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This article waslast modified on 17 February 2019.

This article discusses genetic testing; that is, testing that looks at a person’s genetic makeup for a variety of reasons. An increasing number of genetic tests are becoming available as a result of recent and rapid advances in biomedical research. It has been said that genetic testing may revolutionise the way many diseases are diagnosed but genetic testing does not just help a doctor diagnose disease. There are a number of different reasons genetic tests are performed. These include the following:

1. clinical genetic testing for diagnosing current or future disease

2. pharmacogenomics for the assessment of therapeutic drug treatment

3. identity testing for criminal investigations or forensic studies (sometimes referred to as “DNA fingerprinting”)

4. paternity testing (now sometimes called parentage testing)

5. tissue typing for transplantation

6. infectious disease testing

This article will briefly discuss each of these tests but will focus more on the medical aspects of genetic testing. It is helpful to first understand the basics of human genetics before looking at genetic testing itself.

About Genetic Testing
  • The Basics

    The Basics

    The total of an individual's genetic information is called their genome. The genome consists of structures called chromosomes that are composed of very long double strands of DNA. Each human cell contains 23 pairs of chromosomes. One of each pair is inherited from an individual's mother and the other of each pair is inherited from an individual's father. Twenty-two of the 23 pairs of chromosomes are called autosomes; the other pair are the X and Y chromosomes that determine a person’s sex and are therefore termed the sex chromosomes. Normal males have one X and one Y chromosome while normal females have two X chromosomes.

    Chromosomes are located in the part of the cell called the nucleus. The long, double strand of DNA (sometimes called “nuclear DNA”) contained in each chromosome is organised into many subunits of genetic information, the most important of which are referred to as genes. Genes are made up of nucleotides which are composed of phosphates, a sugar, and a nitrogen-containing base. There are four kinds of bases in DNA; adenine, guanine, thymine, and cytosine. It is the difference in the order (or sequence) of these bases on each strand of DNA that leads to the uniqueness of each person’s genetic makeup. The sequence of bases in each gene is used to produce messenger RNA that is, in turn, translated into proteins and the other components of our bodies. There are an estimated 22,000 genes in the human genome and expression of these genes controls the development and maintenance of the individuals we each become.

    There is also a tiny bit of DNA that is not located in the nucleus of the cell but in the mitochondria that are located in the cytoplasm of every cell. Mitochrondria are the energy providers for cells and contain their own circular piece of DNA. This DNA is called “extra-nuclear DNA”, or more simply “mitochrondrial DNA,” and the mitochondrial genes, in collaboration with nuclear genes, make the proteins that are needed for the mitochrondria to function properly.

    A person’s genotype is their genetic identity, the specific combination of genes that they have in their cells. Observable traits or characteristics, such as hair colour or height, are considered a person’s phenotype. Phenotype is the physical expression of the genotype. People’s phenotypes are different because their genotypes are different. Although human genotypes are alike in many ways, small differences make us unique beings in both genetic makeup and appearance. These differences are called polymorphisms. Genetic polymorphisms in both nuclear DNA and mitochrondrial DNA help to identify us as individuals. Sometimes, but not always, these differences in our genotype are related to disease or to the inability to metabolise or break down drugs normally. These kinds of polymorphisms are called genetic variations, or mutations, and they are either inherited or they can occur spontaneously. Some of these genetic variations occurred over time in an attempt by our bodies to protect us from disease. These variations will be discussed under the specific “Conditions and Diseases” that have a genetic component, such as cystic fibrosis. Sometimes only one nucleotide in a gene is different, and this is referred to as a “single-nucleotide polymorphism.” On other occasions, the number of copies of a particular stretch of DNA can vary leading to the term “copy number variation”. This will be explained in greater detail in the section on “Clinical Genetic Testing". It is important to remember that not all genetic variations, or mutations, are harmful or lead to disease.

