Eli5: why can't apes reproduce with humans who have Down Syndrome?
By - Easy-Fan7144
Down syndrome is caused by a duplication of chromosome 21 rather than a fully functional independent pair of chromosomes with new information. Chromosomes come in pairs and the full pair doesn't get duplicated, so chromosome 21 becomes more of a triad than a pair
Bonus fact: Apes can actually get a disorder similar to down syndrome but it is their chromosome 22 that gets duplicated rather than chromosome 21
What are the typical differences between normal apes and apes with a duplicate 22?
It's actually very similar still to human down syndrome, especially the facial and other physical characteristics. From what I read it's hard to determine if there's similar mental deficiencies because they often don't live very long, especially in the wild, which could partially be because of common heart defects or health issues but also because sometimes the mother abandons them
Chromosomes aren't commodities. It isn't just a matter of having the same amount, they are actually unique. Humans have fewer compared to other apes because chromosome 2 is actually a fusion of what were once two different chromosomes. Down syndrome comes from a third copy of chromosome 21.
So while not having the right number of chromosomes is a sign organisms definitely can't reproduce, having the same number isn't *sufficient*. It is like how you know that you can't make a V8 engine work if you only have six pistons, but that doesn't mean that having eight pistons of any kind is going to work.
Humans generally have 23 pairs of chromosomes. Humans with Down syndrome have 22 unique pairs plus 2 that are copies of each other.
Most apes have 24 unique pairs of chromosomes.
Even though the numbers are close, the genetic information is organised a lot differently amongst the chromosomes, so even though you can spot the similarities, there is no direct correlation between the chromosomes of the two species.
That said, some studies suggest that before our evolutionary lines parted and our genes evolved, there was procreation between the ancestors of what we how call humans and chimpanzees.
Down's is also known as "trisomy 21" - an extra chromosome 21 - not "multiple copies of some and missing others completely" as your post suggests.
(My source, admittedly, is Wikipedia - and Google is your friend)
Down syndrome is caused by the existence of an extra copy of the chromosome 21 (https://www.ndss.org/about-down-syndrome/down-syndrome/) meaning that instead of one chromosome with that index, people with down syndrome have 2 (admittedly it doesn't have to be the entire chromosome, it can be an extra part of it). I didn't say "multiple" I said 2, and I didn't mention "missing others completely".
The chromosomes don't match. It's thought that we have one fewer because in an ancestor two small chromosomes analogous to what we call 2A and 2B fused end to end to make one big chromosome, which is our chromosome 2. In a person with down syndrome they may have the right number of chromosomes, but the wrong types. Human chromosome 2 is still unpaired, and the extra copy of chromosome 21 which causes down syndrome would also not have a pair. They don't match ape chromosome 2a and 2b.