A healthy human cell contains 46 chromosomes. These chromosomes are composed of deoxyribonucleic acid, or DNA. A gene is a short section of DNA, and each of our chromosomes carries hundreds, sometimes thousands, of genes.
Genes contain the information required by our bodies to make proteins. Proteins form the body’s physical structure and are vital in the processes which keep us alive. It is the differences in our genetic makeup that make us all unique.
Sometimes genes may change. This is known as a mutation, and it creates the risk of us developing a particular disorder or passing it on to our children.
Genetic carrier screening
Carrier screening is used to identify those people who carry one copy of a gene mutation which, when present in two copies, can cause a genetic disorder. In many countries, this type of testing is routinely performed on expectant mothers to evaluate the risk of their unborn child being born with a genetic disorder.
How are genetic disorders passed on?
Reproductive cells (i.e., sperm and ova), contain 23 chromosomes each—a half-set of genetic information. When the conception occurs, the 23 chromosomes from the mother’s ova join with the 23 chromosomes from the father’s sperm, culminating in 46 chromosomes, the genetic blueprint required to make a new person. We carry two copies of our genes, one from our mother and one from our father.
When a person is a carrier of a genetic disease, he or she carries one normally functioning copy of a gene and one faulty copy. The majority of genetic diseases are “recessive,” meaning that the normal gene in the pair will override the faulty gene. Carriers generally do not show symptoms, but pass the faulty gene on, meaning that recessive gene faults can pass through the generations without causing any symptoms, and can, therefore, go undetected.
Of our 23 pairs of chromosomes, 22 are non-sex and one pair are the sex, or X, chromosomes. These chromosomes determine the sex of the child. A gene mutation that occurs in one of the 22 pairs of non-sex chromosomes is called an autosomal recessive mutation, while a mutation that occurs in the one pair of X chromosomes is referred to as an X-linked mutation.
Autosomal recessive disorders
An autosomal recessive disorder only presents symptoms in a child when both parents are carriers of the same faulty gene and the child has inherited a mutated gene copy from each of them. Children of parents who are both carriers of the same faulty gene have:
- A 50 percent possibility of inheriting a faulty gene copy from only one parent and becoming a carrier.
- A 25 percent possibility of inheriting two faulty gene copies, one from each parent, and developing the disorder.
- A 25 percent possibility of inheriting two unaffected gene copies, one from each parent, and neither developing nor carrying the disorder.
X-linked recessive disorders
In X-linked disorders, where the gene mutation occurs in the X chromosome, the condition can be inherited via just one copy. An X-linked condition is generally inherited from a mother who is a carrier of a faulty gene. Carrier females have a faulty gene on one X chromosome and a functioning copy on the other X chromosome.
Women pass on one copy of their X chromosomes in each ovum. When a man passes on a Y chromosome in a sperm, a male child will be conceived (XY), and when he passes on an X chromosome, a female will be conceived (XX). Because a male has just one copy of the X chromosome, which he inherited from his mother in her ova, if she passes on a faulty X chromosome there is no healthy X chromosome from the father to offset it; therefore, the male will be born with the condition. However, when a female inherits a healthy X chromosome from her father, this will offset the faulty gene. As such, the female will not have the disorder, but she will be a carrier. It is in this way that X-linked recessive disorders generally only affect males, with females as unaffected carriers. Nevertheless, X-linked inheritance can be incredibly complex, depending upon the disorder and genetic mutation. A carrier female may sometimes exhibit mild symptoms while an affected male may remain symptom free.
How are faulty genes detected?
In genetic testing, doctors take a blood sample, usually from the prospective mother. The sample is sent to a laboratory, where the DNA is analyzed. Scientists inspect certain points on the DNA chain for mutations linked to genetic disorders and return a screening report to the medical practitioner for analysis.
A screening report is not a simple yes or no answer. It is a very complex set of data. It will not provide a definite positive/negative result, but it will tell the medical practitioner what the chances are that the patient is a carrier. If none of the data indicates a risk of the mother being a carrier, testing usually ends there.
Treatment of genetic disorders
While there are no known cures for genetic disorders, gene therapy offers high hopes for future. In this treatment, a healthy gene is introduced into the patient’s body to alleviate or slow symptoms caused by the faulty gene. Over 1,800 gene therapy clinical trials have been approved worldwide in an ongoing battle with a wide range of disorders.