|Type of Down syndrome||Parents' chromosomes||Chance of recurrence|
|Trisomy 21||Normal||1 in 100 if mother < 40. If mother > 40, twice usual chance for her age (see Table 1)|
|Translocation with chromosome 13, 14, 15, or 22||Normal||Usual chance for mother's age (see Table 1)|
|Mother a carrier
Father a carrier
|1 in 8
1 in 40
|Translocation with another chromosome 21||Normal||Usual chance for mother's age (see Table 1)|
|Either parent a carrier||100%|
|Mosaicism||Normal||Unknown, probably the usual chance for mother's age (see Table 1)|
|Young mother with previous Trisomy 21 live birth, miscarriage or stillbirth||About 1%|
|Occurance of Trisomy 21 in 2nd- or 3rd-degree relative; maternal age < 35||Somewhat increased but still less than 1%|
|Rare families with two or more cases of Trisomy 21||Risk markedly increased for 1st- and 2nd-degree relatives|
|Mother with 21/13, 21/14 or 21/15 translocation||About 15%|
|Father with 21/13, 21/14 or 21/15 translocation||About 5%|
|Mother with 21/22 translocation||About 10%|
|Father with 21/22 translocation||About 12%|
|Either parent with 21/21 translocation||100%|
|1. Hook E & Chambers GM Birth Defects, 13 (3a): 123, 1997.||Revised: December 18, 2007.|
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3. Down syndrome prevalence at birthUnited States, 1983-1990.. Teratology 1997;56:31-6.
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13. Cited in Tuck CM, Bennett JW, Varela M. 1984. Down's Syndrome and Familial Aneuploidy in Berg JM (Ed.) Perspectives and Progress in Mental Retardation Vol. IIBiomedical Aspects, p. 168. Baltimore: University Park Press.
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Compiled by Victor J. Bishop