An extra copy of chromosome 21 causes Down syndrome, the most common genetic cause of intellectual disability.1PubMed Central. Classification of human chromosome 21 gene-expression variations in Down syndrome: impact on disease phenotypes The effects reach almost every part of the body, from how the brain wires itself to how the heart forms and how the immune system behaves. It is not all added risk, though, because the same extra genes seem to protect against most solid cancers.2PubMed Central. Ten Reasons Why People With Down Syndrome are Protected From the Development of Most Solid Tumors -A Review The effects vary widely from person to person, and support that starts in infancy changes how a child develops.3Physical Therapy. Evaluation of Children with Down Syndrome Who Participated in an Early Intervention Program: Second Follow-up Study
Why One Extra Chromosome Changes So Much
Chromosomes carry genes, the instructions cells use to make proteins, and most people have two copies of each chromosome. Doctors call having three copies of chromosome 21 “trisomy 21,” a term you may see on test reports. The features of Down syndrome come from genes on chromosome 21 being read more often because there are more copies of them.4PubMed Central. Gene-dosage effects in Down syndrome and trisomic mouse models
Not every extra gene copy is overactive. Only about 3 in 10 active chromosome 21 genes are actually overactive in Down syndrome.5PubMed Central. Classification of human chromosome 21 gene-expression variations in Down syndrome: impact on disease phenotypes The body evens out the output of more than half of them back toward normal levels,6PubMed Central. Classification of human chromosome 21 gene-expression variations in Down syndrome: impact on disease phenotypes which is why the effects are not 50% stronger everywhere.
A few overactive genes act on the brain, others on the heart, others on blood cells, so many different effects arise at once. Because genes act in different tissues and at different stages of development, some effects appear before birth, such as those in the heart, while others show up in childhood or adulthood.
The effects also spread beyond chromosome 21. The extra chromosome changes chemical tags that switch genes on and off on every other chromosome.7PubMed Central. Distinct DNA methylation patterns of cognitive impairment and trisomy 21 in down syndrome Cells in people with Down syndrome show strained energy production and about three times the normal level of a damaging oxygen by-product.8PubMed Central. Adaptive Downregulation of Mitochondrial Function in Down Syndrome
Everyone with Down syndrome has the same extra chromosome, but the rest of their genes differ. Those background genes change which effects show up and how strongly, so one child may have a heart defect and another may not.
How It Affects Learning, Movement and Appearance
Intellectual disability is the most consistent feature of Down syndrome.9PubMed Central. Trans effects of chromosome aneuploidies on DNA methylation patterns in human Down syndrome and mouse models Brain connections strengthen less easily with experience, which is a key part of how memories form.10PubMed Central. From Abnormal Hippocampal Synaptic Plasticity in Down Syndrome Mouse Models to Cognitive Disability in Down Syndrome In animal studies, too much of the brain’s calming signal appears to hold back that strengthening.11PubMed Central. The GABAergic Hypothesis for Cognitive Disabilities in Down Syndrome
The disability is usually mild to moderate and varies widely. Many people with Down syndrome read, work and live semi-independently.
In school-age children, learning often goes best through seeing and doing. Pictures, routines and demonstration usually work better than spoken instructions alone. Short-term verbal memory is often a weak spot, so a long spoken instruction may be lost while a visual schedule works. Progress tends to come in steps, with repetition helping skills stick.
Movement is affected too. The brain area that coordinates movement is smaller in infants, children and adults with Down syndrome.12PubMed Central. Cerebellar Calcium-Binding Protein and Neurotrophin Receptor Defects in Down Syndrome and Alzheimer’s Disease That smaller size is thought to contribute to trouble with fine movement control.13PubMed. Down syndrome gene dosage imbalance on cerebellum development Muscle tone is often low, so sitting and walking come later than usual. A baby may feel floppy when held and take longer to hold up their head. Children usually reach milestones in the same order as other children, only later. Speech often lags behind understanding, so a child may understand far more than they can say.
Typical facial and skull features include a flatter face, a shorter, broader head and a smaller jaw.14PubMed Central. Down syndrome: searching for the genetic culprits They arise from several extra genes working together, not from one gene.15PubMed Central. Down syndrome: searching for the genetic culprits
Heart Defects at Birth
About half of newborns with Down syndrome have some kind of heart defect.16PubMed Central. Multidimensional definition of the interferonopathy of Down syndrome and its response to JAK inhibition About 3 in 4 of these defects are holes in the walls that separate the heart’s chambers.17PubMed Central. Cardiovascular Complications of Down Syndrome: Scoping Review and Expert Consensus The center of the heart, where the walls and valves form, develops abnormally before birth.18Journal of Medical Science at Texas A&M Health Science Center. Genetic and Mechanistic Drivers of Atrioventricular Septal Defects in Down Syndrome
A hole between chambers lets blood flow the wrong way, so the heart and lungs handle extra blood. Over time, that extra flow can strain the lungs, which is why larger holes are repaired early. Babies with a significant defect may tire during feeding, breathe fast or grow slowly, which is how doctors and parents notice a problem.
