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Down syndrome or Down's syndrome, [12] also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. [3] It is usually associated with developmental delays, mild to moderate intellectual disability , and characteristic physical features.
An additional differential diagnosis in some cases is Down syndrome. Some cases of tetrasomy X have been described as having "a false air of trisomy 21" (the underlying chromosomal aneuploidy in Down's), and karyotyping resulting in a diagnosis of tetrasomy X has been performed due to suspicion of Down syndrome.
A guide to Down syndrome including symptoms and diagnosis of this genetic condition. ... Mosaic Down syndrome: Only some cells in the person have an extra copy of chromosome 21. This genetic cause ...
An anaphase lag of a chromosome 21 in a Down syndrome embryo leads to a fraction of euploid cells (2n cells), phenomenon described as "aneuploidy rescue". There is considerable variability in the fraction of cells with trisomy 21, both as a whole and tissue-by-tissue. This is the cause of 1–2% of the observed Down syndromes. [4]
Down syndrome and fragile X syndrome are examples of syndromic intellectual disabilities. Intellectual disability affects about 2–3% of the general population. [11] Seventy-five to ninety percent of the affected people have mild intellectual disability. [11] Non-syndromic, or idiopathic cases account for 30–50% of these cases. [11]
"Differential diagnosis" is also used more loosely to refer simply to a list of the most common causes of a given symptom, to a list of disorders similar to a given disorder, or to such lists when they are annotated with advice on how to narrow the list down (French's Index of Differential Diagnosis is an example). Thus, a differential ...
Brushfield spots are a characteristic feature of the chromosomal disorder Down syndrome or trisomy 21. They occur in 35–78% of newborn infants with Down syndrome. [2] Brushfield spots tend to be obscured by pigmentation of the anterior border layer of the iris in patients with darker irides.
When a syndrome is named after a person, there is some difference of opinion as to whether it should take the possessive form or not (e.g. Down syndrome vs. Down's syndrome). North American usage has tended to favor the non-possessive form, while European references often use the possessive. [19]