That is a very good question. The quick and dirty answer is that there are many genes and many different ways for a gene to go wrong. And once a gene goes wrong, it can be passed on to the babies.

Unfortunately, I cannot give a better answer without going into a lot of background material that cannot go in these forums. So I am going to give you a link to a genetics home reference publication on the web named Help Me Understand Genetics. After you go over the first three entries in the Table of Contents (Cells and DNA, How Genes Work, and Mutations and Health), you will have a lot of the background.

Paul Hollander
Help Me Understand Genetics

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