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When talking about partial trisomies its very hard not to raise the subject of translocations. There are two basic types of translocations, reciprocal and Robertsonian. |
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Chromosomes are actually very sticky and very fragile. Sometimes during mitosis the chromosomes break and then join themselves back together. Occasionally they do not join back up where they originally broke. This results in a condition called a balanced translocation. |
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Diagram from Tokyo Medical University |
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This animation shows how a balanced translocation occurs. |
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Normal |
Balanced Translocation |
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These two diagrams show the end result. The first depicts the normal chromosome arrangement. The second the balanced translocation. Diagrams from Tokyo Medical University |
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Under normal circumstances a balanced translocation causes no problem. All the necessary genetic material is there, it is just rearranged. |
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The very first time that this happens it is called "de novo" De novo comes from Latin and simply means "from the beginning". |
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A person who has a balanced translocation is often called a "carrier". However because it causes no problems most people are unaware that they have a balanced translocation until they have children or until a sibling is diagnosed with a translocation. |
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When a person who carries a balanced translocation has children it is possible that the egg or sperm may have an unbalanced distribution of the genetic material. |
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This is usually in the form of a loss of genetic material, called a monosomy and a gain of genetic material, which is a trisomy. |
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This diagram shows one of the possibilities of an unbalanced translocation. There is a monosomy (loss) of part of the genetic material contained in the purple chromosome and a gain (partial trisomy) of the blue. |
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It is of course also possible that the child would have a normal arrangement of chromosomes or also that they would also have the balanced translocation. However the risks of having a child with a chromosomal disorder are greatly increased. |
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Because translocations are incredibly complex I strongly recommend that individual cases be discussed personally with a genetic counsellor. This information is definitely only a brief outline. |
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Robertsonian Translocations |
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A Robertsonian Translocation is a different type of translocation and only ever involves chromosomes 13, 14, 15, 21 and 22. It is actually quite a common rearrangement and is found in approximately 1 in every 1000 of the population. |
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If you look at the picture of the chromosomes you will see that these chromosomes actually have very short "p" arms. As a result sometimes the two long arms (q) join together to form one long chromosome. |
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Like other types of translocations the person who carries a Roberstonian translocation has the normal complement of genetic material and so is not effected in anyway. |
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However when that person produces either egg or sperm cells it is possible that they will have a loss or gain of genetic material. |
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Usually this does not result in a partial trisomy however, but a complete trisomy or a complete monosomy. There are no known cases of a complete monosomy that have survived. |
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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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