Is Down syndrome an autosomal dominant gene? Please advise!

Not autosomal. Down syndrome happens when an extra 21 chromosome is present in the cells of the body. It happens before conception when the donor egg fails to shed one of its 21 chromosomes in the formation of the egg & the male 21 joins the female pair & creates an embryo with 3 number 21's. It has nothing to do with autosomal traits or mutations in the gene code.