Genetic conditions come from the information within the cells of each parent that combine to form a new life. This is also how eye and hair color, for example, are determined. Genes in cells hold an organism’s DNA, which is a like a blueprint for all the body parts and systems. Sometimes there are mistakes or mutations in these genes that are passed from mother to child or father to child when they are transferred and/or combined. These mistakes may cause problems in the cells function. Different genes control different types of cell functions like protein formation, activity, cell death etc. It really depends on the gene involved, type of cell, and how large the genetic mutation or mistake is.
I hope this starts to answer your question. If you have a follow-up specific condition you want to ask about let me know.