disease arises from structure disease arises from alteration in structure chromosome 21 (Human) syndrome caused by partial chromosomal duplication Down syndrome partial segmental duplication https://github.com/monarch-initiative/mondo/issues/5137 segmental duplication A chromosomal disorder consisting of the partial duplication of chromosome 21. MONDO:0700130 partial trisomy 21 The INCLUDE DCC proposes partial trisomy 21 should be known as partial segmental duplication of chromosome 21, but the term partial trisomy 21 is contradictory because trisomy means three, and there are not three chromosomes. GARD:0026371