Why are genetic diseases inherited?

A genetic disease means that the condition is due to one's DNA. That is, the DNA in every cell of your body (excluding blood cells) has the genetic makeup that lends to the disease, and hence cannot be changed. Genetic diseases arise due to abnormalities or mutations that occur during or before conception. For example, the gamete that gives rise to the offspring may have chromosomal abnormalities when it was produced during meiosis. Similarly, spontaneous mutations can occur during DNA replication or other cellular processes. Some genetic diseases are caused by faulty mitochondria; that is, mutations in the DNA of mitochondria lead to life long issues. Recall that the mitochondria is responsible for creating cellular energy, hence it is important for all of life's processes. Nevertheless, when mutations occur in critical genes involved in normal cellular functions or when they disrupt the balance of gene expression, they can lead to genetic diseases.

Now to answer the question... Genetic diseases are inherited because they are inscribed into our DNA, and thus they get passed on during sexual reproduction. If a genetic disease is inherited, it is because either the offspring's father or mother passed it on during meiosis. To reduce the chance of your offspring inheriting the faulty gene(s), you need to select for a partner in which you are sure doesn't have the condition and is not a carrier. However, if the genetic disease you have is a dominant hereditary disorder, then a single faulty allele will cause your offspring to have it too.