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Down syndrome -- level of mental disability

  1. Apr 19, 2017 #1
    I've read in my biology book "children with down syndrome develop different levels of mental and physical disability."
    But they all have an extra 21st chromosome... My question is what causes this difference?
  2. jcsd
  3. Apr 19, 2017 #2
    I am not a specialist. But my son has Down syndrome. I understand it this way:

    First of all: Normal children develop very different levels of mental and physical ability. So in that respect, it is maybe not surprising that "children with down syndrome develop different levels of mental and physical disability."

    But also I imagine the microbiology of our cells differ a little from person to person. So the extra genes do not influence every person's cells in the exactly the same way.

    The development of the brain might be disturbed more or less from person to person. The same goes for development of limbs, joints, heart, guts and so on. Also the functioning of each type of cell in the body might be disturbed more or less severely.

    I imagine this is the reason trisomy 21 persons can be more or less effected by their illness. And I also imagine it to be the reason levels of physical and mental disability can vary in persons with Down syndrome.

    For example: We know this other boy with Down syndrome who is only mildly affected mentally. He talks and understands quite a lot. But he is troubled by very loose joints. He can hardly walk. My boy on the other hand, is quite good physically - he can run and jump - his joints are only mildly affected by his trisomy. But his mental disability is severe. He has enormous trouble communicating verbally.

    So the extra chromosome 21 causes the same type of problems for the two boys, but at different levels in different cells/functions.

    Hope it helps.
    Last edited: Apr 19, 2017
  4. Apr 19, 2017 #3

    jim mcnamara

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    Staff: Mentor

    A short answer is: differences are caused by the same things that cause differences in non-Downs populations: Heredity and Environment.
  5. Apr 19, 2017 #4
    While DNA replication is a wonder to behold, it doesn't work sometimes, so there are peculiar results on some occasions.
    Some of those replication mistakes are recognisibe, Downs syndrome is one of them.
  6. Apr 20, 2017 #5

    jim mcnamara

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    Staff: Mentor

    @rootone - When a female baby is born, a fixed number of "eggs" she will ever have are already mostly developed and are on "hold". They have been in the Diplotene stage of Meiosis 1 for four months at birth. At age 30 the same unreleased ova are still at the same stage 30 years later. Or 45 years later.


    Trisomy 21 (most common form of Downs') is the result of meiotic failure. The rate of failure is directly related to the age of the female at conception. Failure is the result of being on "hold" for half a lifetime; this is not viewed as replication error. Humans are adapted for having children within a few years after the female reached menses. Which, in 1900, used to be age 18 or more worldwide. [aside] It is now 11 years of age in the US., and is contributory to teen pregnancy. And to increased growth rate of US and worldwide human population -- due to shorter generation times.[/aside]


    15-21 Robertsonian translocation is a very rare form of Downs, and is replication error.
    There are tons of links on this subject.
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