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There are actually a number of different types of trisomy. The ones most commonly encountered are |
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"Full" Trisomy means that as far as anyone can tell there are three complete copies of a chromosome in every cell of the body. So for example in the case of Trisomy 18 there are three complete 18th chromosomes.
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This is a photograph of the karyotype of a boy with Trisomy 18. Note the three 18th chromosomes indicated by the arrow. In the case of "full" trisomy 18 this karyotype is what would be seen if done on any cell within the child's body. |
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In reality there should be no need to use the term "full" - it is actually confusing because children with mosaic trisomy 18 also have a "full" extra chromosome. But people often do use the term "full" to differentiate between the other types of trisomy. However it is assumed, unless indicated otherwise, that when the term trisomy is used it refers to a "full" trisomy. |
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In reality, no one is absolutely certain that any child has a true "full" trisomy. That is why I prefaced this page with the wording "as far as anyone can tell". Some doctors believe that any child with a trisomy that survives until term or older "must" be mosaic, and that its just that the percentage of "normal" cells is so low that it cannot be detected by standard testing as it exists today. When the doctors determined your child's karyotype they took a random sample of cells - usually blood cells, or more correctly peripheral lymphocytes. However it may also have been amniocytes from the amniotic fluid or fibroblasts from a skin sample or even a buccal swab from a cheek. These cells were then sent a laboratory, cultured and looked at closely. Generally they look at between 5 and 20 cells, depending on the protocol of the laboratory. If all of the cells that they look at have the additional chromosome, and it is a complete extra chromosome then it is thought to be "full" trisomy. If even one of the cells contained the normal complement of two chromosomes instead of the three, the diagnosis is usually changed to mosaic trisomy and a percentage calculated accordingly. |
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The "as far as anyone can tell" is always the qualifier because 5, 20 or even a 100 cells is only a very small sample of the many billions of cells in our bodies. Quite logistically you can understand that they cannot look at every cell. As well, the cells that they look at are only from one body system. It is very possible that the cells in each body system can be different. |
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Generally the answer to that question is no. It really does not make a great deal of difference in most cases whether a baby is mosaic or "full". It is not the trisomy itself that causes the early deaths of our children, but the problems that the extra genetic material creates. Children with mosaic trisomy often have exactly the same medical problems, that are as equally life threatening as children with "full" trisomy. The real difference between mosaic and full trisomy happens later because children with mosaic trisomy tend to, but not always, do better developmentally. However, some doctors will more actively intervene if they believe that the baby has mosaic trisomy. Similarly some parents would be more likely to continue on with a pregnancy if they think that there is a chance of mosiacism and some just want to know. If any of those situations apply to you, you may want to discuss further testing with your doctor. In most cases it is enough to ask the lab to look at more cells from the original test. My personal recommendation is that they should look at a minimum of 100 cells. There are other tests however and some of these are outlined in the section on mosaicism. It is not a comprehensive list by any means so you really do need to discuss it with your own doctor. |
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What is a Trisomy Index |
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| Home | What is a Trisomy? | What is Trisomy 18? | What is Trisomy 13? || Alex's Story | Stories From Special Friends | In Memory of Special Friends | | Special Occasions | Awards | Shapiro's Syndrome | SOFT Australia | Trisomy Listerv | |
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Visitors Since 19th November 2000 FastCounter by bCentral |
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