Human race is not destroying its DNA at current pace for long. Deterioration may be more pronounced in a few generations. And as to why the child is not formed

Human race is not destroying its DNA at current pace for long. Deterioration may be more pronounced in a few generations. And as to why the child is not formed from bad copies of worn-off cells of a mature parent - it's been some time since I read the book but I think that there was some reset mentioned when a whole new person is formed inside another organism. Fetal stem cells are known to have superior properties.