    Patterns of Inheritance
    There are several ways in which an individual’s polymorphisms or mutations are inherited. These are called “patterns of inheritance” and result in the transmission of a polymorphism or mutation from one generation to the next.

    One pattern is referred to as autosomal dominant because a single variant or mutated gene on one chromosome “dominates” the normal copy on the other chromosome so that a certain trait or disease appears. The variant or mutation may be inherited from either an individual's mother or father. Individuals with an autosomal dominant trait or disease have a 50-50 chance of passing the same variation or mutation on to their children. Examples of autosomal traits are brown eyes or the ability to roll one’s tongue; examples of autosomal dominant diseases are Familial Hypercholesterolemia or Huntington disease.

    An unusual concept of dominant genes is referred to as codominance, in which the genes on both chromosomes are expressed together. An example of this is the blood type AB, in which the A antigen protein and the B antigen protein are both located on an individual’s red blood cells.

    A second pattern of inheritance is termed autosomal recessive because it requires the inheritance of two genetic variant or mutated copies of the same gene for the trait to appear or the disease to develop. One copy is inherited from an individual's mother and the second copy is inherited from an individual's father. If the individual inherits only one of the variant or mutated genes, he or she will not develop the disease but instead will be an unaffected “carrier”, like his or her parent, and can in turn pass the variant or mutant gene on to his or her children. An example of an autosomal recessive trait would be blue eyes; examples of autosomal recessive diseases include cystic fibrosis, sickle cell anaemia, and haemochromatosis.

    There are also patterns of inheritance in which the variant gene is on either the X or Y sex chromosome, and these are referred to as sex-linked patterns of inheritance. With X-linked recessive diseases, a female carries the abnormal gene on one of her two X chromosomes, but because she possesses one normal copy of the gene, she is not normally affected. However, since males have only one X chromosome, a single abnormal copy of the recessive gene on his X chromosome (inherited from his mother) is sufficient to cause the disease. Examples include Duchenne muscular dystrophy and haemophilia. If a disease is X-linked dominant, a single abnormal gene on the X chromosome can cause that disease to develop so that a female is affected and the condition is often lethal in males. This is a rare pattern of inheritance.

    It is interesting to note that mitochondrial DNA (or “extra-nuclear DNA”) is inherited only from our mothers and the Y chromosome only from our fathers. This is referred to as a “maternal” or “paternal” mode of inheritance. However, there are a number of factors that can obscure or complicate inheritance patterns by affecting the way a gene is inherited or expressed.

  • Clinical Testing

    CLINICAL GENETIC TESTING

    Clinical genetic testing refers to the laboratory analysis of DNA, RNA or chromosomes to aid in the diagnosis, management and prevention of genetic disease. Clinical Genetic testing can:

    1. Provide or rule out a particular diagnosis suspected in an individual or family.

    2. Predict the likelihood of developing a particular disease before symptoms appear.

    3. Tell if a person is carrying a specific genetic change that could be passed on to his or her children.

    4. Determine whether some treatments are more or less likely to work before a patient starts therapy.

    These are definite advantages but there are also some qualities of genetic testing that should be carefully thought through and discussed with a medical professional or genetic counsellor before undergoing any test. These aspects are reviewed in the section entitled  Pros and Cons of Genetic Testing. In an era of patient responsibility, it is important that you can obtain information to fully appreciate the value as well as the drawbacks of genetic testing.

    Testing Genetic Material
    Testing of genetic material can be performed on a variety of cellular specimens. DNA can be extracted from these tissues and examined for possible genetic changes or cells can be cultured for chromosome analysis. Looking at small portions of the DNA within a gene, or copy number changes across the genome, requires specialised techniques and specific laboratory testing. This is done to pinpoint the exact location of genetic errors or copy number changes. This section will focus on the examination of a person’s genes and chromosomes to look for the changes responsible for a particular disease.

    There are four basic reasons that genetic material is tested for clinical reasons:

    1. Diagnostic genetic testing is performed on a symptomatic individual with a phenotype sufficiently suggestive of a genetic disorder. This assists the individual’s physician in making a clear diagnosis and preventing a recurrence of the same disorder by prenatal diagnosis (see below).

