in the genes, proteins and chromosomes. There are about 20,000 - 25,000 genes in a human genome. Genetic tests are conducted to diagnose any abnormalities in the genomic content of an individual that are responsible for causing genetic disorders.
In addition to the study of genes and chromosomes, genetic testing involves biochemical tests to identify proteins that signal anomalous genes. In genetic testing, genes of an individual (from blood or other body fluids samples) are examined for any alterations. The possible results are; there can be deletion or addition of bases in the DNA strand.
Sometimes, a part of chromosome or an entire chromosome may get added, deleted or translocated. In such situations, the genes may be suppressed or over expressed. An individual can carry abnormal genes without expressing them or the genes can be expressed leading to genetic diseases.
Prenatal Genetic Tests
As the name suggests, prenatal genetic tests are conducted to identify genetic disorders in a fetus during pregnancy. Genetic disorders such as Down syndrome, neural tube defects and other chromosomal abnormalities can be detected with this test before birth.
If there is a positive result for any of the disorders, parents can opt for termination of pregnancy. It can also be used in determining the sex of the fetus. However, it is illegal, if abortion is done after determining the sex of the fetus.
Preimplantation Genetic Diagnosis (PGD)
Preimplantation diagnosis or embryo screening is an alternative to prenatal testing. Here, the testing is done on embryos before implantation and sometimes, on oocytes before fertilization.
It helps in identifying the embryos, which pose a risk to certain genetic disorders. Many couples opt for PGD instead of prenatal diagnosis, to diagnose genetic disorders beforehand so as to avoid the dilemma of abortion.
Newborn Genetic Screening
Newborn genetic screening is done to identify genetic disorders in newborn infants. In this procedure, DNA sample of the baby is studied for defects. Studies are also done for confirming the absence of key proteins that are essential for proper functioning of the body.
Early diagnosis of any genetic disorders will be helpful in reducing symptoms that can cause disability and certain chronic diseases.
Carrier testing is a genetic test performed to identify hidden or recessive alleles that can cause genetic disorders. Many of the hereditary disorders such as sickle-cell anemia, cystic fibrosis and color blindness are caused due to passing of the recessive genes from parents. Blood samples are tested for DNA and chromosomal abnormalities.
Couples who have a family history of genetic diseases opt for this test for better family planning. In case, if both the couples have the same recessive allele, then there is a high risk of having children with the disease.
Predictive Gene Testing
Predictive tests are done in order to look for disorders that have been inherited in the family. In many cases of genetic disorders, members of a family suffer from the same disease. In other words, the defective gene runs in the family. In predictive gene testing, DNA samples of the family members are studied to mark any common genetic disorders.
Usually, a comparison is made between the genomic content of an individual suffering from the disorder and an individual that is free of the disease. Thus, the defective gene is marked for further treatment and study. It has been found that when there is familial history of cancer, genes causing cancer are present amongst most of the family members.
There are both positive and negative effects of genetic testing. One of the benefits is to confirm a diagnosis that can help in the correct treatment of the disease. Uncertainties about any genetic abnormality of an individual can be verified, which will help in planning his/her future.
A genetic counselor can help in explaining the pros and cons of undergoing a genetic test. The counselor can also help the individual in taking a proper decision, if the result is negative for a genetic test.