https://www.earth.com/news/crispr-used-to-remove-extra-chromosomes-in-down-syndrome-and-restore-cell-function/

Scientists in Japan have used CRISPR, a gene editing tool, to remove the extra chromosome that causes Down Syndrome from cells in the lab. While this is a long way from a working cure for humans, it is a promising sign that the technology could someday correct the condition and others like it.

Eliminating conditions such as Down Syndrome has proven a controversial topic, with some arguing that doing so amounts to eugenics.

This can't be applied to humans retroactively?

Honestly, I wouldn't know. Maybe it would need to be done during pregnancy when the condition is detected.

Who knows far in the future, but for now you could only do it to a test tube zygote and finding one has extra chromosome it's easier and safer to just use another. We're far from having any practical use from this technology.

Ethically speaking, I don't think there's an eugenics problem because we're talking about removing extra copy, with the same genes still remaining.
Though I don't know if if it's always 100% identical chromosome? I imagine there'd have to be some theoretical chance of mutation, but that'd still 99.999... Except that would kind of make it eugenics problem same as removing a single gene, but still in lesser sense.

Cool, gene therapy is about to become reality. I don't think you could do that retroactively, though. What would that even look like, the person would transform in front of their eyes?

If AI is even half what they say it'll be, I expect most things like this, including cancer, to be extinct in ten years, tops. If not that, at least a full-blown cure.

I'm not an expert myself on this or Down Syndrome, but (remembering back to biology): it would require extensive and careful therapy after the embryonic stage that we have nowhere near the technology to implement. It's probable much of the gene expression that occurs from Down syndrome is already established by the time of birth. In other words, altering the instructions isn't going to change the wiring that's already implemented.

Just get proper prenatal healthcare and abort the fetus. I'm not even being mean or edgy. Healthy adults are struggling to get proper healthcare everywhere in this world, even when they're healthy enough to advocate for themselves. Do we really think someone with Down syndrome Is going to be just okay in a world like this? Health inequality is real, and patients with Down syndrome are most susceptible to it.

Abort early and focus on improving the lives of the living.

You're not entirely wrong but that perspective is very dark and grim. Instead of trying to change people who are different and don't mesh with the system, we need to change the system itself to serve the majority of the human population. The two are not in sync with each other at all. The system as it stands is so broken, that it only stands to benefit the top 15-20% of society and that number has been on a bell curve of shrinking since the pandemic and we've made every mistake towards mass scale "social suicide".

Unlikely, although I am no expert. But every cell in your body contains a complete copy of your genome, so you have to fix much more genomes in a grown body, then in an embryo or even only the one cell that is a germinated egg. Also, as Jumpin noted, the genes are basically recipes for enzymes, that in turn control certain developments in you growing up. So it could well be, that changing the genes after the fact will not have any effect or not a substantial one.