[Retric]

All species that sexually reproduce occasionally have the wrong number of chromosomes show up in an offspring. In humans it occurs in ~0.6% of live births. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3681172/

Down syndrome is an example of that anomaly rather than some mutation that’s passed down. It’s an extra partial or complete copy of chromosome 21, and therefore can happen to any couple.

[User3456335]

My point is that perhaps there is a reason that this chromosome causes this issue when an additional copy is present. If other chromosomes do not cause the same issues, why not? Why would that same reason not hold for chromosome 21?

[Retric]

You have it backwards here, Down syndrome is unusual for being viable not because the symptoms are so terrible. Most chromosomal anomalies result in a non viable egg / sperm which doesn’t result in a live birth. The majority don’t even last long enough for someone to notice they were potentially pregnant.

Turner syndrome is another example without the mental issues: https://en.wikipedia.org/wiki/Turner_syndrome

In the larger context, there may be some long term advantages to chromosomal anomalies because their number varies so much between closely related species.

[stvltvs]

In other words, Down syndrome isn't inherited therefore not something evolutionary forces can act on. The only way around that is if the base rate of chromosomal anomaly is heritable, yes?

[lazide]

Or general human reproductive ‘cost’ is heritable.

Think r/K evolutionary strategies [https://en.m.wikipedia.org/wiki/R/K_selection_theory]

A species that can produce 1000+ offspring (or more) per mating (with multiple matings per adult lifespan) - like most insects, seaturtles/squids, etc. has little incentive to have high precision/accuracy reproduction.

One that does a small handful (whales, elephants, etc), has a lot of incentive to do so (or self abort early on if there is an issue).

This gets complicated though, because while a single individual in a species which is predominantly r strategy (few offspring), can be ‘more’ on the K side, and vice versa.

And a species which has too much consistency, both genetically and ‘approach’ is very susceptible to inbreeding/mono-cropping issues where a single event/disease can wipe them out, or they can even destroy themselves due to recessive traits.

So there is a general (but diffuse!) evolutionary pressure towards a degree of mutation/error in reproduction in even the most hardcore ‘r’ species (which will necessarily produce a lot of noise/‘wasted paths’) at the species level, alongside a hard evolutionary pressure to reduce it for individuals.

In for example humans. Or elephants. Or whales.

This is also why things like rich/poor, healthy/sick, pretty/ugly, strong/weak, etc. will never go away - they’re outcome distributions along a probability curve due to fundamental different approaches by individual humans due to the necessity of how humans have to be in order for humanity (the species) to survive.

Individuals have strong incentives to ‘tip the scales’ in various ways of course, and societies (at a minimum!) have strong incentives to stop them. It’s why aborting based on gender is illegal in India, for instance. [https://pubmed.ncbi.nlm.nih.gov/7920120/]

Because if everyone had the same attributes in order for everyone to be ‘pretty’ or ‘strong’ or ‘rich’ or whatever, then some hypothetical weight-lifting-infectious-disease (or a famine, or an attack by jealous anti-weightlifters) would wipe out the whole population. And if everyone optimized for ‘rich’ genes (whatever that means), then society would implode, because that literally couldn’t work.

And those types of situations, albeit less light hearted, have, do, and will continue to happen eventually.