Why are some chromosomal abnormalities (Downs Syndrome) allowed to continue through a normal pregnancy where almost all other chromosomal abnormalities are miscarried?

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How does the body know which chromosomal abnormality is good enough to become a person and others are better to be ended?

In: Biology
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There are tons of chromosomal abnormalities that go through, not just downs. Color blindness, missing limbs, red hair… 🤣

Some abnormalities, prevent the embryo from living. It’s not the mother’s body that determines this, but the embryo itself. If the cells do not replication and function within certain bounds then they die.

Some genes are more important than others. Let’s say one chromosome carries genes for some brain development and facial structure, etc. and another chromosome controls genes crucial to fetal growth and development, significant brain structure, etc. one you can live with but one you can’t.
It’s like if I went to a car factory to sabotage it. I got to an assembly line that builds the engine and break it. Can’t make the foundational parts of a car, so no car. Let’s say instead I break a machine that controls the shape of the steering wheel, still a operational vehicle, just imperfect.
People with Down syndrome are still operational human beings with the building blocks of life, just have some hurdles.

It isn’t like the body checks whether the baby will be able to live long before it decides to give birth. It is just that some anomalies are so detrimental to life that the fetus/embryo just starts to die.

The mother’s body recognizes the dying offspring and flushes it out to prevent damage to the uterus.

But Down Syndrome children are physically viable. Their heart works, their blood vessels work, etc. To the mother’s body, it is perfectly normal. It cares not how smart or capable the child will end up being.

And even with that basic check, sometimes it fails, with some mothers giving birth to dead children (1 in 160 “completed” pregnancies are stillborn in the US)

Down syndrome is also called trisomy 21, this means that you got three instead of the usual two copies of your 21st chromosome in body cells, resulting in all the genes on that chromosome being over-expressed. Trisomy 13 and trisomy 18 individuals also get past birth with a reasonable frequency although it is still much rarer than Down’s syndrome and they have much shorter lives. If you have too many sex chromosomes that is not much of a serious condition at all, the Y chromosome is not that relevant and the excess X chromosomes (also the second one in normal females) are for the most part silenced anyway. This is why you can even survive with just one X chromosome (an example of monosomy, only chromosome pair in humans where it is non-lethal). Monosomies are more severe than trisomies partly because having one copy of your genetic material will result in a lot of genetic damage being expressed.

Notice that your chromosomes are numbered from long to short, and trisomy 21 is thus affecting your second-to-last shortest chromosome (sex chromosomes don’t get a number). Now it just happens that there are no quintessential genes located on this short chromosome and that you can survive with all of them expressed about 1,5 times what they usually are. The trisomy has the combined effects of all of these, it really results in a wholly different recipe for a human and with chromosome 21 it just happens to be viable. This is pretty much a coincidence because of all the things that happen to lie on that chromosome, genes are spread pretty randomly. Chromosomal abnormalities are pretty extreme things genetically because they often affect an entire chromosome at a time, but also many mutations in individual genes will be ‘lethal’, although also often only if it’s present in both copies.

You’re asking the wrong question. The right question is: why does a miscarriage happen?

It’s because the fetus died, sadly. So obviously if some “abnormalities” kill you, and some don’t, the ones that kill you will cause a miscarriage.

The OP unintentionally uses eugenics thinking, which is common the internet, by assuming that “abnormalities” “SHOULD” be eliminated by the body, and then asks WHY does anomaly X get eliminated. That’s false. ALL OF EVOLUTION happened because of “abnormalities” and changed and variation in offspring. Without a tolerance for abnormalities, no species would have been able to evolve.