Does an av canal defect cause downs syndrome?

No. Av canal does not cause down syndrome, but av canal is one of the abnormalities that make up Down syndrome (a syndrome is a group of abnormalities that appear together in affected persons). The cause of Down syndrome is the presence of an extra chromosome #21.
No. Down syndrome is due to chromosomal abnormality where there is an extra copy of chromosome 21, and is also known as trisomy 21. Many children with Down syndrome have heart defects and the av canal defect is among the most common.