    2. Carrier testing of genetic material to see whether predisposing factors can be identified in the parents of an affected child that confer a risk of having further children with a genetic condition. Analysis of parental DNA or chromosomes may also be necessary to decide on the significance of some genetic changes.

    3. Presymptomatic testing identifies the presence of a variant or mutant gene that can cause disease even if the physical abnormalities associated with the disease are not yet present in an individual.

    4. Prenatal testing can be used to determine the genetic status of the unborn child because of parental risk factors, physical abnormalities detected in the child using ultrasound or a risk of a serious condition identified by biochemical screening.

    To test DNA for medical reasons, some type of cellular material is required. This material can come from blood, urine, saliva, body tissues, bone marrow or hair, etc. The material can be submitted in a tube, on a swab or in a container but it is important to follow the guidelines for the taking and transport of material appropriate to each specific test. These are usually available from the laboratories to which genetic tests are sent. If the test requires RNA, the same materials can be used but may need rapid handling and despatch. Once received in the laboratory, the cells are used for the extraction of DNA (or RNA) from the nuclei or cell culture for chromosome analysis.

    The lab professionals who perform and interpret these tests are specially trained physicians and scientists usually working in accredited laboratories. The extracted DNA is manipulated in different ways in order for the scientist or technologist to see what might be missing, mutated or extra in such a way as to cause disease. The results can be broadly divided into:

    1. Changes in the number of copies or the structure of a chromosome or chromosomal segment.

    2. Changes in the sequence of DNA bases or the copy number of a gene, part of a gene or a factor that controls its expression.

    These changes may be present in an individual from the beginning of their life (termed “constitutional”) or take place during the development of the body or a cancer (called “acquired”).

    Specific Genetic Diseases
    There are many diseases that are now thought to be caused by alterations in DNA or chromosomes. These alterations can either be inherited or can occur spontaneously. Some diseases that have a genetic component to them include:

    Alzheimer's Disease

    Bone Marrow Disorders

    Breast Cancer

    Ovarian Cancer

    Bowel Cancer

    Cystic Fibrosis

    Down Syndrome

    Haemochromotosis

    Leukaemia

    Lupus erythematosis

    Lymphoma

    Osteoarthritis

    Pre-senilin Mutation


    Sickle Cell Anaemia

    Thalassaemia

    Several things can go wrong with the genes that make up the DNA or the chromosomes resulting in these and other diseases. The section below discusses what can happen to DNA, genes or chromosomes that might lead to a disease.

    Genetic Variation and Mutation
    All genetic variations or polymorphisms originate from the process of mutation. The constitutional genetic variation you are born with arises during meiosis, the specialised cell division that that leads to the formation of each sperm or egg. Acquired genetic variation arises after conception during mitosis, the normal somatic cell division that all cells undergo in growing tissues, or as a result of exposure to environmental factors. Some variations are passed down through the generations while others appear in a particular individual for the first time (termed “de novo”). It is important to realise that copy number variation and mutation are natural processes and that only certain changes lead to disease whereas others may have no detectable effect. Genetic variations can be classified into different categories including:

    1. Unbalanced chromosome abnormalities or copy number variations: a whole extra chromosome 21 can cause Down syndrome while a whole missing X chromosome can cause Turner syndrome. An extra copy of part of chromosome 17 causes Charcot-Marie-Tooth disease while a missing part of chromosome 22 leads to DiGeorge syndrome. Copy number variation of the defensin genes on chromosome 8 can increase the risk of the common skin complaint, psoriasis, but many other copy number variations are phenotypically neutral.

    2. Balanced rearrangements: chromosomes may exchange segments with each other such that the “carrier” of the translocation is phenotypically normal but may have a risk of producing children that are not.