The extra chromosome alone is not enough to cause a defect, because other genes help decide which babies are affected.19Journal of Medical Science at Texas A&M Health Science Center. Genetic and Mechanistic Drivers of Atrioventricular Septal Defects in Down Syndrome Extra copies of immune-signal receptor genes disrupt heart development in cell and mouse studies.20Journal of Medical Science at Texas A&M Health Science Center. Genetic and Mechanistic Drivers of Atrioventricular Septal Defects in Down Syndrome
Babies with Down syndrome are routinely given a heart ultrasound, either before birth or soon after. Some holes close on their own, while larger ones need surgery, often in the first months. Repaired hearts usually allow normal activity and growth afterward.
An Immune System Stuck on Alert
Chromosome 21 holds four of the six receptor genes for interferon, a key alarm signal in the immune system.21PubMed Central. Down Syndrome and Autoimmune Disease So people with Down syndrome have a constantly raised interferon response,22PubMed Central. Down Syndrome and Autoimmune Disease which can make the immune system more likely to turn on the body’s own tissues.23PubMed Central. Down Syndrome and Autoimmune Disease
Interferon normally rises during an infection, then settles down once the threat has passed. When it stays elevated, the body acts as if it were always under attack, which helps explain why autoimmune conditions appear more often and sometimes at younger ages.
This raises the chances of thyroid disease, celiac disease, type 1 diabetes and some skin conditions.24PubMed Central. Down Syndrome and Autoimmune Disease Stronger interferon activity is linked to a history of both heart defects and autoimmune thyroid disease.25PubMed Central. Multidimensional definition of the interferonopathy of Down syndrome and its response to JAK inhibition Gut bacteria are often less varied and more inflammatory, which may add to bowel inflammation.26PubMed Central. Endocrine and Digestive Disorders Arising in Childhood in Down Syndrome and Their Cross-Talk
Some signs are easy to miss. An underactive thyroid can look like tiredness, weight gain or slower progress. Celiac disease may show as poor growth, bloating or diarrhea. Regular blood tests for thyroid function are standard throughout life.
Higher Leukemia Risk but Fewer Solid Cancers
The extra chromosome changes gene switches in blood-forming cells early in life, which may explain frequent blood problems.27PubMed Central. The genome-wide impact of trisomy 21 on DNA methylation and its implications for hematopoiesis Some newborns develop a temporary surplus of immature blood cells, which almost always carries a change in one particular gene.28PubMed. Analysis of GATA1 mutations in Down syndrome transient myeloproliferative disorder and myeloid leukemia The same gene change is found in a leukemia that can follow, and testing for it cannot predict which babies will go on to develop leukemia.29PubMed. Analysis of GATA1 mutations in Down syndrome transient myeloproliferative disorder and myeloid leukemia
For families, this newborn condition is often found on a routine blood count in the first weeks. It usually clears up without treatment within months. Because leukemia can follow, children get regular blood checks for the first few years.
A few cells carrying the gene change can linger and later pick up further changes, which is how leukemia can follow in some children. Most babies with the temporary condition never develop leukemia.
Warning signs of leukemia include unusual paleness, bruising or bleeding easily, repeated fevers and tiredness that is out of the ordinary. These have many ordinary causes, so they call for a blood test, not alarm. Childhood leukemia in Down syndrome is treated, and doctors adapt the treatment because these children can be more sensitive to chemotherapy side effects.
Solid tumors go the other way. People with Down syndrome have higher blood levels of a natural blocker of new blood vessel growth.30PubMed. High serum endostatin levels in Down syndrome: implications for improved treatment and prevention of solid tumours Tumors need new blood vessels to grow, so this may help explain why many solid cancers are less common.31PubMed. High serum endostatin levels in Down syndrome: implications for improved treatment and prevention of solid tumours The same effect matches lower rates of diabetic eye disease and hardened arteries.32PubMed Central. Ten Reasons Why People With Down Syndrome are Protected From the Development of Most Solid Tumors -A Review
Alzheimer’s Disease in Later Life
Chromosome 21 carries the APP gene, the source of amyloid, the sticky protein that clumps in the brains of people with Alzheimer’s.33PubMed. Chromosome 21 gene dosage and Alzheimer’s disease in Down syndrome: DYRK1A mechanism and its translation to blood-based biomarkers Having three copies is the main driver of the heavy amyloid buildup in people with Down syndrome.34PubMed. Chromosome 21 gene dosage and Alzheimer’s disease in Down syndrome: DYRK1A mechanism and its translation to blood-based biomarkers Lab-grown nerve cells with the extra chromosome produce more amyloid even at an early stage.35PubMed Central. Amyloid precursor protein dosage normalization rescues neurogenesis and Alzheimer’s Disease phenotypes associated with Down Syndrome Switching off one APP copy brought amyloid back to normal levels in those cells.36PubMed Central. The Impact of APP on Alzheimer-like Pathogenesis and Gene Expression in Down Syndrome iPSC-Derived Neurons So Alzheimer’s often begins earlier in adults with Down syndrome.37PubMed Central. Trans effects of chromosome aneuploidies on DNA methylation patterns in human Down syndrome and mouse models The sticky protein builds up quietly for years, and brain changes can appear well before anyone notices a change in thinking or behavior.
