What Are Single-Gene Diseases? Inheritance Risks & PGT-M for Family Planning
What are single-gene diseases? If you’re planning a family or have a family history of a genetic condition, you may have worried about single-gene disease. But what does it actually mean? And why does it matter for your family planning journey?
This guide explains everything you need to know about single-gene diseases—what they are, how they’re inherited, and how PGT-M testing can help prevent transmission.
The Short Answer: What Is a Single-Gene Disease?
What is single-gene disease? A single-gene disease, also called a monogenic disorder, is a genetic condition caused by a mutation or change in just one gene. Genes are made of DNA and contain the instructions for how your body grows, develops, and functions.
Single-gene disorders are the best-understood genetic conditions, following clear, predictable inheritance rules. Humans average roughly 400 abnormal genes, yet most do not trigger disease, as they are recessive or remain unexpressed.
It is estimated that approximately 1% of people in the general population have a monogenic disorder. Single-gene disorders collectively affect millions of people.
Key distinction from other genetic disorders
Unlike chromosomal disorders (e.g., Down syndrome), stemming from alterations in whole-chromosome number or structure, single-gene diseases arise from defects within a single specific gene.
They also differ from multifactorial disorders, which develop from the interplay of genetic mutations and environmental triggers, including diet, chemical exposure, and lifestyle.
Why Do Single-Gene Diseases Matter for Family Planning?
Many people are unaware they are carriers of a single-gene mutation. Understanding these disorders is particularly important for family planning because of the inheritance risks they pose.
How Are Single-Gene Diseases Inherited?
Single-gene disorders follow predictable patterns of inheritance, often referred to as Mendelian inheritance. The three main types are:
1. Autosomal recessive conditions: When both parents are carriers of the same autosomal recessive condition, each child has a 25% chance of being affected and a 50% chance of being a carrier (Mayo Clinic).
2. Autosomal dominant conditions: When one parent has an autosomal dominant condition, each child has a 50% chance of inheriting the condition (MedlinePlus).
3. X-linked conditions: Carrier mothers have a 50% chance of passing the affected gene to their sons (MedlinePlus, NHS).
4. Some single-gene diseases occur as new (sporadic) mutations in a child when neither parent carries the abnormal gene; these are called de novo mutations.
A family health history is a valuable tool for understanding how conditions are passed down through generations.
If you have a family history of genetic disease, such as Huntington’s disease, cystic fibrosis, or BRCA-related cancers, genetic testing and carrier screening can help identify inheritance risks before pregnancy.
How Can Single-Gene Diseases Be Detected Before Pregnancy?
For couples at risk of passing on a single-gene disorder, Preimplantation Genetic Testing for Monogenic Disorders (PGT-M) offers a powerful solution. PGT-M is a specialized genetic test that screens embryos created through IVF for specific single-gene disorders before transfer.
Who is PGT-M for?
By testing embryos before pregnancy begins, PGT-M helps identify which embryos are unaffected by that condition, allowing families to move forward with greater clarity and confidence. PGT-M may be recommended if you or your partner:
1. Individuals with a personal or family history of a single-gene disorder;
2. Couples who are both carriers of the same recessive condition (e.g., cystic fibrosis, sickle cell anemia);
3. Individuals who are carriers of or affected with an X-linked condition (e.g., Fragile X syndrome, Duchenne muscular dystrophy);