    3. Mutations: these come in a bewildering variety of forms that include:

    a. Deletions, in which one or more nucleotides are lost such as the delta 508 mutation in cystic fibrosis.

    b. Insertions, in which one or more nucleotides are inserted into a gene.

    c. Indels in which both insertion and deletion occur.

    d. Substitutions, in which one nucleotide is replaced by another.

    e. Triplet variations which have a range of variation in normal individuals but may expand to cause diseases such as Fragile X syndrome or Huntington disease.

    4. Splice site variations that result in one or more different messenger RNAs and proteins being produced by a single gene.

    5. Single nucleotide polymorphisms: these are often phenotypically neutral but are useful to track changes in copy number or the pattern of inheritance of a linked gene.

    Silent genetic variations are those mutations or changes in a gene that do not change the protein product of the gene. These mutations rarely result in a disease.

    Testing for Products of Genetic Expression
    Many inherited disorders are identified indirectly by examining abnormalities in the genetic end products (proteins or metabolites) that are present in abnormal forms or quantities. As a reminder, genes code for the production of thousands of proteins and, if there is an error in the code, changes can occur in the production of those proteins. So, rather than detecting the problem in the gene, some types of testing look for abnormalities in the amount or structure of the proteins themselves or their precursors.

    An example of testing for genetic products includes those widely used to screen newborns for a variety of disorders. For instance, instead of looking for the gene mutations in the autosomal recessive condition, Phenylketonuria(PKU), a biochemical test can be used on a tiny blood sample from a baby’s heel to look for the presence of the extra phenylalanine that is characteristic of this disease. Too much phenylalanine up in the blood can lead to mental retardation but, once detected, this can be avoided by treatment with a special restricted diet.

     

  • Pharmacogenomics

    PHARMACOGENOMICS

    In some cases, individuals who are given a certain therapeutic drug to treat symptoms or to keep symptoms from occurring, have a very violent reaction to the drug or feel no affect whatsoever. In some cases, this can be traced to the genetic makeup of the individual and the study of this phenomenon is called “pharmacogenomics” or “pharmacogenetics.”

    As an example, a woman who had surgery to remove a tumour was given codeine as a pain reliever. Although she was doing well after the surgery, as soon as she began treatment with codeine she developed a full-body rash, difficulty breathing, and an irregular heartbeat. When she was taken off the codeine, her reaction disappeared. Upon further study, it was found that she lacked the enzyme in her blood that metabolised (broke it down into different components) the codeine into morphine and other substances, so she was essentially being overdosed with codeine. The lack of the enzyme was directly related to a polymorphic variation in one of the cytochrome genes (CYP2D6) gene that produced it. Sometimes these polymorphisms can cause a very serious reaction in an individual that could lead to death.

    In other cases, individuals “hyper metabolise” drugs. This occurs when there is too much of an enzyme present that breaks down the helpful drug too quickly, leading to a lack of response to the drug. This can happen when too many copies of the gene are present and too much enzyme is produced. In other cases, the special receptor that the drug binds to on cells or tissues is missing, again because of a variation in the gene that makes the receptor protein. When there is no receptor to bind the drug, the drug may not have any affect on the cells or tissues that it should.

    Genetic testing to determine the polymorphisms that play a role in our response to a drug is now available for a number of conditions including an individual’s resistance or sensitivity to the effectiveness of drugs used in viral therapy for e.g. HIV or Hepatitis C.

  • Identity Testing

    IDENTITY TESTING

    Identity testing is sometimes referred to as “DNA testing”, a term most frequently used in relation to criminal investigations. "DNA testing" is an unfortunate misnomer as all types of genetic analysis, whether for disease or identification or for tissue typing, involves assessment of DNA or RNA.

    Identity testing focuses on the identification of an individual through the analysis of either nuclear or mitochondrial DNA extracted from blood, tissue, hair, bone, etc. Any material that contains cells with nuclei can be used for nuclear DNA extraction and eventual identity testing. Mitochondrial DNA, which is “extra-nuclear,” is used when a sample is severely degraded or if only hair shafts with no attached cells are available.

