English English Wikipedia English Chr. 21 English English English English chromosome 21 English English English English English English 21st chromosome chromosome 21 English 21st chromosome chromosome 21 Wikipedia chromosome 21 English English English English English English Wikipedia chromosome 21 English chromosome 21 English English English Wikipedia English chromosome 21 Chr. 21 English English English English English Chr. 21 Chr. 21 English English English English English كروموزوم شماره ۲۱ Wikipedia English English English Wikipedia English English English English English