Down syndrome is caused by an extra number 21 chromosome. But what does this really mean? To really understand what causes Down syndrome, you need to have a thorough understanding of what chromosomes are, and what they do.
Packages of genetic information
The easiest way to think of chromosomes is as packages of genetic information. The human body is made up of all different types of cells and inside each of these cells are our genes. Genes are the units of inheritance that are passed from one generation to another. They are located in every cell of our bodies and they provide the instructions for every structure and function of our body. Humans have about 25,000 different genes. Rather than having all 25,000 bouncing around inside cells individually, our genes are located or packaged on chromosomes rather like pearls on a necklace.
Genes are made of DNA which is a molecule made up of four different chemicals called bases, which are usually referred to by their initials - A for adenine, T for thymine, G for guanine and C for cytosine. These 4 bases make up the "genetic code" - a sort of hereditary alphabet. Genes are made up of small sections of DNA and these genes are located on chromosomes. If you think of bases as an alphabet, then genes can be thought of as sentences. Each sentence (or gene) provides a specific instruction for our bodies. One pair of genes might tell what color our eyes will be and another might tell how small our little finger should be! When scientists want to refer to all of our genes together, they call it a "genome".
Chromosomes, as well as the genes located on them, come in pairs. Humans have a total of 46 chromosomes arranged into 23 pairs. The first 22 pairs of chromosomes are called "autosomes" and are numbered 1 to 22, from largest to smallest. The 23rd pair is called the "sex chromosomes" and consist of an X and a Y in males and two X chromosomes in females. Chromosomes can be seen under a microscope but the individual genes located on them cannot.
Most people have 46 chromosomes or 23 pairs of chromosomes. People with Down syndrome have an extra number 21 chromosome for a total of 47 chromosomes. This condition is also called “Trisomy 21.” Trisomy comes from the Latin word "tri" which means three and "some" which means body and refers to chromosomes. People with trisomy 21 have three bodies 21, or three number 21 chromosomes.
Because chromosomes are the packages of genetic information and contain all of our genes or instructions, people with Down syndrome actually have three copies of all of the genes located on chromosome 21. It is estimated that there are 400 genes on chromosome 21. So basically, people with Down syndrome have 400 extra instructions. While having extra instructions sounds like it should be a good thing, it is actually like adding extra ingredients to a recipe. You will still get the same basic dish (a human body) but the form and function is different.
Causes of Trisomy
Trisomy for a specific chromosome is the result of a misdivision in the sperm or the egg prior to conception. There is absolutely nothing that anyone can do to prevent a trisomy, and there is nothing that someone can do to cause a trisomy. It is an accidental occurrence that happens before conception in either the egg or the sperm. We do not know what causes a trisomy to occur.
While trisomies can occur in people of any age, there is an association between a mother’s age and the chance to have a baby with a trisomy. An association is simply scientific term for a link. An association is not a cause. This is an important point, because people are often under the mistaken belief that maternal age causes Down syndrome. This is not true. We do not know what causes trisomies to occur. We simply know that as women get older, the risk for having a pregnancy with a trisomy increases.
While there is a link between maternal age and the occurrence of trisomies, most babies with Down syndrome are born to mothers under the age of 35. This is due to the fact that there are far more younger women having babies. In fact, 80% of babies born with Down syndrome are born to women younger than 35 and the average age of a mother of a baby with Down syndrome is 28 years. Overall, the likelihood of a woman under age 30 giving birth to a child with Down syndrome is less than 1:1000. The risk increases as a woman gets older, with an incidence of about 1:112 at 40 years of age. However, this is still less than 1%.
The bottom line is that we know that there a link or association between a mother’s age and the risk to have a child with a trisomy such as Down’s syndrome, but no one knows the specific cause of trisomies.
National Institute of Child Health and Human Development (NICHD), Facts about Down Syndrome.www.nichd.nih.gov/publications/pubs/downsyndrome.cfm%5C 2008.
Newberger, D., Down Syndrome: Prenatal Risk Assessment and Diagnosis, American Family Physician. 2001.www.aafp.org/afp/20000815/825.html