Down Syndrome: An Overview

Down syndrome occurs when a person has a full or partial extra copy of chromosome 21. The human body is made up of little cells, and in each there is a nucleus or center where genetic material is stored in genes. Genes carry the codes responsible for all our inherited traits such as hair color and height. The chromosomes are the rod-like structures that our genes are grouped upon.

Typically, the nucleus of each cell contains 23 pairs of chromosomes that make 46 in total. Half of these chromosomes are inherited from each parent. Both parents give us distinct features. There are changes in the course of development when there is an additional genetic material of chromosome 21 being added.

A few common physical traits of Down syndrome are low muscle tone, small stature, an upward slant to the eyes, and a single deep crease across the center of the palm. Each person with Down syndrome is unique and may have these characteristics to different degrees, or not at all.

Achievement Center of Texas Down syndrome Center
DSCO1947 by Annika Leigh via CC

Down Syndrome Causes

Regardless of the type of Down syndrome a person may have, the main thing to note is that every person that has the disorder will have an extra chromosome (chromosome 21) in all or some of their cells. The cause of the extra chromosome is still unknown. However, research has shown that the majority of cases are caused by the father.

Different Types of Down Syndrome

Trisomy 21 (Nondisjunction)

An error in cell division called "nondisjunction" causes Down syndrome. Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. Trisomy 21 is this type of Down syndrome. It accounts for 95% of cases.


In translocation, which accounts for about 4% of cases of Down syndrome, the total number of chromosomes in the cells remains 46; however, an additional full or partial copy of chromosome 21 attaches to another chromosome, usually chromosome 14. The presence of the extra full or partial chromosome 21 causes the characteristics of Down syndrome.


Mosaicism (or mosaic Down syndrome) causes a mixture of two types of cells. Some contain the usual 46 chromosomes, and some contain 47.  Those cells with 47 chromosomes contain an extra chromosome 21.

Mosaicism is the least common form of Down syndrome and accounts for only about 1% of all cases of Down syndrome.  Research has indicated that individuals with mosaic Down syndrome may have fewer characteristics of Down syndrome than those with other types of Down syndrome.  However, broad generalizations are not possible due to the wide range of abilities people with Down syndrome possess.

How ACT Helps Those with Down Syndrome

At Achievement Center of Texas, we have many students at our Down syndrome Center. Our Day Habilitation program and our extracurricular classes, including the Arts Exploration and Hip-Hop Dance Classes, help our students to achieve their fullest potential. Not only do they spend quality time with their friends at the center, they learn new skills to take home. Thus, they are able to build confidence and grow to become more independent adults over time.

Services we offer

Sign up for Classes at Our Down Syndrome Center

Achievement Center of Texas has many students with Down syndrome, and we have plenty of space to accommodate all of your individual needs and goals. Click the button below to begin the enrollment process.