Discuss the use of human genetics in medical science with particular reference


Discuss the use of human genetics in medical science with particular reference to (a) diagnosis, (b) gene therapy and (c) DMA finger printing.

Ans : Use of human genetics in medical science : If knowledge about genetics of human disease is available, it can be used in a variety of ways to avoid or reduce the incidence of some of these diseases. This can be achieved in a variety of ways and we will describe five of them, namely (a) genetic counselling, (b) antenatal diagnosis, (c) gene therapy (d) making choice of baby’s sex, and (e) DNA finger printing in forensic science.

a) Amniocentesis and antenatal diagnosis : When a pregnant woman is known to have a chance of bearing a child with a genetic defect, it may be desirable to diagnose the condition in the fetus, this can be done by taking some cells from the fetus by drawing a few millilitres of amniotic fluid with the help of a hypodermic needle; the technique is called amniocentesis (see fig.), and is usually performed at 15th week of pregnancy, to allow enough time for safe abortion if recommended.

human genetics in medical science
Fig.: Technique of amniocentesis, used to test for hereditary or developmental defects.

The amniotic fluid has free cells of fetal origin, which can be cultured and tested in various ways e.g. karyotype, enzyme production and restriction site pattern analysis of its DNA. At least 35 diseases which can be identified by this technique are known. If disease is detected through such an antenatal diagnosis, abortion of fetus can then be recommended. However, if abortion is not acceptale to parent, there is no point in carrying out antenantal diagnosis.

It is possible to identify the disease now within 2 months of pregnancy. Unlike an 18 week period required earlier. The number of disease specific DNA probes is also increasing at a fast rate, so that antenatal diagnosis by- DNA analysis or linkage should be possible for all single gene defect. In recent years, the incidence of the disease thallassaemia in cypriot community in Britain has fallen from 30 to 2 per year, due to the use of antenatal diagnosis. In U.S.A, on the other hand, there is a. campaign against abortion and, therefore, also against antenatal diagnosis. In such cases there will be births of defective children and these may become patients for gene therapy discussed in the following section.

b) Gene therapy : If a child is diagnosed to carry a defective gene leading to disability, one may like to get this gene replaced by a normal functional gene. This is gene therapy in theory. One would like to ask that if there is a need and demand for gene therapy, can it be done? The answer to this question has changed from ‘no’ to ‘yes’ in recent years. The possibilities are being explored and treatment by transfusion of cells with functional gene, if not by replacement of defective cene, are being suggested and tried. Gene therapy can be used at two difierent levels :

Patient therapy, in which cells with healthy gene may be introduced in the affected tissue, so that the healthy gene overcomes the defect, without affecting the inheritance of the patient and (ii) embryo therapy, in which the genetic constitution of embryo at the post-zygotic level is altered, so that the inheritance will be altered. It is believed that in future, gene therapy of both types will be possible.

i) Patient therapy : Patient therapy will involve the following steps : (i) the defective gene should be identified, (ii) normal healthy gene should either be isolated or synthesized, (iii) isolation of cells of the tissue, where the normal healthy gene will need to function, (iv) the normal gene should be placed into a cell, where it can function. The gene will have to be placed into the correct site on the host chromosome, so that the gene may function, or even one may have to delete the defective gene. There are three main problems in this connection. First, the introduced gene may not function, second, that when corrected cells are reintroduced, these may be outnumbered by the non-curved, resident cells, and third, there are only a few diseases affecting only a single tissue.

Utilizing the above approach, first clinical gene transfer (approved in U.S.A.) was achieved in 1989. It was a marker gene (neomycin resistance = NeoR) introduced into tumor-infiltrating lymphocytes (TIL). These neoR/ TIL were transferred into patients with advanced cancer, to ensure that the approved protocol really works. The first approved gene therapy

orotocol for correction of adenosine deanunae (ADA) deficiency, however GG GG by the end of 1992 two dozen acttye clinical protocols „„ So continents became available lor trials. However the technology „ still very expensive and specialized to be used extensively. Other less expensive techniques (involving delivery of gene through vectors are being developed.

ii) Embryo therapy : This will involve the following steps which have been tried in case of mouse or rabit only, (i) in-vitro fertilization of the egg, (ii) insertion of normal gene into embryo at post zygotic evel, eitner with viruses or directly by microinjection, (iii) integration of inserted gene in host DNA, where it may or may not function. The inserted genes have been found to be inactive generally, but in a few animals, genes have been switched on in a tissue in a specific way but their activity is at a very low level. However, it is not yet possible that the therapeutic newly inserted genes function under normal control in the animal, in time, space or quantity.

c) DNA finger printing : The technique of DNA fingerprinting was developed for the first time (1985, 86) by Alec Jeffreys and his colleagues at Leicester University in U.K. In this field, establishment of the identity of a person with the help of blood stains, semen (sperms) stains or hair roots will be possible with almost absolute certainty. In this technique, DNA will be isolated from blood stains, semen stains or hair roots and will be subjected to southern blotting. DNA, hybridization with the help of specific DNA probes. This will reveal polymorphism in DNA, which has a very stable inheritance. For this purpose, DNA from blood, semen or urine stains may also be amplified using PCR technique.

It is speculated that the above technique I will allow the identification of rapists in rape cases, and of mother and or father in case of doubtful percentage (see fig.). This technique will allow identification even which the stains on victim’s clothes etc. are several years old and with much more certainty than has hitherto been possible through techniques of blood groups etc., since the number of blood groups available becomes a limitation. The technique of DNA fingerprinting reveals such a great polymorphism that the possibility of two persons having same pattern

In India, DNA fingerprinting tests are carried sequences inherited by out at the Centre for Cell and Molecular Biology (CCMB), Hyderabad. For this purpose, a test with the Bkm probe (banded krait minor satellite DNA) earlier used for identification of sex chromosomes has been found to cost one-tenth

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