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

Question

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

In: Biology

Answer 1

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.