Genetic traits passed from parents to children occur naturally, but if a parent has an abnormal DNA, they may pass on an abnormal gene to the child, which can cause genetic diseases. These diseases cannot be completely cured but can be prevented, such as through in vitro fertilization or appropriate care, which can alleviate symptoms. Genetic screening (Test Heritage Carrier Screening) is very important because it helps reduce risks and prevent the transmission of genetic diseases from parents to children.
What is DNA?
DNA is a genetic substance with a length of up to 3 billion units or 3 billion bases, in which a portion called a gene gives orders for cell functions. Normally, the entire 3 billion base DNA strands of humans are wrapped around protein to form a rod called a chromosome, of which there are 46 rods, 23 from the father and 23 from the mother.
What is a genetic disease?
A genetic disease is a disease caused by abnormalities in the genetic material or DNA, which controls the working of different cells in the body. When DNA is abnormal, it may lead to abnormalities in bodily functions. If the abnormality occurs in a crucial part of the DNA, it can lead to severe diseases. Genetic diseases can be passed down to offspring because children receive half of their DNA from each parent, such as in Glucose-6-phosphate dehydrogenase (G6PD) deficiency and thalassemia.
How do genetic diseases occur?
Genetic diseases can occur in several ways, whether as chromosomal abnormalities, which involve the whole rod being missing or added, such as in Down syndrome, group gene abnormalities, where many genes must be abnormal simultaneously to cause the disease, or single-gene diseases, where a single gene abnormality can cause disease. Gene abnormalities also manifest as either dominant or recessive genes. In the case of dominant genes, a child inheriting one set from either parent can result in the disease. In the case of recessive genes, the child must inherit a set from both parents to manifest the disease.
What is a Single Gene Disorder?
A Single Gene Disorder is a genetic disease resulting from a mutation in a specific gene location (Single Genes Disorder), with each location controlling the functioning of different organs. They are divided into three categories:
- Dominant gene abnormalities (Autosomal Dominant), where inheriting a mutated gene from either the father or mother alone can cause symptoms of the disease, such as in Neurofibromatosis, Marfan syndrome, etc.
- Recessive gene abnormalities (Autosomal Recessive), where inheriting mutated genes from both carrier parents is necessary, such as in Thalassemia.
- Diseases due to sex chromosome (X-linked) abnormalities, with dominant and recessive genes passed through the X chromosome, such as in Hemophilia.
How are genetic diseases transmitted?
Humans normally have 46 chromosomes or 23 pairs, comprising 22 pairs of body chromosomes and 1 pair of sex chromosomes. Men have XY sex chromosomes, and women have XX. Single-gene genetic diseases are primarily transmitted in two ways:
- Recessive gene on body chromosomes, when a couple carries the same genetic disease, their child has a 25% chance of having the disease, or 1 in 4, and a 50% chance of being a carrier, or 2 in 4.
- Recessive gene on the X chromosome, even though the male is not a carrier, if the woman is a carrier, the son has a 50% chance, or 1 in 2, of having the disease, even with a single set of recessive genes, as men have only one X chromosome set. Daughters have a 50% chance of being carriers.
How are carriers of genetic diseases different from those who have the disease or are normal?
Normally, humans have 2 sets of DNA, one from the father and one from the mother. Most single-gene genetic diseases arise from what is called recessive DNA abnormalities on chromosomes. If both parents carry the same disease, the child has a 25% chance of inheriting 2 sets of recessive disease genes from each parent and showing disease symptoms. However, if the child inherits just one set of recessive disease genes, a 50% possibility, they will be a carrier, showing no symptoms, not having the disease, leading a normal life, but able to pass on the recessive disease gene to offspring.
Why is genetic carrier screening important?
Because carriers of genetic diseases show no symptoms but can pass the disease to children, resulting in a 25%-50% chance of the child having the disease. Genetic diseases have diverse symptoms; some are not life-threatening but require dietary or medication adjustments for a normal life, while others are life-threatening, affecting intelligence, reducing lifespan, or causing disabilities that hinder normal living. Therefore, genetic screening (Test Heritage Carrier Screening) is important to reduce risk and prevent genetic diseases.
The benefits of genetic screening
Genetic screening (Test Heritage Carrier Screening) assesses the risk of genetic diseases in children, revealing whether the husband and wife have recessive genes and the probability of the child having the disease. The best time for screening is before pregnancy to understand the potential genetic abnormalities in the child. In cases of desiring pregnancy with gene abnormalities, IVF may be chosen to select a healthy embryo without abnormal genes, making the chance of disease almost 0%. If already pregnant, genetic screening can determine the percentage chance the child has the disease, enabling amniotic fluid tests to confirm gene abnormalities and consider termination before 24 weeks of pregnancy. If continuing the pregnancy, medical advice for monitoring, careful drug use, avoiding certain foods or chemicals that might affect the baby, to live normally, under close specialist supervision, is advised.
Genetic diseases in Thailand
In Thailand, several genetic diseases are prevalent, most notably alpha and beta thalassemia and G6PD deficiency, where consuming fava beans causes blood breakdown. In 2024, it was found that 39% of Thais were carriers of at least one genetic disease, with the most common group being blood genetic disorders at 31%, followed by metabolic disorders at 8%, causing sensitivity to certain nutrients that may be life-threatening, and other system effects such as weak muscles and intellect at 7%. Thus, genetic screening is crucial.
According to N Health Laboratory, part of BDMS, among those screened for genetic disease carriers, 75.5% were carriers of at least one genetic disease, and 11.7% of couples were at risk of passing genetic diseases to their children. In the case of recessive gene transmission on body chromosomes, there is a 25% chance or 1 in 4 that a child will have a genetic disease, a rate higher than Down syndrome, which all pregnant mothers are now screened for.
The top 5 most prevalent diseases are alpha thalassemia at 18.6%, beta thalassemia at 15.5%, hereditary deafness at 14.4%, G6PD at 10%, and Stargardt disease causing vision loss at 5.9%.
Bangkok Hospital has offered genetic carrier screening since mid-2024, with 53.1% of individuals screened found to be carriers of at least one genetic disease.
Currently, over 400 gene abnormalities can be detected. Most parents do not know they carry a recessive gene because the disease is asymptomatic but can be passed to children. Some diseases manifest immediately after birth, some when the child grows up or reaches adolescence. If husband and wife have the same gene abnormality or disease, the child has a 25%-50% chance of being affected, depending on the type of disease. Therefore, it is important to prioritize genetic disease screening before pregnancy.
Specialist in genetic disease treatment
Dr. Romsai Lerdpienpitayakul, Maternal and Fetal Medicine specialist, Women’s Health Center, Bangkok Hospital.
You can click here to schedule an appointment yourself.
Hospitals specializing in genetic disease treatment
The Women’s Health Center, Bangkok Hospital, offers guidance, cause identification, and genetic disease screening with a team of specialists, nurses, multidisciplinary teams, and modern treatment technology to reduce the risk of genetic disease transmission from parents to children, promoting healthy growth at every stage of life.
