With respect to the Merle gene and
ocular dysgenesis, these abnormalities are congenital, which means they are present at birth.
Merle
ocular dysgenesis also occurs, but since we know this is the result of breeding two merles together the source of the problem is obvious and easy to avoid.
Most common is probably merle
ocular dysgenesis — the complex of defects observed in dogs that have inherited two copies of the merle gene.