How can chromosomal abnormalities cause genetic diseases and be harmless?

it depends on if it is recesive or dominant. for example. 2 people can have the same mutation one person has 2 copies one from each parent. while the other person has one copy. both people may show symptoms or the person with only one copy will show mild to no symptoms never knowing that they have a disease causing genetic defect that could cause there children or grand children problems. this is seen alot when talking about fragile x. A person can be in what is called the gray zone. they do not have the illness but their grandchildren have a higher odds of being born with it.

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