I AM 37FEMALE AND HAVE A 2 YEAR OLD AND

I AM 37FEMALE AND HAVE A 2 YEAR OLD AND 8 MONDSH OLD. I HAVE NOTICED OVER THE PAST FEW YEARS THAT THE SKIN ON MY ELBOWS KNEES FINGERS AND DTOES IS GETTING SO DARK. IT IS EMBARRASSING AND OTHER PEOPLE ASK ME WHAT I HAVE GOTTEN IN DTO, THIN ITS DIRTINESS. I HAD 3 BIOPSIES DONE IN THESE AREAS AND SHOWED CONSISTENT WITH ACANTHUSES NIGRA ALBICANS. BUT I DON'T HAVE THE RETICULAR PATTERN AND IN AREAS IN ARMPITS AND BENECK AND GROIN , THE WAY THIS CONDITION IS USUALLY DESCRIBED. DOCTOR PRESCRIBED MINOCYCLINE. DO YOU HAVE ANOTHER EXPLANATION FOR WHY A CAUCASIAN WOMAN WOULD HAVE THE DARK AREAS ?