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Home » Biology Homework Help » Biotechnology » Detection of Genetic Diseases
Detection of Genetic Diseases
Human beings suffer from several hundred genetic diseases almost all of which are produced by single recessive mutations. Many of these ailments can be managed but there is no cure for them, except for the fast emerging option of gene therapy. Their incidence can be minimized by an early detection of the afflicted foetuses which are then aborted. Therefore, when a woman gets pregnant the probability of her having a child suffering from a genetic disorder is estimated based on the histories of her and her husband’s families, and from the knowledge of previous births, if any. In case of risk, further investigation is carried out for a clear cut and specific diagnosis.

Obtaining Foetal Cells: Earlier foetal cells were obtained by amniocentesis, i.e. withdrawal of amniotic fluid (which has free cells of foetal or developing human embryo origin) with the help of a hypodermic syringe. But amniocentesis is applicable only 18 weeks or later after the pregnancy, which is rather late for abortion. Therefore, foetal cells are now obtained from biopsies of trophoblastic villi which are an external part of human embryo and later form a part of the placenta. The biopsy is performed during 6th or 8th week of pregnancy (using an endoscope passed through the cervix of uterus), usually provides 100 of pure foetal DNA.

Foetal cells present in the amniotic fluid obtained by amniocentesis are recovered by centrifugation and cultured to obtain sufficient cells for various analyses. But the tissue obtained from biopsy of trophoblastic villi is usually enough for assays and in vitro culture may not be necessary.

Disease detection: The foetal cells are used for detection of the genetic disorders in one of the following ways:

1. Determination of karyotype of cells provides information on various syndromes produced by gross chromosomal aberrations.

2. Most of the genetic disease produce defective proteins/enzymes or no enzymes; many of these proteins have been identified and some of these can be assayed. The foetal cells are used to assay the concerned enzymes activities to detect such genetic diseases. Atleast 35 genetic diseases can be detected by assaying activities of specific enzymes.

3. In case of some genetic diseases, the concerned gene mutation may alter (either abolish or produce) the recognition site for a restriction enzyme. The RFLP so produced can be detected by Southern hybridization; a sequence of the concerned gene is used as probe. For example in case of sickle cell anemia, the mutation from GAG to GTG elimination a recognition site for the restriction enzyme Mst II (CCTNAGG) in the gene of hemoglobin. DNAs from a normal and the test individuals (including foetal samples) are digested with Mst II, subjected to gel electrophoresis and probed with a sequence of gene. If the test individuals have normal gene, the bands detected in their Southern blots will be comparable to those of normal DNA. A sickle cell mutant of the gene will change this pattern in a detectable manner. Heterozygotes will show the bands present in both normal and stickle cell DNAs.

This approach is applicable to only those disorders in which the gene mutation changes the restriction pattern. As a result, this approach is not of general application.

4. A more general approach utilizes oligonucleotides probes representing the sequence altered by the gene mutation causing the genetic disease. Typically a set of two separate probes are used for each disease: one probe is complementary to the normal sequence, while the other complementary to the mutant sequence. The probes are radiolabelled and used to probe Southern blots; under appropriate conditions the probes can distinguish the normal and mutant DNA samples.

A set of two 19-mer (19 bases long) oligonucleotide probe has been successfully used to detect sickle cell anemia. One of the two probes contains the sequence complementary to that changed by the sickle cell mutation, while the other is complementary to the same segment of the normal allele. The Southern blots of normal individuals hybridize only with the probe; those of sickle cell homozygotes only with the probe, those of the heterozygotes hybridize with both and probes. Similarly, other mutant genes (e.g. -antitrypsin gene implicated in pulmonary emphysema) differing from the normal allele gene for a single base could be detected using this approach.

This approach is of more general application than detection of RFLPs. But this assay can be used only in such cases where the base sequence of the gene segment containing the mutations (for both normal and mutant alleles) is known to allow the synthesis of the two oligonucleotide probes. Probes can be prepared for mutations due to base substitution, insertion or deletion in the concerned genes.  

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