Genetic screening is a routine diagnostic procedure devised to detect those who are carriers of, or who are themselves affected by, a hereditary disease. Genetic screening applies to populations rather than to individuals.
Genetic screening helps the future generation to get rid of many undesirable inherited diseases. Screening may be done at the intrauterine diagnosis and of the newborns. Amniocentesisis the method by which many biochemical disorders and most chromosomal abnormalities can be detected at prenatal stage. If the diagnosis indicates certain defects in the fetus, then the parents are advised to think of considering abortion. A pregnant woman over the age of 35 years may have a higher risk of carrying a baby with Down’s syndrome.
The most-widespread application of genetic screening in the United State is for phenylketonuria (PKU). Phenylketonuria is an inborn error of metabolism which is an autosomal recessive trait caused by mutation affecting the mechanism of metabolizing phenylalanine. All hospitals in the United States screen newborn babies for PKU by a blood test called Guthrie test. After genetic screening if both the parents are heterozygous for a genetic disease and the genotypes of both the prospective parents become known and they can decide to produce a child or not. It is simple matter to work out the probability of their child inheriting the disease. This can be done through appropriate tests around 2-3 months after conception. This is achieved through amniocentesis or through chorionic villus biopsy; the cultured fetal cells may be used for determining their karyotype, levels of the critical enzymes and restriction patterns of DNA. Such an antenatal diagnosis is now available for several genetic diseases and for a variety of chromosomal defects. The purpose of such a diagnosis is premature termination of abnormal fetuses.
Genetic counselling and antenatal diagnosis provide definite relief to the prospective parents and reduce the frequency of genetically defective individuals in the population. However, it is unlikely that these measures would eliminate the deleterious alleles from a population. This is so because most genetic defects are recessive and heterozygotes for such alleles. Thus even after a total ban on reproduction by the homozygotes for such recessive alleles they would remain in the population through the heterozygots, therefore even such an extreme selection would lead to only a slow decline in their frequency. Further, it is not likely that all the couples in any society will willingly submit themselves, at least in the foreseeable near future, to these procedures. But genetic counselling has become a routine aspect of medical practice in most developed countries. It has been advocated that defective genes may be corrected through sophisticated genetic techniques either during the early stages of embryo development (embryo therapy) or in specific tissues of the adult patient (patient therapy); such an approach is referred to as genetic surgery. Embryo therapy would involve:
1) In vitro fertilization of egg.
2) Production of several copies of the normal allele of the defective gene.
3) Introduction of this DNA into the zygote or in the cells of the developing embryo.
4) Integration of DNA, preferably in place of the defective allele, so that it may function normally.
The aim of patient therapy is to introduce the normal gene into the critical tissue of the patient that is affected by a genetic disease, i.e., the tissue where the concerned gene is required to express itself the most e.g., pancreas in the case of diabetes. The steps involved in patient therapy would be similar to those of embryo therapy. But in this case, cells from the concerned tissues may have to be exercised and treated in vitro to correct their genetic defects and then reintroduced into the tissue where they may function normally. Techniques for isolation, identification and multiplication of many human genes are now available, and for many others they are likely to be developed soon. The techniques for gene transfer in eukaryotes are being refined and it may not remain a great problem in the near future.
The correction of genetic defects by DNA technology is known as genetic engineering or genetic manipulation. There are different techniques involved, viz., transformation, transduction and replacement. Transformation is concerned with the introduction of genetic information from the external sources. Free DNA is put into the cells and integrated into the DNA which receives the same, hence the DNA is transformed leading to the correction of the defective genes. In certain cases where foreign DNA is not accepted by the recipient DNA, RNA is isolated and DNA transcription with RNA dependent DNA polymerase is prepared. In humans, DNA technology has corrected many enzyme defects. Transduction is one where viruses mediate the DNA that is introduced to the host. Genetic information from the host DNA is incorporated into the virus DNA. Then the bacterial material is transferred to the new host and becomes active in the new cells. Functionally homologous viral genes replace defective genes. Presence of viruses or the association of viral infection with certain metabolic traits may be beneficial for humans. However it is a dream that may be more difficult to fulfil the patient and embryo therapies through DNA mediated genetic modifications.
Genetic screening and counselling may also lead to certain problems. The cases of mistaken paternity, the problem of confidentiality, delayed counselling are important among them