Why does the addition of one extra chromosome cause Down syndrome, and how come that people of all backgrounds with Down syndrome have the same distinct features? How does the addition of one extra chromosome cause Down syndrome?
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Each chromosome contains genes. Extra copies of chromosomes result in those genes being expressed (that is, producing whatever proteins actually “do the thing the gene does”) more often.*
For most chromosomes, three copies result in some gene that needs to be expressed at a very specific level being expressed too much. This is usually fatal. In humans, only three copies of chromosome 21 (Down syndrome) and chromosome 18 (the rarer Edwards syndrome) are normally survivable; most embryos/fetuses with three copies of one of the other chromosomes die before they’re born (in rare cases, they can sometimes survive with three copies of 8, 9, or 13 too).
The specific symptoms of Down syndrome come from the genes that happen to be on chromosome 21. The exact mechanisms aren’t known, but two genes that are present code for a protein that is common in the brain (and which, in Down syndrome, results in a buildup similar to that of Alzheimer’s disease) and some important enzymes. There are 310 genes on chromosome 21, though, and it’s likely that some complicated mix of many of them contributes to the specific traits of people with Down syndrome.
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* The exception is the X chromosome. During early development, a [genetic] woman’s cells disable one of their two copies of the X chromosome, leaving only one expressed in each cell (though sometimes different ones in different cells). This means that [genetic] women with two X chromosomes don’t over-express X-related genes relative to [genetic] men with only one X chromosome. Not related to Down syndrome, but cool.
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Chromosomes are what cells use as blueprints to make stuff. More of those chromosomes means more of that stuff. Body functions are like baking recipes. If you dont stick to the recipe exactly, but instead use too much of some ingredients, things will go wrong.
Fun fact: that “trisomy” is a third chromosome, also called a triploid.
Strawberries used to be super tiny, but the big ones at the store we buy now have a chromosome that’s an octoploid. 1 extra chromosome causes Down’s syndrome. 6 extras make a strawberry bigger.
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