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Down’s syndrome is a genetic condition caused by an extra chromosome 21. In most cases it is not inherited and occurs as a random event. However, translocation Down’s syndrome can sometimes run in families. This guide explains the difference and when genetic counselling is relevant.
Down’s syndrome is a genetic condition, meaning it arises from a change in a person’s genetic material, specifically the presence of an extra copy of chromosome 21. However, being genetic does not mean being hereditary. In the vast majority of cases, Down’s syndrome is not passed from parent to child and does not run in families. The chromosomal error that causes trisomy 21, the most common form, occurs randomly during the formation of an egg or sperm cell and is not related to any genetic change in either parent that could be passed on.
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A genetic condition is one caused by a change in a person’s genes or chromosomes. A hereditary condition is one that is passed from parent to child through the genes. Down’s syndrome is genetic because it involves a chromosomal difference. But in most cases it is not hereditary because the chromosomal error arises freshly in the affected individual rather than being inherited from a parent.
Trisomy 21 accounts for around 94 percent of Down’s syndrome cases and is not inherited. The extra chromosome 21 arises from a random error in cell division during the formation of the egg or sperm, not from any heritable genetic variant in either parent. A parent with Down’s syndrome could potentially pass the condition to their child, but most people with trisomy 21 do not have biological children, and the transmission risk in those who do is complex and requires specialist advice.
Translocation Down’s syndrome, which accounts for around 4 percent of cases, is the exception. In this form, extra chromosome 21 material is attached to another chromosome. Some parents carry a balanced translocation, meaning they have the right amount of genetic material overall but arranged differently. A parent with a balanced translocation has a significantly elevated risk of having a child with translocation Down’s syndrome. The exact risk depends on which parent carries the translocation and which chromosomes are involved.
Mosaic Down’s syndrome occurs due to an error in cell division after fertilisation and is not inherited from a parent. Like trisomy 21, it occurs as a random event. There is no evidence that mosaic Down’s syndrome runs in families, and the recurrence risk for parents of a child with mosaic Down’s syndrome is not significantly elevated above the background age-related risk.
For parents who have had a child with trisomy 21, the recurrence risk in a subsequent pregnancy is estimated at approximately 1 percent above the age-related risk. For parents of a child with translocation Down’s syndrome where genetic testing confirms that one parent carries a balanced translocation, the recurrence risk is considerably higher and genetic counselling is essential to understand the specific figures and the options available.
Genetic counselling is recommended for any family where Down’s syndrome has been diagnosed, particularly where translocation has been confirmed as the chromosomal mechanism. Genetic counsellors can explain the specific chromosomal findings, calculate the recurrence risk for future pregnancies, describe the prenatal testing options available and support the family in understanding the implications of the diagnosis. Referral to a genetic counselling service through the NHS is available and families should ask for this if it has not been offered.
For related information see our articles on Causes of Down’s Syndrome and The 3 Types of Down’s Syndrome.
Yes. Down’s syndrome is a genetic condition caused by the presence of an extra copy of chromosome 21 in a person’s cells. However, in most cases it is not hereditary: the chromosomal error occurs as a random event rather than being passed from parent to child. Translocation Down’s syndrome is the one type that can sometimes be inherited.
For trisomy 21, which accounts for 94 percent of cases, the recurrence risk in a subsequent pregnancy is approximately 1 percent above the age-related risk. For translocation Down’s syndrome where a parent carries a balanced translocation, the recurrence risk is significantly higher. Genetic testing after any Down’s syndrome diagnosis can clarify the chromosomal type and help estimate recurrence risk.
If your child has translocation Down’s syndrome, both parents should be offered chromosomal testing to check whether either carries a balanced translocation. If a balanced translocation is identified in a parent, genetic counselling can explain the specific recurrence risk and the prenatal testing options available in future pregnancies. For trisomy 21, carrier testing is not applicable because the condition arises from a random error rather than a heritable variant.
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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