The first signs often look like personality shifts or the loss of daily skills rather than forgetfulness. New seizures in an adult can be an early sign. A baseline record of skills in early adulthood helps doctors spot change later.
Why the Extra Chromosome Happens
Most cases happen when the chromosome 21 pair fails to separate as an egg or sperm forms.38PubMed. Risk factors for nondisjunction of trisomy 21 For errors in the egg, the main risk factors are the mother’s age and how the chromosome pair exchanged pieces.39PubMed. Risk factors for nondisjunction of trisomy 21 In younger mothers, the way the pair crossed over is the bigger risk, while in older mothers wear on the eggs seems more important.40PubMed Central. Association between maternal age and meiotic recombination for trisomy 21
Sometimes extra chromosome 21 material is attached to another chromosome, which is called a translocation.41PubMed. A jumping Robertsonian translocation: a molecular and cytogenetic study A healthy parent can carry such a rearrangement and pass Down syndrome to children.42PubMed. A jumping Robertsonian translocation: a molecular and cytogenetic study
In mosaic Down syndrome, only some cells carry the extra chromosome. It often arises after conception, and features may be milder depending on which cells are affected.
Nothing a parent did during pregnancy causes the extra chromosome. When a translocation is found, the parents’ chromosomes are worth testing, and a genetic counselor can estimate the chance of recurrence.
Finding It Before Birth
NIPT is a blood test in pregnancy that looks at fetal DNA in the mother’s blood.43PubMed Central. A Case of False Negative NIPT for Down Syndrome-Lessons Learned Despite advertised accuracy above 99%, it is still a screen, and both false positives and false negatives happen.44PubMed Central. A Case of False Negative NIPT for Down Syndrome-Lessons Learned Confirm any result with a diagnostic test before making decisions.
The chance that a positive result is correct depends on how common the condition is. When it is rare in a group, such as younger mothers, a larger share of positives are false alarms. That is why the positive predictive value, not the headline accuracy, tells you what a positive screen means.
Two diagnostic tests can settle it. Chorionic villus sampling (CVS) takes placental cells, usually around 10 to 13 weeks. Amniocentesis samples the fluid around the baby, usually after 15 weeks. Both count the chromosomes directly and carry a small risk of miscarriage.
After a high-risk screen, ask what the positive predictive value is for your age, what the timing and risks of confirmation are, and for a referral to genetic counseling.
What Helps Development
Children in early intervention programs scored higher on thinking and daily-living skills and avoided the decline usually seen with age.45Physical Therapy. Evaluation of Children with Down Syndrome Who Participated in an Early Intervention Program: Second Follow-up Study Physical therapy started before age one gave better movement scores than therapy started later.46PubMed Central. Effects of early physical therapy on motor development in children with Down syndrome Infants given treadmill practice walked independently about 100 days sooner.47PubMed. Treadmill training of infants with Down syndrome: evidence-based developmental outcomes Ask about referral to early intervention soon after birth.
These programs usually involve physical, speech and occupational therapists who visit the home or a center. Parents learn exercises to practice daily. In many countries the programs are free from birth to age three.
Early practice of skills such as sitting, grasping and babbling builds the base for later ones. Starting later still helps, though starting early tends to help more.
Treatments are also being explored. Drugs that block the overactive DYRK1A gene are being studied to ease thinking difficulties.48PubMed Central. DYRK1A, a Dosage-Sensitive Gene Involved in Neurodevelopmental Disorders, Is a Target for Drug Development in Down Syndrome In lab-grown cells, a gene called XIST can silence the extra chromosome and correct blood-cell problems.49PubMed Central. Trisomy silencing by XIST normalizes Down syndrome cell pathogenesis demonstrated for hematopoietic defects in vitro This work is still in cells, not people.
Sleep and Breathing
Children with Down syndrome have narrower throats,50PubMed. Obstructive sleep apnea in children with Down syndrome: contribution of upper airway and chemosensitivity and their bodies respond less strongly to low oxygen levels.51PubMed. Obstructive sleep apnea in children with Down syndrome: contribution of upper airway and chemosensitivity So breathing is often obstructed during sleep.
Snoring, pauses in breathing and restless sleep are warning signs, and daytime tiredness or behavior changes can follow. An overnight sleep study confirms it, and surgery or a breathing machine can help.
It can go unnoticed because a child may seem to sleep through the night even though breathing is repeatedly interrupted. It can be present even in a thin child, since airway shape, not weight, is a main cause in Down syndrome.