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There are three types of Down’s syndrome: trisomy 21, translocation and mosaicism. Each arises through a different chromosomal mechanism. This guide explains what each type is, how common it is and whether the type affects the presentation of Down’s syndrome.
Down’s syndrome is categorised into three types based on the chromosomal mechanism that causes it. The three types are trisomy 21, translocation Down’s syndrome and mosaic Down’s syndrome. All three types result in extra chromosome 21 material being present in the person’s cells, which causes the characteristic features of Down’s syndrome. The vast majority of people with Down’s syndrome have trisomy 21. The type of Down’s syndrome is determined by karyotype analysis and has implications for genetic counselling and recurrence risk.
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Trisomy 21 is the most common form of Down’s syndrome, accounting for approximately 94 percent of all cases. In trisomy 21, every cell in the body has three complete copies of chromosome 21 rather than the usual two. This arises from a process called non-disjunction during the formation of an egg or sperm cell, in which chromosome 21 fails to separate correctly, resulting in a reproductive cell with two copies rather than one. When this cell is fertilised, the resulting embryo has 47 chromosomes in every cell.
Translocation Down’s syndrome accounts for approximately 4 percent of cases. In this type, the extra chromosome 21 material is not present as a separate chromosome but is attached to another chromosome, most commonly chromosome 14. The total count of chromosomes in the cell may be 46 but the extra chromosome 21 material is present as part of another chromosome. Translocation can occur as a random event or it can be inherited from a parent who carries a balanced translocation. Karyotype analysis is needed to distinguish translocation from trisomy 21.
Mosaic Down’s syndrome accounts for approximately 2 percent of cases. In mosaicism, only some cells in the body have the extra chromosome 21: others have the typical 46 chromosomes. The mosaic pattern arises from an error in cell division that occurs after fertilisation, during the early stages of embryonic development. The proportion of cells affected and where in the body those cells are located varies between individuals. Some people with mosaic Down’s syndrome have a slightly milder presentation of features, though this varies considerably and cannot be predicted from the percentage of affected cells alone.
For most practical purposes, the three types of Down’s syndrome produce broadly similar characteristics and abilities. The learning disability, physical features and health considerations associated with Down’s syndrome are present in all three types. Mosaic Down’s syndrome may sometimes be associated with a slightly milder presentation, but this varies considerably and the type alone cannot predict an individual’s abilities, health needs or potential. Each person with Down’s syndrome is an individual, regardless of type.
The type of Down’s syndrome has significant implications for recurrence risk in future pregnancies. Trisomy 21 carries a recurrence risk of approximately 1 percent above the age-related risk. Mosaic Down’s syndrome has a low recurrence risk. Translocation Down’s syndrome may carry a significantly higher recurrence risk if one parent carries a balanced translocation, and genetic testing of both parents is recommended. The recurrence risk figure specific to each family requires genetic counselling to calculate accurately.
The type of Down’s syndrome is determined by karyotype analysis, a chromosomal test that analyses the chromosomes in a blood or tissue sample from the baby. The karyotype shows the number and arrangement of chromosomes, allowing the laboratory to identify whether the extra chromosome 21 is present as a separate chromosome (trisomy 21), attached to another chromosome (translocation) or present in only some cells (mosaicism). This test is carried out routinely following all new diagnoses of Down’s syndrome.
For related information see our articles on Causes of Down’s Syndrome and Down’s Syndrome and Genetics.
The three types are trisomy 21, where every cell has three copies of chromosome 21 and which accounts for about 94 percent of cases; translocation Down’s syndrome, where extra chromosome 21 material is attached to another chromosome and which accounts for around 4 percent of cases; and mosaic Down’s syndrome, where only some cells contain the extra chromosome and which accounts for around 2 percent of cases.
All three types produce broadly similar characteristics and the type alone cannot reliably predict an individual’s abilities or health needs. Mosaic Down’s syndrome may sometimes present slightly less strongly in terms of some features, but this varies considerably between individuals. The type is most important for its implications for genetic recurrence risk and for the genetic counselling advice that should be given to the family.
The type is determined by karyotype analysis, a chromosomal test on a blood sample. Following a diagnosis of Down’s syndrome, whether at birth or prenatally, karyotype analysis is routinely carried out. The results will identify the chromosomal mechanism and allow the clinical team to advise on recurrence risk. Where translocation is identified, genetic testing of both parents is recommended.
Visit our Understanding Down’s Syndrome hub for more guides on causes, diagnosis, education, health conditions and independent living.
The information in this article is provided for educational purposes only and is not intended as medical advice. If you have concerns about Down’s syndrome or any health condition, speak to a qualified healthcare professional. SENDhelp Education Limited accepts no responsibility or liability for any loss or damage arising from reliance on this content. Any links to third-party websites are provided for convenience only and do not constitute endorsement of their content.
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