The Food and Drug Administration on Friday cleared a first-of-a-kind blood test that can help diagnose mental disabilities in babies by analyzing their genetic code. The laboratory test from Affymetrix detects variations in patients’ chromosomes that are linked to Down syndrome, DiGeorge syndrome and other developmental disorders. About 2 to 3 percent of U.S. children have some sort of intellectual disability, according to the National Institutes of Health. …