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Genetics for Parents of Trisomy Children
by Heather Morgan

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?
Full trisomy (be it trisomy 18, 13, or some other type) means that there is one extra chromosome in every cell. 95% of all cases of trisomy are “full” in this manner. When doctors simply say that a child has “Trisomy 18” they are referring to this “full” type. There are, however, some other, very rare forms of trisomy.

What is Partial Trisomy?
Partial trisomy is the next type of trisomy, and there are two different types of partial trisomy. A partial trisomy occurs when only part of the extra chromosome exists in every cell. Chromosomes have two main parts - a "p" arm and a "q" arm. The "p" arm is the shorter of the two. Because of this division between the two parts, it is sometimes possible for only half of the chromosome to be present as extra information.

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?
A "mosaic" trisomy indicates that a child has two or more different chromosomal "sets" - in other words, their karyotype may look like this:


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?
Perhaps the biggest question parents ask is "did I do something – or fail to do something – that is responsible for my child having a trisomy". The answer is very distinctly "no." Although a few individuals are carriers of this condition, they are rarely aware of this prior to conception. If your chromosomal analysis comes back clear, then you must accept that what doctors know today about trisomy is this: there was nothing you could do to cause it and there was nothing you could do to prevent it. Even if you are a carrier you must not blame yourself. As parents we do everything possible to look after ourselves and give our future children the very best potential for life. The only way to prevent the conception of a trisomy child for any of us is to screen every single sperm and every single egg we ever produce prior to allowing them to unite – and then monitor every cell throughout the first days of development to ensure that each and every cell division is accomplished properly. The moral? You did not cause your child's condition. You could not prevent your child's condition. All that your child now needs from you is this – your unconditional and continuous love and approval – even beyond the extent of their life – for as long as you live.

What are the chances that this will happen to us again?
Officially the risk of any problem occuring during pregnancy for any person is 2% - 3%. Having a child with a trisomy (if that trisomy is not caused by a translocation or Robertsonian translocation) increases your potential risks by 1% - that is, 1% on top of the normal 2% - 3%. The most current statistics put the rate of trisomy 18 at 1:3,000 live births. It is estimated that the frequency among miscarried children is one-hundred-fold these numbers. There is, however, a connection between maternal age and incidence rate. To calculate your own risk factor, visit:

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?
Its all well and good to try to understand all of the genetics behind trisomy – our need for knowledge, particularly in the early stages of this journey, demands that we gain this information. As parents, though, we need to know more. Answers to questions such as, “what is this going to mean for us? For our baby? For a subsequent pregnancy?” Those answers you'll find not here, but in our community boards, with real people who have real experiences and real, very special, children.

last updated August 7th, 2003

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