Genes are like the instructions that tell our bodies how to develop and function normally. Genes are comprised of a long row of DNA molecules which are like the letters that make up the words or instructions. The alphabet of DNA has only 4 letters, A, T, G and C. Each gene has a function. Perhaps it makes a product, like an enzyme that helps us to digest food, or a molecule that helps our nerves communicate with each other. The specific combination of the letters in our genes controls how our genes function. When an instruction is misspelled, the gene might not function normally, and this is when genetic disease occurs.
A chromosome is a string of many genes all lined up one after another. Human beings have 46 chromosomes, each covered with thousands of genes. Each chromosome contains thousands of genes, each of which is several thousand base-pairs long. A gene is any section along the DNA that has instructions encoded that allow a cell to produce a specific product, usually a protein that serves one particular function in the body. There are between 20,000 and 30,000 genes, and every single gene is made up of thousands, even hundreds of thousands, of DNA molecules.
Back to top
Scientists say that a gene is a basic unit of heredity in a living organism, meaning that every living creature or plant or bacteria is made up of genes. Our genes are given to us by our parents. When the egg and sperm combine to make a new human being, each provides 23 chromosomes, providing us with a total of 46 chromosomes. Of the 46 chromosomes, 44 are known as “autosomes” and are present in both men and women. . Autosomes typically contain two copies of each gene, one contributed from each parent. Some of us might resemble one parent more than the other, but we still inherit about half of our genetic information from each one. The remaining two chromosomes are called sex chromosomes, which are designated X and Y. Women inherit two X chromosomes, one from each parent, and , men inherit one X chromosome from their mother and one Y chromosome from their father. Whilst women have two copies of each gene on the X chromosome, men only have genes they inherit from their mother on the X chromosome and only genes they inherit from their father on the Y chromosome.
Back to top
When there is a change in the sequence of DNA letters that make-up a gene, this can change the instructions communicated by that gene and interfere with its ability to do its job, or make its protein. This misspelling is considered a mutation. Just like changing a letter in a word can change the word’s meaning, a mutation can change the instruction contained in the gene. Mutations can range in size from a single DNA ”letter”, which may have little effect, to a deletion of a large piece of a chromosome causing the gene not to function at all, and make none of its required product.
Back to top
Some mutations result in proteins that do not function normally, and may end up causing disease. There are several ways that a gene mutation can change the way a protein functions, including:
The human genome is believed to contain around 20,000 - 30,000 genes. Genetic testing identifies changes in chromosomes, genes, or proteins and can confirm or rule out a diagnosis, suggest an increased risk to develop a particular disorder, or determine that an individual is a carrier for a particular condition and might pass that gene on to their children.
A pediatrician or a general physician may refer a child to a clinical geneticist because of a suspicion that a child has a genetic disorder based on facial features,or other physical characteristics observed in the child.
Back to top
Genetic tests are used for a variety of reasons, including:
There are three general categories of genetic tests:
What is a chromosomal test?
Chromosomes are the large DNA-containing structures in the nucleus of a cell. Chromosomel tests look at features of a person's chromosomes, including their structure, number and arrangement. These tests look for changes, such as pieces of a chromosome being deleted, expanded, or being switched to a different chromosomal location.
Types of chromosomal tests include:
Gene tests look for signs of a disease or disorder in DNA or RNA taken from a person's blood, other body fluids like saliva, or tissues. These tests can look for large changes, such as a gene that has a section missing or added, or small changes, such as a missing, added, or altered chemical base (subunit) within the DNA strand. Gene tests may also detect genes with too many copies, individual genes that are too active, genes that are turned off, or genes that are lost entirely.
The types of gene tests include:
Biochemical tests look at the amounts or activities of key proteins. Since genes contain the DNA code for making proteins, abnormal amounts or activities of proteins can signal genes that are not working normally. These types of tests are often used for new-born screening.
Genetic tests are performed on a sample of blood, hair, skin, amniotic fluid (the fluid that surrounds a fetus during pregnancy), or other tissue. For example, a medical procedure called a buccal smear uses a small brush or cotton swab to collect a sample of cells from the inside surface of the cheek. Alternatively, a small amount of saline mouthwash may be swished in the mouth to collect the cells. The sample is sent to a laboratory where technicians look for specific changes in chromosomes, DNA, or proteins, depending on the suspected disorder. The laboratory reports the test results in writing to a person's doctor or genetic counselor.
Back to top
The results of genetic tests are not always straightforward, making interpretation challenging, this is why a geneticist will consider not only the test results, but also a person’s medical history, family history, and the type of genetic test that was done.
A positive test result means that the laboratory found a change in a particular gene, chromosome, or protein of interest. Because family members have some genetic material in common, a positive test result may also have implications for relatives. It is important to note that a positive result cannot always establish the exact risk of developing a disorder or its severity.
A negative test result means that the laboratory did not find any abnormality of the gene, chromosome, or protein under consideration. Depending on the type of test, further testing may be required, as not all genetic changes that can cause a particular disorder can be detected.
A result may also be uninformative, indeterminate, inconclusive, or ambiguous. This can occur because everyone has common, innocent variations in their DNA, called polymorphisms that do not affect our health. In some cases, testing other affected and unaffected family members can help clarify this type of result.
Back to top
The physical risks associated with most genetic tests are very small, particularly for those tests that require only a blood sample or buccal smear (a procedure that samples cells from the inside surface of the cheek).
Rapid advances in genetic medicine are constantly improving doctors' ability to detect mutations and diagnose disorders.
However, genetic testing does have limitations: