••••••>Genetic Testing and Screening
Genetics, a discipline of biology, is the science of heredity and variation in living organisms. The fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding. However, the modern science of genetics, which seeks to understand the process of inheritance, only began with the work of Gregor Mendel in the mid-nineteenth century. Genetic testing is one of the important techniques of medical science for detecting alterations in the genes and chromosomes of an individual. It helps in diagnosing genetic disorders, predicting the risks of disease occurrence and also in identifying carriers of genetic disorders. 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 cells (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. Let’s look into the details of some of the common genetic tests, 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, genetic testing is done on embryos before implantation and sometimes, on oocytes before fertilization. Usually, an embryo results from in vitro fertilization (IVF) is tested before implanting it to the uterus. PGD 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. 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 family members.