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.
There are normally 46 chromosomes in human cells. When eggs and sperm cells are manufactured, half the chromosomes are removed to leave only 23. The 23 in the sperm and 23 in the egg add up to 46 chromosomes, which is how you inherit genes from both your mother and father. Sometimes in the process of forming the sperm or egg cells, the process malfunctions, leaving the wrong number of chromosomes, either more or less than the normal 23. If these abnormal sperm or egg cells happen to win the lottery and start a pregancy, then there is the potential for a genetic condition caused by abnormal chromosome count. Most of these conditions are fatal to the zygote before the pregnancy is noticed, but in some cases an abnormal chromosome count can still result in a viable pregnancy.
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