Turns out that this already happens: https://rarediseases.info.nih.gov/diseases/12480/tetrasomy-21 https://rarediseases.info.nih.gov/diseases/12480/tetrasomy-2.

Turns out that this already happens: https://rarediseases.info.nih.gov/diseases/12480/tetrasomy-21 https://rarediseases.info.nih.gov/diseases/12480/tetrasomy-2... The bad news is that the prognosis is even worse than for the trisomy.