WASHINGTON — Don't expect designer babies any time soon — but a major new ethics report leaves open the possibility of one day altering human heredity to fight genetic diseases, with stringent oversight, using new tools that precisely edit genes inside living cells. What's called genome editing already is transforming biological research, and being used to develop treatments for patients struggling with a range of diseases. The science is nowhere near ready for a huge next step that raises ethical questions — altering sperm, eggs or embryos so that babies don't inherit a disease that runs in the family, says a report Tuesday from the National Academy of Sciences and National Academy of Medicine.