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Updated: Jun 30, 2023

Genetic disorders that are caused by three copies of one chromosome being present in one or more cells, instead of the usual two copies.

This article is pending medical review.


Written by Sophie Oppelt

Reviewed by Alizeh Ahsan and Julian Zeegers

Edited by Juliëtte Gossens


All of our genetic information is stored in our genes as part of our DNA, which coils up to form separate chromosomes. A healthy human being has 23 pairs of chromosomes in every cell. Each healthy cell therefore contains 46 chromosomes in total. The last pair of chromosomes, pair 23, contains the sex chromosomes. These are usually XX in females and XY in males, but they may be a little bit different in some people. Chromosome pairs 1 to 22 are called the autosomal chromosomes, which just means that they are not the sex chromosomes. The structure of the autosomal chromosomes is the same across the sexes.

During the production of new cells (cell division), the mechanism of correctly pairing the new chromosomes or chromatids is not flawless. This can lead to chromosomal abnormalities and aneuploidies, which are instances of chromosome sets with more or less than 46 total chromosomes. In this article, we’ll be focusing on a specific type of aneuploidy called trisomy. Trisomies occur when a chromosome is present three times in each affected cell instead of the normal count of two.

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We can distinguish between full and partial trisomies. A full trisomy is characterised by an entire extra chromosome being present in addition to the normal pair. Three complete copies of the chromosome are then present in each cell. In a partial trisomy, only an extra piece of the chromosome is present in each cell. There will then be two complete copies of the chromosome, plus a little bit. (1, 2)

Because more of the total DNA is affected in full trisomies compared to partial trisomies, they have a bigger effect on the health and development of the baby. The symptoms and effects of partial trisomies depend on their size and location, but they are often not lethal. The exact symptoms depend very much on the specific part of chromosome involved.

Autosomal Trisomies

There are only three full autosomal trisomies, with which the baby can develop further and survive birth. These are trisomies of chromosome 13 (Patau syndrome), 18 (Edwards syndrome) or 21 (Down syndrome). Full trisomies of any other chromosomes usually result in a spontaneous miscarriage. (2, 3)

Patau syndrome - Trisomy 13

Patau syndrome, which is defined by a trisomy on chromosome 13, brings many physical and intellectual disabilities with it. Often, the baby suffers from brain and spinal cord defects, muscle weakness and heart issues. Lip clefts and additional fingers and toes are also common symptoms. Most babies with Patau syndrome only survive the first couple of days, and only 5-10% survive their first year of life. This syndrome occurs in 2 per 10,000 births (or 0.02%). (3, 4)

Edwards syndrome - Trisomy 18

Edwards syndrome is caused by a chromosome 18 trisomy and occurs in 5 per 10,000 births (or 0.05%). This aneuploidy is characterised by underdeveloped bodies during and after pregnancy. This leads to a different structure of internal organs like the heart, lungs and kidneys, which also impairs their function. Furthermore, trisomy 18 babies tend to have a smaller-sized head and suffer from muscle weakness, which causes a weaker cry and less movement of the baby. Less than 10% of babies with Edwards syndrome will survive for longer than a year. (3, 4)

Down syndrome – Trisomy 21

Down syndrome, the trisomy of chromosome 21, is the most common autosomal trisomy. It occurs in 22 per 10,000 births (or 0.22%). Babies with Down syndrome are known for their flat facial features with a shorter neck and a specific eye shape. Affected children may be slower to learn, caused in part by a very short attention span, and they may have poor judgement. People affected by trisomy 21 are often more prone to medical conditions, such as heart defects, hearing loss, leukemia, dementia, and obesity.

Trisomy 21 can manifest itself in many different ways. Many people with Down syndrome live flourishing, fulfilling lives. Others deal with severe health issues and need constant intensive support from loved ones and healthcare professionals. Fortunately, the life expectancy for people with Down syndrome has risen over the past few years to up to 60 years. (3, 4)

Sex Chromosome Trisomies

It’s also possible for trisomies to occur on the sex chromosomes. Most of the time, these trisomies will only affect the ability to reproduce. They will rarely cause other severe developmental impairments. The most common sex chromosome trisomies are XXX (Triple-X-syndrome), XXY (Klinefelter syndrome) and XYY- syndrome.

Triple-X-syndrome occurs in females. It can easily go unnoticed and is often not diagnosed, as it mainly affects reproduction. However, it can also be related to autoimmune issues, very mild intellectual impairments, or an abnormal heart structure.

Klinefelter syndrome occurs in males. This trisomy is also often not diagnosed until adult age due to the accompanying infertility problems. Klinefelter syndrome is characterised by small testicles and penis, low sperm count, low sex drive, and weaker bones. Affected people may also be less muscular compared to other males.

Above average height, difficulties with motor skill development, and learning disabilities are symptoms of XYY-syndrome. This syndrome also affects males.

In general, sex chromosome trisomies don’t bring severe impairments with them and are often not diagnosed or discovered at all. Most affected people can live a normal life. Having biological children may be more difficult, however. (6, 7)

Some trisomies show distinct features, like trisomy 21, which can be observed in an ultrasound examination. If your doctor notices such features, they may advise that you undergo more accurate testing, for example non-invasive prenatal testing (NIPT) (which you can read more about here) or invasive testing methods like amniocentesis or chorionic villus sampling (3, 5). Invasive testing methods are explained in more detail in the article Invasive prenatal testing methods.



  1. Witters G, Van Robays J, Willekes C, Coumans A, Peeters H, Gyselaers W et al. Trisomy 13, 18, 21, Triploidy and Turner syndrome: the 5T's. Look at the hands. Facts Views Vis Obgyn. 2011;3(1):15-21.

  2. Brenner S, Miller JH (eds.). Encylopedia of Genetics. 1st ed. Philadelphia, USA: Elsevier; 2001.

  3. Medline Plus. Noonan syndrome. Available from: [Accessed July 1st, 2022]

  4. Loane M, Morris JK, Addor MC, Arriola L, Budd J, Doray B et al. Twenty-year trends in the prevalence of Down syndrome and other trisomies in Europe: impact of maternal age and prenatal screening. European Journal of Human Genetics. 2013;21(1):27–33. DOI: 10.1038/ejhg.2012.94

  5. Health Quality Ontario. Noninvasive Prenatal Testing for Trisomies 21, 18, and 13, Sex Chromosome Aneuploidies, and Microdeletions: A Health Technology Assessment. Ont Health Technol Assess Ser. 2019;19(4):1-166.

  6. Skuse D, Printzlau F, Wolstencroft J. Sex chromosome aneuploidies. Handb Clin Neurol. 2018;147:355-376. DOI: 10.1016/B978-0-444-63233-3.00024-5.

  7. Berglund A, Stochholm K, Gravholt CH. The epidemiology of sex chromosome abnormalities. Am J Med Genet C Semin Med Genet. 2020;184(2):202-215. DOI: 10.1002/ajmg.c.31805

Please note: the information we provide to you here is for educational purposes only. If you’re experiencing any discomfort or have any complaints or questions about your health, please contact your doctor or other relevant health professional. We don’t provide medical advice.


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