Down syndrome is one of the most frequently occurring birth defects in the world. It is a chromosomal abnormality which results in 47 chromosomes, instead of 46, with an extra chromosome being 21. Because the majority of Down syndrome cases affect the twenty-first chromosome, it is commonly called "trisomy 21." Down syndrome is usually caused by a cell division error, which is known as non-disjunction. This occurs when an embryo develops with three number 21 chromosomes instead of two. This excess genetic material alters the course of dev ....