    Increasingly, identity testing is used to identify a suspect in a criminal investigation by comparing the DNA found at a crime scene to that of the suspected individual. If the suspected individual is convicted of the crime, his or her DNA polymorphisms are put into a data bank that is accessible to the police force.

    Other uses of identity testing include finding the identity of those who cannot be distinguished by other means, as with decomposed bodies. In this type of forensic genetic testing, patterns of polymorphism specific to an individual are used to produce a “DNA fingerprint.” DNA sequences that are particularly susceptible to polymorphic variation are useful for this purpose including DNA sequences known as microsatellites, minisatellites, short tandem repeats (STRs) and variable number tandem repeats (VNTRs).

    Other applications of this type of testing include the determination of an individual’s parent or parents, often called “parentage testing”, and identifying organ donors by using genetic testing for tissue transplantation, called "tissue typing".

  • Paternity Testing

    PATERNITY (PARENTAGE) TESTING

    The primary goal of parentage testing, often called paternity testing, is to identify the biological parent of a given child. It is done to determine an individual’s parent or parents in, for example, cases of adoption or alleged paternity. This determination must be looked at very carefully and must identify the alleged parent with at least 99% certainty.

    Many different types of laboratory tests can be done to assess parentage, including examination of red blood cell antigens (blood typing), polymorphic serum protein genes or patterns of genetic polymorphism. The DNA testing techniques used are similar to the “DNA fingerprinting” techniques used for forensic science.

    If, after testing multiple systems, the parent in a dispute is not excluded as a possible parent, a mathematical estimate of the possibility that the tested person could be the biological parent can be calculated. This mathematical testing combines the results of the genetic tests with other “non-genetic events” (location of the alleged parent at the time of conception, phenotype of the parent and child, etc.) and results in a “parentage index.” This index is a percentage of the likelihood of parentage. Results of these tests are admissible as evidence in court.

  • Tissue Typing

    TISSUE TYPING FOR TRANSPLANTATION

    In the past, it was difficult to tell exactly whether an organ or tissue, such as a kidney, lung or bone marrow, was an exact match for the transplant between a donor and recipient. If it was not, a serious rejection reaction could sometimes occur between the recipient patient and the transplanted organ.

    Basic laboratory testing for tissue transplantation involves mixing the white blood cells (leucocytes) from the donor (or the donor tissue) and the recipient together and observing whether an immune response occurs. Proliferation of a specific population of leucocytes signals the onset of an immune response and the likely rejection of the tissue by the recipient’s body. Although this technique is still commonly used, analysis of the DNA in both the donor and the recipient (tissue typing) is used to diminish the likelihood of rejection in the case of tissue transplantation. In bone marrow transplants, DNA testing (or sometimes chromosome analysis) is done to determine whether the leucocytes and their precursors repopulating a recipient’s bone marrow are his own or those of the donor.

    A very specific set of genes is examined when DNA testing is used for tissue typing. On chromosome 6, a large set of genes called the “Major Histocompatibility Complex,” or MHC, resides. These genes are very polymorphic (different) between individuals, and they code for the production of specific glycoprotein antigens on the surface of many cells. It is these antigens that “recognise” our own organs and tissues from those of another individual. These antigens have the ability to begin an immune system response that results in organ or tissue rejection if the tissue looks foreign.

    A distinct region within the MHC on chromosome 6 is used in the DNA analysis of tissue that could be used for transplantation. This region called the  human leucocyte antigen, or HLA-D, region, and the sets of genes located there are further subdivided into HLA-DR, HLA-DQ, and HLA-DP depending on the type of glycoprotein antigen they code for. Polymorphisms in these genes are carefully compared between donor and recipient to determine the appropriateness of the transplant.

