Chromosomal Basis of Down Syndrome
| Percentage . | Chromosomal Basis . |
|---|---|
| 96 | Meiotic nondisjunction (95% occur in egg, with recurrence risk of 1% until mother’s age risk exceeds 1% at age 40, and it then increases according to maternal age) |
| 3–4 | Translocation (usually occurs with 1 chromosome 21 attached to chromosome 14, 21, or 22) |
| 14/21 translocation (1/3 of patients have a parent carrier with balanced karyotype) | |
| 90% have mother as the carrier parent, with a recurrence chance of 10%–15% | |
| 10% have father as the carrier, with a recurrence chance of 2%–5% | |
| 21/21 translocation (1/14 of patients have parent carrier with a balanced karyotype); carrier parent equally likely mother or father, with recurrence chance of 100%13 | |
| 1–2 | Mosaicism: number of affected cells vary between individuals; clinical findings vary widely |
| Medical complications fewer and intellectual disability often less severe | |
| Partial trisomy: duplication of delimited segment of chromosome 21 present; extremely rare | |
| Percentage . | Chromosomal Basis . |
|---|---|
| 96 | Meiotic nondisjunction (95% occur in egg, with recurrence risk of 1% until mother’s age risk exceeds 1% at age 40, and it then increases according to maternal age) |
| 3–4 | Translocation (usually occurs with 1 chromosome 21 attached to chromosome 14, 21, or 22) |
| 14/21 translocation (1/3 of patients have a parent carrier with balanced karyotype) | |
| 90% have mother as the carrier parent, with a recurrence chance of 10%–15% | |
| 10% have father as the carrier, with a recurrence chance of 2%–5% | |
| 21/21 translocation (1/14 of patients have parent carrier with a balanced karyotype); carrier parent equally likely mother or father, with recurrence chance of 100%13 | |
| 1–2 | Mosaicism: number of affected cells vary between individuals; clinical findings vary widely |
| Medical complications fewer and intellectual disability often less severe | |
| Partial trisomy: duplication of delimited segment of chromosome 21 present; extremely rare | |
Adapted from Bull MJ. Down syndrome.14
Information regarding meiotic nondisjunction and translocation is from Hook,13 information regarding mosaicism is from Papavassiliou et al,15 and information regarding partial trisomy is from Pelleri et al.16