Several human diseases are due to genes in the human genome; how is it that genetic diseases have come about?

Sherita Gallati: It is a common misconception that we have genes for diseases. The truth is, we have the disease because of a defect in a normal gene that has some function in our body. The defect may be because of mutation or because of heredity.

Star Gollnick: Also, many genetic diseases that persist in populations are those that occur AFTER our optimal reproductive years. For example, if I get cancer, heart disease, or diabetes when I'm 30 it really doesn't matter because my children will still be predisposed to that disease because it hasn't affected my reproductive fitness and will remain in the population.

Joni Ziak: In some cases it was just a random mutation of the genes that was passed down to further generation, maybe with further mutation as time passed. Nature is not perfect and thing will go wrong sometimes.