    The exact techniques used to test DNA for tissue typing are similar to those mentioned in the sections above. DNA is extracted from donor and recipient cells, then manipulated and fragmented in such a way as to isolate a specific region on a chromosome and within a gene. The fragments are subjected to further analysis that allows for comparison of the polymorphisms in the HLA-DP between the donor’s tissue and the recipient’s blood. This careful analysis of genetic material results in fewer rejection reactions and a better chance of a successful transplant.

  • Infectious Disease Testing

    INFECTIOUS DISEASE TESTING

    When we hear the term “infectious disease”, we usually think of something that can infect us and cause a disease process to begin. That “something” can be a bacteria, virus, parasite, or fungus obtained from many different sources (other infected individuals, poor hygiene, transfusion with infected blood, shared needles between drug users, etc.). Disease-causing bacteria and viruses are known as infectious agents, and some of them can be quickly identified by using genetic testing techniques; however, common infectious agents, such as certain bacteria and viruses, are usually much less expensive to identify using standard laboratory methods that don’t involve genetic testing techniques.

    Bacteria are one-celled organisms that contain their own DNA and in some cases can cause serious disease. Even those bacteria that live harmlessly inside our bodies and are involved in beneficial chemical processes can become mutated under unusual conditions and cause us to be unwell. By analysing the DNA, viruses, bacteria and their mutated variants can be identified with great speed and precision. Some of the bacteria that can be quickly identified using these genetic testing techniques include: Chlamydia trachomitis, which is an organism that causes a sexually-transmitted disease; Neisseria gonorrhea, which causes gonorrhoea, Borrelia burgdorferi which causes Lyme Disease, Legionella pneumophilia which causes Legionnaire’s disease, Mycoplasma pneumoniae which leads to “walking pneumonia,” Mycopbacterium tuberculosis which can cause tuberculosis and Bordetella pertussis which causes whooping cough. Specimens that might contain these bacteria include urine, blood, sputum, cerebrospinal fluid and others.

    Viruses are unusual organisms that sometimes insert their DNA into a host’s genome. The viral RNA or DNA utilises the host’s cells to produce proteins and make more viruses. Viruses such as hepatitis C virus (HCV) and human immunodeficiency virus (HIV) are examples of RNA viruses.

    Other disease-causing viruses that contain DNA instead of RNA include Herpes simplex virus, cytomegalovirus, Epstein-Barr virus, parvovirus, and varicella-zoster viruses. All of these viruses can be identified by first removing the suspected viral DNA or RNA from a patient specimen and then using it to provide a “fingerprint” of the suspected virus.

    Specimens usually include blood, cerebrospinal fluid, sputum, other body fluids, amniotic fluid, tissue or bone marrow. Much of the testing on donor blood that will be used in a blood transfusion utilises genetic testing to inspect the blood for viral contamination.

    Determining how many copies of virus RNA are present in an individual’s blood is another use of infectious disease genetic testing techniques. The number of copies present is typically referred to as the “viral load” or “viral burden”. This testing is usually done after a drug therapy is initiated to assess whether it is working or not to remove or decrease the viral RNA load. The most common viral load tests are for HCV or HIV, and the tests require a sample of blood.

    A parasite is a complex multi-cellular organism. Parasites usually infect an individual through the saliva of a biting insect, such as a mosquito, or through infected material. An example of a parasite that can be identified using genetic tests is Toxoplasma gondii which can cause encephalitis or congenital infections that can lead to severe damage of an unborn child (foetal toxoplasmosis).

