Some maladies are cause by chromosomes being deleted (see Prader-Willi). How does that work?
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The sperm carries 23 chromosomes from the father, the egg carries 23 chromosomes from the mother. They combine to create 46 chromosomes that become part of every cell in the fetus and then the human that’s born.
But, it’s not a perfect process; things go wrong. Sometimes, the sperm or the egg are missing part of all of one of the chromosomes, or they have an extra copy of one of them.
Sometimes the sperm and egg are fine, but a mistake happens in one of the first few cell divisions, so too many of the resulting cells have a chromosome difference.
Chromosomes carry all of the genetic material necessary to create a human being, so when there’s a chromosome problem, the vast majority of the time the fetus does not develop. It just leads to a miscarriage.
However, there are a few rare cases where a chromosome difference is still viable – the fetus is still able to grow and survive – such as Prader-Willi, which is caused by a deletion of part of Chromosome 15.
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