Common Genetic Diseases in Turkey

What is a Genetic Disorder?

A genetic disorder is a disease caused by anomalies in genes or chromosomes inherited from parents to children. These hereditary diseases appear as mutations in our DNA or chromosomal abnormalities.

In the genetic code, which serves as the building block of human existence, we inherit one strand of the double helix of DNA from our mother and the other from our father. Genetic disorders occur due to pre-existing mutations or mutations acquired later.

These genetic diseases can manifest at birth or later in a person's life. A person's intelligence, emotions, thoughts, and all other information are already encoded in their DNA.

What Causes Genetic Disorders?

Genetic disorders are classified into chromosomal and gene disorders. The DNA structure, passed down from generation to generation, contains all the information necessary for an individual's growth, development, intelligence, emotions, and thoughts and is present from birth.

In healthy individuals, the genetic material inherited from both parents results in the birth of healthy babies without any chromosomal anomalies or gene mutations. However, sometimes mutated genes or chromosomal anomalies inherited from parents are passed down as a genetic legacy.

Globally, about one in every 30 individuals develops a genetic disorder. While many can be treated with early diagnosis, some currently have no treatment. In many cases of consanguineous marriages, individuals with genetic diseases from the same family line may pass pathogenic mutated genes to their children.

Symptoms of Genetic Disorders

The process beginning with pregnancy, including risks identified in screening tests, age-related risks, ultrasound findings, or a history of multiple miscarriages, can raise suspicions of a genetic disorder in the baby. Therefore, between the 12th week (chorionic villus sampling - CVS),the 16th-18th weeks (amniocentesis),or the 20th week (cordocentesis),tests can diagnose whether the baby has a genetic disorder. If genetic disorders are not detected during pregnancy, symptoms like developmental delays, muscle weakness, hearing loss, or concentration issues after birth can prompt further tests using a blood sample from the baby to confirm a genetic disorder.

Common Genetic Disorders in Turkey

• Sickle Cell Anemia
• Alpha Thalassemia
• Beta Thalassemia
• SMA
• Fragile X Syndrome
• DMD
• Phenylketonuria
• Biotinidase Deficiency
• 21-Hydroxylase Deficiency
• Cystic Fibrosis

Sickle Cell Anemia: An autosomal recessive disorder, detectable by mutations in the beta-globin gene.

Alpha Thalassemia: A common disease in this region, caused by deletions in the alpha-globin gene. Four alpha-globin genes are present, and individuals planning to marry or have children should undergo thalassemia carrier testing to prevent passing on the disease to their children.

Beta Thalassemia: An autosomal recessive disease typically caused by mutations in the beta-globin gene. It is diagnosed through DNA sequencing.

SMA: A genetic disorder found in approximately one in 40 individuals in Turkey, with one in every 250 children being diagnosed. Couples can undergo carrier screening tests to ensure healthy offspring. SMA is a severe muscle disease, with the most common type 1 SMA manifesting as muscle weakness in newborns.

Fragile X Syndrome: The second most common cause of intellectual disability after Down syndrome, mainly passed from carrier mothers to their male children.

DMD: A muscle disease passed from carrier mothers to male children, starting with progressive muscle weakness before age five and leading to wheelchair dependence by around age 12.

Phenylketonuria: A metabolic disorder caused by mutations in the PAH gene. If the amino acid phenylalanine is not restricted through diet, it can lead to intellectual disability.

Biotinidase Deficiency: A genetic disorder that can present within days of birth, leading to developmental delays and skin issues.

21-Hydroxylase Deficiency: Causes excessive androgen production, resulting in symptoms like male-pattern hair growth and deepening of the voice.

Cystic Fibrosis: A genetic disorder affecting one in every 30-40 individuals in Turkey, leading to serious lung and pancreas diseases as well as infertility.

Each genetic disorder can be diagnosed with specific tests, or they can be screened using a carrier panel.

Chromosomal Disorders

Humans have 46 chromosomes, two of which are sex chromosomes. Males have 46XY chromosomes, and females have 46XX chromosomes. Chromosomal disorders can arise from chromosomal abnormalities like missing chromosomes (e.g., Turner syndrome 45,X) or extra chromosomes (e.g., Down syndrome, trisomy 21, Klinefelter syndrome 47,XXY),as well as deletions, duplications, or translocations of chromosome segments (e.g., leukemia).

Single Gene Disorders

Single gene disorders result from mutations in a single gene, leading to various diseases like organ failure, intellectual disability, or muscle diseases.

Rare Genetic Disorders

Around 80-85% of rare diseases are caused by genetic factors. The remaining 15-20% are thought to involve environmental, viral, or allergic factors. These chronic diseases often manifest at birth or in childhood and are estimated to affect over 7,000 people in Turkey. Examples include phenylketonuria, progeria, tuberous sclerosis, Rett syndrome, congenital adrenal hyperplasia, adrenoleukodystrophy, capillary leak syndrome, and osteogenesis imperfecta.

Will the Children of Couples with Genetic Disorders Inherit the Same Disease?

If both partners carry the same genetic disorder, there is a 25% chance that their child will also have the disorder. For instance, if both partners are carriers of SMA, there is a 25% chance their child will have SMA, a 50% chance the child will be a carrier, and a 25% chance the child will be healthy.

Are Genetic Disorders Mostly Caused by Consanguineous Marriages?

Consanguineous marriages occur between individuals with a blood relation. In such marriages, there is a higher risk of genetic disorders in children due to the shared genetic heritage. In first-degree consanguineous marriages (e.g., cousins),about 5% of children have a genetic disorder.

How Can Genetic Disorders Be Prevented?

In Turkey, common genetic disorders can be detected through carrier screening tests, while rare genetic disorders can be identified through DNA analysis. Genetic counseling based on family history is recommended for individuals planning to have children.

Can Genetic Disorders Be Prevented Through IVF?

If a person is found to be a carrier of a chromosomal or genetic mutation, they can still have a healthy child through preimplantation genetic diagnosis (PGD),which involves selecting healthy embryos for implantation.