  • Pros and Cons

    Pros and Cons of Clinical Genetic Testing

    Genetic testing holds great potential for the future of medical care. It offers many benefits, including the provision of important information that can be used when making decisions about having a family and taking care of one’s own health. However, there are also limitations. For this reason, it is important to understand the nature of genetic testing and the information that it can and can’t provide. For example:

    • Clinical genetic tests are not just descriptive as many laboratory tests are (such as describing the glucose level in your blood), but may be predictive as well. Predictive tests may not give a yes/no answer, but instead will tell what the chances are of developing a particular genetic condition. Such results may not be definitive and may leave a person wondering what to do with those results, particularly if available treatments or therapies are limited. However, the profession of Genetic Counselling exists to provide individuals and their families with accurate information and help interpret genetic results so that individuals and their families can make their own decisions.
    • Many genetic tests will only look for the presence or absence of a specific genetic mutation; the test cannot always guarantee that the disease will develop nor can the test provide information about other genetic diseases not being specifically looked for by that test.
    • While the test may detect a particular problem gene, it cannot predict how severely the person carrying that gene will be affected. Again with cystic fibrosis, symptoms may be mild bronchial abnormalities or may extend to severe lung, pancreatic and intestinal problems.
    • Many genetic tests cannot detect all of the variations that can cause a particular disease. For instance, with genetic testing for cystic fibrosis, most genetic testing panels only look for the more common mutations and further specialised testing may be needed to identify rare causes of this disease. Similarly, some diseases are caused by mutations in more than one gene and financial constraints may limit the degree to which multiple genes can be analysed using current techniques.
    • Some diseases are the result of an interaction between one’s genes and one’s environment. The way in which these interactions cause disease is not clearly understood. Examples of these diseases include coronary heart disease, type 2 diabetes, obesity and Alzheimer disease.
    • Legal issues, such as patient privacy, use of genetic testing to determine insurance coverage and the use of archived patient samples are some of the broader social issues to be considered.

    Because of these limitations, genetic test results can be a mixed blessing. An absolutely essential component of clinical genetics testing is giving your informed consent to do the tests and knowing what you want to do with the results of these tests. Knowing your legal rights and making certain that your privacy is respected may also be useful. Educating yourself about genetic tests and talking to your medical provider, and/or genetic counselling service, may help you decide whether you think you should have genetic testing performed.

    It is important to remember that genetic testing is different from other types of laboratory testing in the sense that the results may have implications, not only for you the patient, but also for family members who may need to be tested as well. Genetic education and counselling is therefore often advised to help understand and cope with the results of genetic tests.

  • The Future

    The Future: Advances, Potential, Conclusions

    With the completion of the Human Genome Project, we have learned that the word “normal” has a more qualified meaning when it comes to a person’s genetic makeup. Genetic variations occur in great numbers in our genome (our total genetic makeup). We are all unique, not only in our personalities and appearance, but in our genotype as well.

    Scientists continue to work on ways to better understand the structure of our genetic makeup, which could allow for important advances in the prevention and treatment of many diseases. There are promising new screening tests available, such as those for ovarian cancer or cystic fibrosis, that researchers are trying to replicate for other disorders as well. Knowledge of the genetics can also assist in the creation of “designer drugs” targeted at a specific mutation as the use of “Glivec” to treat chronic myeloid leukaemia.

    Gene therapy is an approach to treating potentially lethal and disabling diseases that are caused by single gene deficiencies. With specialised techniques, gene expression can be manipulated to correct the problem in the particular patient, although the correction will not be passed on to the offspring of that patient. That is, corrections are made at the DNA molecule level to compensate for the abnormal gene so that the detrimental symptoms of the disease are not expressed in the patient. This is still highly experimental but clinical trials are being conducted and some promising results are beginning to emerge.

    The application of other new technologies can be expected. Microarrays, or gene chips, have already extended chromosome analysis into the sub-microscopic world of copy number variation so that the genetic basis of diseases and traits can be identified. Similarly, next generation sequencing is expected to bridge the gap between whole genome analysis using microarrays and the single targeted gene analysis more common with current DNA sequencing techniques.

    Further advances in genetic testing will eventually replace older methods of predicting prognosis, help to treat only those patients who will respond to therapy and guide further research into new therapies. Recent advances are also helping to increase our understanding of some complex cancers, such as multiple myeloma and lymphoma. Without doubt, advances in genetic research will have an impact on the laboratory tests available to all patients for the detection and treatment of an increasing variety of diseases.

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