why some illnesses and diseases “skip” a generation.
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You have two copies of every gene (except for most genes on the X-chromosome, which males only get one of). If you get two “bad” copies of a certain gene, obviously that’s going to lead to a bad outcome. But what happens if one copy is functional and the other isn’t? That depends on what the gene does. Sometimes, one bad copy can screw everything up. But sometimes, the bad copy just does nothing and the good copy can “fill in” for the bad one.
If just one bad copy is enough to cause problems, then anyone who carries that bad gene will have these problems. In that case, the disorder cannot skip a generation: if neither of your parents have the disorder, then they cannot have the bad gene (if they had it, it would show).
However, if you need *two* copies to experience the ill effects, then it is possible to carry a single copy of the gene without experiencing any issues. This means both your parents could be carriers without having the disorder, and if they both pass their bad gene copy onto you, now you have two bad copies and you get the disorder. And thus the disorder has skipped your parents’ generation. And maybe it skipped your grandparents too and a bunch more generations. Only those people in your family tree who received two bad copies would have had the disorder.
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