A Look at Translocation, Trisomy and Mosaic Down Syndrome

The 3 Different Types of Down Syndrome Explained

chromosomes, human karyotype, down's syndrome, trisomy 21
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Reciprocal translocation is the genetic cause behind translocation Down syndrome, which is one of the three primary types of Down syndrome. Reciprocal translocation is a special type of chromosome abnormality caused by pieces of separate chromosomes breaking off of their original chromosomes and switching places. The two resulting new chromosomes are called derivative chromosomes.

Any chromosome can be involved in a reciprocal translocation.

There are two types of reciprocal translocations -- balanced and unbalanced. When a balanced translocation occurs, the correct amount of chromosome material is present. When an unbalanced translocation occurs, there is either too much or too little chromosome material present.

For example, a reciprocal translocation between the long arm of chromosome 14 and the short arm of chromosome 21, resulted in two new fused chromosomes called derivative chromosomes.

Translocation Down Syndrome 

Translocation Down syndrome occurs when two chromosomes, one of which must be a #21, join together at their ends. Instead of having three independent, separate number 21 chromosomes, a person with translocation Down syndrome has two independent #21 chromosomes and a #21 chromosome that is attached to another chromosome. The attached chromosomes are called a derivative chromosome and can occur for the first time in the person with translocation Down syndrome (de novo translocation), or the derivative chromosome can be inherited from a parent.

When a child is diagnosed with translocation Down syndrome, it is very important that the parents of the child also have karyotype testing to see if they carry this translocation.

While translocation Down syndrome occurs via a different mechanism, a child with translocation Down syndrome will have the same features and symptoms as a child with trisomy 21.

Translocation Down syndrome is one of the three forms of Down syndrome (the others are trisomy and mosaicism). It accounts for 4 to 5% of Down syndrome cases.

Trisomy Down Syndrome 

Normally human beings inherit 23 chromosomes from both the mother and the father for a grand total of 46 chromosomes. However, people with trisomy Down syndrome get an extra copy of chromosome 21, for a total of 47 chromosomes. Having an extra chromosome 21 manifests as Down syndrome.

Mosaic Down Syndrome 

Mosaicism Down syndrome is different from other types of Down syndrome because not all the cells in the body have 46 chromosomes. Instead, a percentage of cells can have 47 chromosomes (with an extra copy of chromosome 21).

For example, a blood sample from a baby with mosaic Down syndrome may examine 20 blood cells: 10 of these cells could have an extra chromosome 21 (47 chromosomes total) and 10 of these cells could have no extra copy of chromosome 21 (46 chromosomes total). Based on these counts, the baby would have a mosaicism of 50 percent.

If this blood sample were representative of the blood as a whole, then half the cells in this child's blood would be normal, and the other half would contain an extra chromosome.

Of note, levels of mosaicism differ based on the sample of tissue take. For instance, the blood, the skin and the brain could all have different levels of mosaicism.

From a clinical perspective, a person with mosaic Down syndrome could have many of the same clinical features as a baby with trisomy or translocation Down syndrome; however, the severity of illness may differ. For instance, some of the more distinct facial features of Down syndrome, like the upward slant of the eyes or skin folds, may be less pronounced.

Mosaicism among baby's with Down syndrome has been an interesting area of clinical research. Specifically, scientists have tried to predict IQ and risk of heart defects based on mosaicism percentages. However, mosaicism hasn't panned out as a good clinical indicator.

Ultimately, there's still lots of variability in the genetic make-up of a person with mosaic Down syndrome, which means that there is no typical presentation or appearance of a child with mosaic Down syndrome. Some children with mosaic Down syndrome may have very few clinical features of the disease; whereas, other children may present with all features of Down syndrome.

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