The Extra Chromosome
The initial cause is a gamete (egg or sperm, more commonly the egg) is formed incorrectly. During meiosis, the now-formed-incorrectly gamete forms two copies of chromosome 21. In the picture to the right, the red box indicates a karyotype (a picture of the chromosomes) with Down syndrome. As you can see, there are three copies of that chromosome. [8]
The originWhen parents hear that their kid will have, or have Down syndrome,most times, the immediate though it, "Who's fault was it?" Well, it's not really anyone's fault. It's just a problem that occurs during meiosis. It's not hereditary. Most times, the nondisjuntioned (the messed up) gamete comes from the mother, only 5% of Down syndrome cases come from men. It also is more likely for older women to have children with Down syndrome. At age 35, there is a .3% chance and at 45, there is a 3% chance. The graph to the right (blue line) shows the increasing risk of being an older women and having a baby with Down syndrome. [1]
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The picture to the left indicates a normal karyotype. As you can see, the 21st chromosome only has to copies. Though this is a minor change, it occurs in every cell. This can lead to changes on every gene. This small change can cause the protein producing levels to be abnormal. Having too much or too little protein can lead to serious problems. [8]
Types of Down SYndromeThough all forms of Down syndrome include chromosome 21, there are different ways to receive the extra chromosome. There also different "levels" of Down syndrome, like how much it affects you and your body.
Trisomy 21 (Nondisjunction): This is the type that I have spoken about all along. It is also the most common, it accounts for 95% of cases. This is when the gamete is formed incorrectly by nondisjunction. The offspring then receives three copies of chromosome 21, rather than two. [1] Mosaicism: This occurs when nondisjuntion occurs in not all of the cells. This means that there is mixture of cells with 46 chromosomes and 47 chromosomes. As you may guess, this means that the child's case of Down syndrome is not as severe. Mosaicism accounts for 1% of all cases. [1] |
Translocation: This is a very interesting type, it accounts for only 4% of all cases. A chunk of chromosome 21 breaks off in the cell division process and connects to another chromosome (usually chromosome 14). Even though the total number of chromosomes is still 46, the presence of the extra part of chromosome 21 brings in some characteristics of Down syndrome. The chart to the right shows the extra chromosome 21 on chromosome 14. [1]
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