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Genetics for parents of Trisomy children
The information provided below is not intended as medical advice, and should not be relied upon as a substitute for consultations with qualified health professionals who are familiar with your individual medical needs.
What are chromosomes?
Chromosomes are thread-like structures composed of DNA and other proteins. They are present in every cell of the body and carry the genetic information needed for that cell to develop.
How does a Trisomy occur?
The basic cause of trisomy is during human cell division, and occurs when egg and sperm cells are initially created.
How was my child diagnosed with a Trisomy?
Doctors use genetic testing (including amniocentesis and chorionic villus sampling) to gain samples of cells from a patient. Having obtained these cells they use a process called karyotyping to determine how many chromosomes the individual has.
By looking at multiple cells they can determine whether the karyotype is full, mosaic or partial.
What is a “Full” Trisomy?
What is Partial Trisomy?
In the second instance, a "partial trisomy p or q" means that the child has only a portion of one or the other half of the chromosome. This is the most common type of partial trisomy, and comes with some crazy numbers. For example, 46XX, dup(18), (p12 -> p11.2) This means that only the portion of chromosome eighteen found between 12 and 11.2 on the short end of the chromosome is found in triplicate.
New genetics research has determined that there are different "qualities" of chromosomal material. Approximately equal amounts of triplicated chromosomal material from the q arm of chromosome 2, for example, cause more abnormalities than the same amount of triplicated material from the p arm. In other words, in our insatiable desire to predict “how affected” our child may be, we should not assume that just because they only have a partial trisomy they will be better off than those with full trisomy.
What is Mosaicism?
In this case, out of 25 cells tested, six cells had a portion of the eighteenth chromosome replicated three times. The portion included from the beginning of the "p" arm of the chromosome to the q12 position. In a further twelve cells the portion went from the beginning of the "p" arm of the chromosome to the beginning of the "q" arm. In the final seven cells the child was chromosomally normal.
I was told I have a translocation what is that?
Sometimes, during the process of meiosis, a section of a chromosome can detach, turn over and reattach. This is called an inversion and doesn't cause any outward (phenotypical) signs. Unfortunately at other times this detached section connects with another chromosome, causing a translocation.
If all of the original information is present (just attached in a different place) there will again be no phenotypical signs. Sometimes, however, the rearrangement is unbalanced (adding or missing information), causing various degrees of trisomy. This can happen completely "out of the blue" (as the doctors call it, 'de nova') or it can be caused by a carrier.
In the second instance, a parent carries in their complement of genes a balanced translocation. This is fine for them, but when they produce offspring, their offspring can "miss out" on, or receive extra of, the chromosomes that are misplaced. This is the first form of a parent being able to "pass on" a trisomy to their child. The parent who has a regular balanced translocation has a one:four chance of producing a child with a trisomy.
Is a Robertsonian Translocation the same as a Translocation?
A Robertsonian Translocation is a little different, in that they only happen in chromosomes where the p arm is very short (13, 14, 15, 21 and 22). The chromosomes "lose" the p arms of both chromosomes and create a balanced karyotype (all genetic information necessary is present) which only contains 45 chromosomes. This again does not cause any problems for the parent "carrier", but it does cause a high risk of abnormality in subsequent children.
Did I Cause my Child's Trisomy?
What are the chances that this will happen to us again?
Unfortunately the data shows that the more advanced the maternal age, the higher the likelihood of conceiving a child with a chromosomal abnormality. This is something which many people know already. What some of us forget from high school biology, however, is that even at 19 we can have a child with a chromosomal abnormality. Furthermore, remember that this data is limited to an extent by the fact that it represents only live-born children, and those with chromosomal abnormalities are much more likely to be miscarried or stillborn than their chromosomally "normal" counterparts. In other words, the chance of conceiving a child with a chromosomal abnormality is much greater than what is listed above.
What else do I need to know?
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