3 Types of Down Syndrome

Did you know that Down Syndrome, otherwise known as Trisomy 21, actually has 3 types? It is a little known fact as most make the assumption when they see someone with down syndrome characteristics that one size fits all, but that is simply not the case. There are actually 3 types of Down Syndrome.

Because of the differing abilities from one individual with Down Syndrome to the next, it is often assumed that Down Syndrome is a spectrum disorder. This is not true. Down syndrome is completely different from Autism and other spectrum disorders; the two diagnoses have nothing in common unless the individual happens to receive a dual diagnosis later in life.

In order for someone to be diagnosed clinically as having Trisomy 21, or Down Syndrome, there has to have been a genetic karyotype test. This test is to determine the presence of an extra copy of the 21^st chromosome in the genetic map that makes each person an individual. This diagnosis, post karyotype test, is important because there are some children who may exhibit soft markers for Down Syndrome, but in fact, may not have the extra copy of the 21^st Chromosome.

If the extra genetic material is not there on the 21^st set, the person will not be diagnosed as having Down Syndrome. If the genetic material is there, it is important to get the diagnosis so that the child can access early intervention services and qualify for any special needs assistance that can be provided.

Let’s start with the basics, what is a chromosome exactly? A chromosome is a threadlike structure that carries genetic details in the form of genes, genes are the blueprint details that determine how you will look, what color eyes you will have, whether you will be a good musician, and so many other fine details that make each person an individual.

3 types of down syndrome — Source genome.gov

But, what happens when there is an atypical event during conception? Typically the baby is conceived with 23 chromosome pairs (46 individual chromosomes), when something atypical happens the baby is conceived and at some point, possibly during cell division, with an extra chromosome or sometimes just a piece of chromosomal material. This seemingly small event has wide reaching consequences and forms the basis for many of our genetic differences today. Specifically when the 21^st chromosome is the one affected there are then 3 types of down syndrome that can happen as the result.

Classic Trisomy 21

In these types of down syndrome all copies of the 21^st chromosome are in triplicate. This is the case for 95% of babies who will be diagnosed with Down Syndrome. The extra genetic material affects each person differently. Some individuals will function very well with no health issues to speak of and the ability to function cognitively very well. Some individuals will have more affects with their physical makeup such as heart issues, intestinal tract difficulties, and other motor and cognitive abilities.

Because there are a variety of ways in which a person is able to function and is affected by the extra material people often think that this is a spectrum disorder. Trisomy 21 is not a spectrum disorder it is a genetic difference at the very basic level of our human makeup.

Translocation

Another of the types of down syndrome is when extra genetic material is attached to the 21^st chromosome, but it may not be a full and separate chromosomal piece. This can be thought of as though it is kind of like a small hitchhiker that is caused by rearranged chromosome material. So instead of a full third strand of Chromsomal material, there is a piece of chromosomal material attached to one of the other full chromosomes. The most interesting part about this specific kind of Down Syndrome is that the extra piece of chromosome 21 can actually attach itself to another chromosomal pair such as the 13^th set or the 15^th set. It does not always attach itself to the 21^st set but it is a 21^st chromosomal piece. (source CHOPedu)

This type can be hereditary but is the only type that is. According to the Children’s Hospital of Philidelphia:

3 to 4 percent of babies born with Down syndrome have translocation Down syndrome. Whenever a translocation is found in a child, the parents’ chromosomes are studied to determine whether the translocation was inherited or not. If one parent has the translocation chromosome, then the doctor knows the baby inherited the translocation from that parent. When a person has a rearrangement of chromosome material, with no extra or missing chromosome material, he or she is said to have a “balanced translocation” or be a “balanced translocation carrier.(CHOPedu)

Mosiac Down Syndrome

The last of the types of Down Syndrome is Mosiac. In Mosiac Down Syndrome, only some of the person’s 21st set of chromosomes are in triplicate and others are completely unaffected. In the microscopic world some samples of the chromosomes in a karyotype test will look like typical children and others will be affected by the presence of a 21^st chromosome.

These cases have a tendency to exhibit fewer of the physical characteristics of Down Syndrome and often it can be difficult to tell, at first glance, that they have Down Syndrome. (1-2% of cases). One if the interesting facts about Mosiac Down Syndrome is that it is not at all uncommon for someone with mosaic down syndrome to go undiagnosed until months or years after birth when some of the more noticable cognitive and developmental delays may show up.