Introduction:
Genes are part of DNA that carries genetic information and is passed down from generation to generation. Genetic testing helps in analyzing the DNA (deoxyribonucleic acid), RNA (ribonucleic acid), chromosomes, and metabolites and can help in identifying the genetic variants which are associated with disorders.
What Are Genes and DNA?
The smallest unit, called cells, makes up the human body. The cell contains a nucleus, which controls everything in the cell. The nucleus contains 23 pairs of chromosomes, and in each pair, one comes from the father and another from the mother. Chromosomes are present in the form of threads containing a single molecule of DNA (deoxyribonucleic acid) and a protein. These DNA molecules are in the form of a twisted ladder. A part of DNA is called genes and contains genetic information. There are about twenty-five thousand genes in a cell.
What Is Genetics?
Genetics is the study of genes, their variations, and how traits are passed down to offspring. Genes carry information that is required to make proteins and must be passed down to the next generation. Genetic testing helps in the study of chromosomes, RNA (ribonucleic acid), DNA (deoxyribonucleic acid), and specific metabolites.
What Is Genetic Testing?
Genetic testing is the test that helps in analyzing the genes, chromosomes, and specific metabolites to identify the variants that are associated with disorders. Genetic testing can identify variants, such as mutations in genes, genes that can increase the risk of the disorder, chromosomal abnormalities such as deletion, duplication, inversion, or rearrangement, and abnormalities in enzymes and proteins that can change the DNA and result in a specific disorder. Direct testing is the gene-specific test of DNA and RNA, and the indirect or linkage-based test is the identification of genetic markers within the family.
What Are the Different Types of Genetic Tests?
The different types of genetic tests are:
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Molecular Tests: These help in analyzing more than one gene. It determines the sequence of DNA. Targeted single variant test analyses specific variants in a gene, for example, detecting the HBB (hemoglobin subunit beta) gene in sickle cell anemia. Single gene test analyses one gene. Gene panel tests analyze more than one gene; for example, many gene variants can cause epilepsy. Whole genome sequencing helps in analyzing variations in an individual's DNA.
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Chromosomal Tests: This test helps in analyzing the changes in chromosomes, such as addition, duplication, deletion, inversion, and rearrangement of a segment of a chromosome. For example, in William's disease (a developmental disorder), there is a deletion in the section of chromosome 7.
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Gene Expression Tests: This test helps in identifying genes that are active or suppressed, with the help of mRNA (messenger RNA).
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Biochemical Tests: These tests help in analyzing the proteins which are made from the information of DNA. For example, low levels of biotinidase are caused by the BTD gene, which can cause biotinidase deficiency.
What Are the Clinical Applications and Uses of Genetics?
The clinical applications of genetics are:
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Diagnosing the Diseases: Diagnosing testing is done on individuals who show the signs and symptoms of the disorder. The advantages are:
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It can help in precise diagnosis. Therefore invasive tissue sampling can be eliminated.
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It can provide information about therapeutic interventions.
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It can provide the basis for anticipatory guidance (providing information to the parents about the development of their child) and genetic counseling.
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It also helps in identifying risk factors.
The different types of diagnosis are:
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Preimplantation Diagnosis: This can help in identifying the risk of having a genetic disorder in the child. It is done in the case of embryos that are created using in-vitro fertilization (IVF).
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Prenatal Diagnosis: This is done during pregnancy to identify the risk of the developing fetus for a particular disorder.
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Carrier Diagnosis: It can help in identifying an individual who carries one copy of the mutant gene, but when present in two copies can result in a particular disorder. This is useful in case of a family history of genetic disorders.
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Predictive Diagnosis: This is done to detect variations in genes that can cause disorders after birth or can have late onset in high-risk family members. It can also help in identifying the risk of developing the disorder.
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Forensic Testing: This helps to identify or rule out a crime suspect and to identify biological relationships for legal purposes.
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Genetic Screening: This is done in case of populations who are at high risk of developing disorders such as black people are screened for sickle cell anemia (a type of anemia that causes rigid red blood cells which are in sickle shape), Ashkenazi Jews are screened for Tay-Sachs disease (a neurological disease that causes the death of neurons), and Southeast Asian, African, Asian Indian, Middle Eastern, and Mediterranean people are screened for beta-thalassemia (a type of blood disorder that causes decreased hemoglobin). Screening is done when the pattern of inheritance is known, and if there is the availability of effective therapy. Screening should be sensitive, non-invasive, specific, safe, reliable, and valid The uses of genetic screening are:
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Helps in identifying asymptomatic parents that carry the recessive genes.
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Screening of newborns can help in identifying genetic disorders and treating them early.
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Helps in identifying risk. In such cases, a prenatal diagnosis should be made, such as amniocentesis, umbilical cord blood sampling, chorionic villus sampling, fetal imaging, and maternal blood sampling.
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Helps in identifying genetic disorders which can be treated and complications can be prevented, such as in the case of phenylketonuria (increased levels of phenylalanine in the blood), hypothyroidism, and galactosemia (incapable of breaking down simple sugar substances), replacement therapy or special diet can be given, and use of corticosteroids by pregnant women can lower the severity of congenital virilizing adrenal hypoplasia (a genetic disorder that affects the adrenal glands, which are present above the kidney).
Treatment of Disorders: By understanding the pathological mechanisms of underlying genetic disorders, therapeutic management can be provided. For example, in the case of steroid-resistant nephrotic syndrome (SRNS), individuals having two pathogenic NPHS2 mutations have an aggressive form and do not respond to second-line or third-line immunosuppressive therapies, and have a low risk of recurrence in renal allografts. In the case of nephrogenic diabetes insipidus (an X-linked disorder that occurs due to mutation in AVPR2 and AQP2 gene), which has severe renal concentrating effects, which can cause dehydration and electrolyte imbalance, therefore identification of these mutations can help in early prevention of dehydration, especially in infants. Pharmacogenomics is a new branch that involves genetics and pharmacology. It helps in understanding how genetic variations can affect the response to drugs.
Conclusion:
Genetic tests are done voluntarily, and it is a personal choice. It has a lot of benefits to the family and society but has its limitations and risks. A genetic counselor or geneticist helps in providing information about these test results.
