The amount of black and white dots are supposed to be equal. In that case there is currently 12 white dots and 18 black dots. The pattern is that there are (including the missing one) 8 different set of dots each with a different number of dots from 1 to 8. The missing one must be 6 dots long. To make the amount of black and white dots equal, you need 6 white dots. So the missing one will be a set of 6 white dots.
It will be vertical because dots with even number of dots are vertical and an odd number of dots is horizontal. (Credit goes to @CarlLöndahl for this one)
The sets of dots interchange in vertical/horizontal. Number of dots in each set increases by one (so the set containing six dots is missing). The number of black and white dots are of equal amount (18 of each, this seems to be the most logical answer).