Down syndrome is autosomal dominat?

No autosomal. Down syndrome happens when an extra 21 chromosome is present in all 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 traits or mutations in the gene code.
No. No. It is chromosomal. Down syndrome is not usually passed from one generation to the next. So it is not dominant or recessive.