4. Couples who have had a previous child with a genetic condition;
5. Couples who have experienced recurrent pregnancy loss due to genetic causes.
6. Have an autosomal dominant condition (e.g., Huntington's disease) or carry a mutation associated with hereditary cancer risk (e.g., BRCA1/2)
How PGT-M works:
How PGT-M testing can lower the chance of passing genetic illness to your child? Because each genetic mutation is unique, every PGT-M test is custom-designed. The PGT-M process involves several steps:
|
Step |
What Happens |
Why It Matters |
|
1. Pre-PGT Workup (Before IVF Begins) |
Genetic counseling and DNA analysisof both partners |
To identify the specific mutation and design a personalized test |
|
2. IVF Cycle |
Eggs are retrieved and fertilized in the lab to create embryos |
PGT-M requires embryos; IVF is the only way to obtain them for testing |
|
3. Embryo Biopsy |
5-10 cells are removed from the blastocyst (Day 5–6 embryo) |
Provides the cell sample needed for genetic analysis without harming the embryo |
|
4. Genetic Testing |
Biopsied cells are analyzed for the specific gene mutation; results typically take 4 weeks |
Identifies which embryos carry the genetic mutation |
|
5. Embryo Freezing |
Embryos are frozen and stored while awaiting test results |
Preserves embryos so they are ready for transfer once results are available |
|
6. Embryo Selection |
Unaffected embryos are prioritized for transfer; Carrier embryos may also be discussed with a genetic counselor |
Allows couples to select embryos free from the genetic condition |
Important: Because each PGT-M test is custom-built, the entire process, from genetic workup to receiving results, can take several weeks to months. Couples should plan accordingly and discuss timelines with the fertility team.
Combined testing and success rate
PGT-M can be performed alongside PGT-A (screening for chromosomal abnormalities) on the same embryo biopsy sample, providing comprehensive genetic information. Though it's important to note that patients undergoing both tests will ultimately have fewer embryos remaining for transfer after testing.
A 2025 study of 572 PGT-M IVF cycles reported clinical pregnancy and live birth rates per embryo transfer of 51.3% and 44.8%. NHS data shows that one in three cycles of IVF with PGT-M will result in a baby.
Common Examples of Single-Gene Diseases
What are single-gene diseases? Single-gene diseases include many well-known conditions. The table below shows some of the most common examples (NIH).
|
Disease |
Inheritance Pattern |
Key Feature |
|
Cystic fibrosis |
Autosomal recessive |
Affects lungs, pancreas, and digestive system; causes thick, sticky mucus |
|
Sickle cell anemia |
Red blood cells become sickle-shaped, causing pain and organ damage |
|
|
Tay-Sachs disease |
Progressive neurological deterioration; most common in certain populations |
|
|
Spinal muscular atrophy (SMA) |
Progressive muscle wasting and weakness |
|
|
Huntington‘s disease |
Autosomal dominant |
Progressive neurological degeneration; symptoms typically appear in mid-adulthood |
|
Marfan syndrome |
Affects connective tissue, skeleton, heart, and eyes |
|
|
Fragile X syndrome |
X-linked |
Intellectual disability and developmental delay |
|
Duchenne muscular dystrophy |
X-linked recessive |
Progressive muscle weakness; primarily affects males |
|
Hemophilia A |
Blood clotting disorder; primarily affects males |
Mutations in approximately 4,000 genes have been identified as causal for thousands of rare genetic diseases. Rare genetic disorders, which include many single-gene diseases, affect fewer than 200,000 people in the U.S..
Conclusion
What are single-gene diseases? These conditions stem from mutations within a single gene and exhibit clear, predictable inheritance patterns. Understanding these patterns is essential for anyone with a family history of genetic conditions or those planning a family.
Pre-pregnancy carrier screening (like PGT-M) uncovers underlying risks, supporting couples to make well-informed reproductive decisions.
If you have a family history of genetic disease or worry about potential carrier status, consult a genetic counselor or fertility specialist. Recognizing your genetic risk is the first step toward making informed choices for your family’s future.
FAQs
1. What is a single-gene disease in simple terms?
A single-gene disease is a health condition caused by a change (mutation) in just one gene. These disorders follow predictable inheritance patterns and can be passed from parents to children.
2. What is the difference between PGT-A and PGT-M?
PGT-A screens embryos for chromosomal abnormalities (aneuploidy), while PGT-M screens for specific single-gene disorders inherited from one or both parents. They can be performed together on the same embryo biopsy.
3. Can single-gene diseases be cured?
Most single-gene diseases cannot be cured, but many can be managed with treatment. Advances in gene therapy are offering new hope for some conditions. Prevention through genetic screening and PGT-M is currently the most effective strategy for at-risk couples.
This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider for personalized recommendations.
