<Li> In a small percentage of cases , Down syndrome is caused by a rearrangement of chromosomal material between chromosome 21 and another chromosome . As a result , a person has the usual two copies of chromosome 21 , plus extra material from chromosome 21 attached to another chromosome . These cases are called translocation Down syndrome . Researchers believe that extra copies of genes on chromosome 21 disrupt the course of normal development , causing the characteristic features of Down syndrome and the increased risk of medical problems associated with this disorder . </Li>
where does the extra chromosome 21 come from
a rearrangement of chromosomal material between chromosome 21 and another chromosome