There are three main typesâ€™ chromosomal anomalies seen in Down Syndrome.
It is important to recognise the different patterns to provide accurate genetic counselling, although this does not influence the severity of the phenotypic manifestation.
Standard or Regular Trisomy 21: All cells have an extra chromosome 21. This is the most common type in 95% of cases. 90% of these cases are attributed to errors in meiosis 1 during oogenesis and 4% are attributed to paternally derived meiotic non-dysjunction. The recurrence risk is low.
Translocation accounts for 4% of cases. This occurs when an extra small arm of chromosome 21 is translocated on an another chromosome, most commonly 13,14, 15, 22. In most cases this is a de novo occurrence. However in third of cases either parent could be a carrier of the balanced translocation. If the translocation is maternally derived, the risk of recurrence is 1:6 and 1:20 if, paternally derived.
Mosaicism This usually occurs post conception mitotic dysjunction. Hence not all cell lines have 21.The phenotype may be milder. The recurrence risk is low.
There have been significant advances and emphasis on the sequencing of the 310Â genes on chromosome 21 to gain a greater understanding in the variation in the manifestation of the syndrome. This may result in providing an evidenced base for effective therapy for example the current well recognised genes are GATA1 gene associated with transient myeloproliferative disorder and megakaryoblastic leukemia and JAK2 gene associated with acute lymphoblastic leukemia.
Presentations at DSMIG Meetings
Materials from meetings are available for members only who need to log in to access them. For details on how to become a member click here.
Book Chapter -Â Advances in medical genetics Â Arjan Bowman and Raoul Hennekam
UK National Down Syndrome Cytogenetic Register
Unique information from the largest existing DS births database. Currently over 14,000 anonymous registrations. At least 95% ascertainment. Up to date information about uptake and outcome of prenatal diagnosis, cytogenetics and a range of demographic factors.
OMIM (Online Mendelian Inheritance in Man)
Site from the National Institutes of Health that gives very comprehensive genetic information. Search the OMIM database for â€œDown syndromeâ€. Access ref #190685.
Â Down syndrome: searching for the genetic culprits.
Lana-Elola E1,Â Watson-Scales SD,Â Fisher EM,Â Tybulewicz VL.
Dis Model Mech.Â 2011 Sep;4(5):586-95. doi: 10.1242/dmm.008078