I think your misconception is quite common so I decided to answer this question. Just a few coexisting possibilities: - New mutations, so they haven't been affected by selection. They happen really really often, most of them don't do anything, some of them give you a horrible disease, and a few might result in a slight improvement in a function. - New chromosomal defects are subjected to a higher degree of selection since they affect a huge bunch of genes, they mostly result in miscarriage, but in rarer cases, the defect is compatible with life, like in the case of Down syndrome. The genetic disease might be inherited if the mutation did not cause too severe symptoms in the parents: - in the case of so-called autosomal recessive diseases, the mutation does not cause a problem if the "pair" of the gene on the homologous chromosome is not mutated, meaning there will still be a healthy protein that can do all the work that is needed for the body. The problems start when both parents of a child have a mutated gene, some of their children might inherit none of the working genes, so they will have a disease. - in autosomal dominant diseases, when only one bad gene copy is enough to wreak havoc, they can be inherited from the parents if the symptoms only appear late in life. - in X-linked recessive diseases, the mutation is present on one of the genes found on the X sex chromosome, and won't cause trouble if there is a working copy on the other X chromosome - meaning, females won't have a problem since they have two X chromosomes, but their sons who inherited the bad copy only have this one nonfunctional one and will have a disease. ...and so on.