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Down's syndrome occurs when either the egg or the sperm cell contain on extra chromosome 21. To my understanding, women are born with all the egg cells in place already, so there's no further cell divisions, and thus no further chance of having one cell with an extra chromosome.

Now, the chance of having a baby with Down's syndrome increase with age:

At maternal age 20 to 24, the probability is one in 1562; at age 35 to 39 the probability is one in 214, and above age 45 the probability is one in 19

Why is this so, when the egg cells are complete at birth?

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It is true that woman are born with all the eggs they will ever release , meiosis is not completed in the egg. There will be a meiotic cell division. Now this might sound confusing to you how there can be cell division without increase of egg number. For this - I will recommend that you read any basic bio textbook covering both the meiosis and gamete formation in females. –  biogirl Dec 7 '13 at 18:35
    
I think by this discussion it is clear that there is a chance of having one cell with extra chromosome. –  biogirl Dec 7 '13 at 18:40
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2 Answers 2

up vote 7 down vote accepted

The frequency rises with maternal age due to a peculiarity of meoisis in female mammals. Meiosis is originated in the fetal ovary, arresting at metaphase I with the homologous chromosomes aligned for segregation. Cells remain in this state until the time of ovulation, often decades later in humans. The longer cells remain in the arrested state, the greater the chance that there will be a nondisjunction event when meiosis resumes.

Source : http://www.madsci.org/posts/archives/2005-07/1121714807.Ge.r.html

I think you have the misconception that egg cells have completed meiosis by the time female is born. This is not true ( as indicated above in the answer). At birth, they are in metaphase I , after puberty, each month some of them divide further but only one survives and get arrested at metaphase II . Only after fertilization is the meiosis complete.

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If you are not familiar with some of the terms or have any other confusion , ask ! –  biogirl Dec 7 '13 at 18:33
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The chance of having a child with Down's Syndrome does not only have to do with cell division, but the mechanism that allows spontaneous abortion to occur within the uterus of the mother.

There is strong evidence for uterine selection against genetically disadvantaged embryos. However, as women approach the menopause and the risk of future infertility increases, this selection, or filtering stringency, is expected to relax.

Neuhäuser and Krackow’s paper provides evidence that older mothers, who give birth to children with Down Syndrome, have a relaxed stringency of quality control of embryos (or relaxed filtering stringency), which increases the probability that these women will bring children with developmental defects to full term. They believe that this relaxed filtering stringency is an adaptive maternal response and it might explain why the rate of Down Syndrome accelerates with increasing maternal age.

Basically, the mechanism that allows for the uterus to select against 'genetically disadvantaged embryos' (which can be Down's Syndrome, or other genetic deficiencies) is not as good at detecting these problems, OR as this paper suggests, is 'less picky' about the genetic quality of the fertilized egg, and will allow it to implant in the uterine walls.

It is also important to note that these studies are all ongoing, and many different reasons for the development of Down's Syndrome are unknown.

Literature Cited

Neuhäuser M & Krackow S (2006). Adaptive-filtering of trisomy 21: risk of Down Syndrome depends on family size and age of previous child. (Naturwissenschaften, DOI 10.1007/s00114-006-0165-3)

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