Your Genome is a Mess
Each of us also is likely to be packing around a number of dangerous recessive alleles - gene copies which don't do much damage unless you inherit a copy from each parent. These are a good reason to avoid excessive inbreeding. Overly inbred populations like Ashkenazi Jews, many Arabs, and some Indian Jati have a heavy load of genetic disease in consequence.
Gene copying in humans is a rather precise process, with only about one mutation in ten billion base pair copies, but with 3 billion base pairs to be copied in every cell division and trillions of cells, a lot of mutation happens. Every child is packing a number of mutations different from her parents.
Alu units are only the most common of a whole host of similarly selfish DNA inserts. Add in all the mutations with every generation and you might wonder how we function at all.
The answer is redundancy. In addition to the two alleles of almost every gene we carry, there is a lot of other redundancy in the genome. Many genes exist in more than one copy in the genome. Other damaged genes still produce proteins that function after a fashion. We are a bit like that old car that still runs even though one cylinder only works 1/4 the time and it can't shift into reverse. Other genes only produce Huntington's disease, cancer, heart disease, or dementia in middle or old age and so don't affect reproduction.
Understanding of the genome now gives us tools for preventing some of these problems. Genetic testing can reveal whether you and your intended share a fatal or other serious recessive, and should probably be considered if you are in a vulnerable population. For IVF, pre-implantation testing of embryos can reveal some dangerous alleles.
Finally, the era of genetic editing is upon us. It is possible in some cases to edit and replace faulty genes. Plants and animals have already been tested and produced. Editing human germ line cells is currently strongly discouraged, and so far as is known, has never been used to produce a human baby, but editing of somatic cells is busily underway, and the future rushes onward.
One animal example of interest to many was the editing in mice of a cell type that tends to break down bone in old age. In mice with osteoporosis, this edit not only halted the osteoporosis but significantly slowed aging and extended life.