What is selfing?
As the title implies, the main drawback to selfing cannabis plants is that you loose the male portion of your population, making future crosses difficult. Some think that by selfing a plant, all the offspring will turn out just like mom. That is only true if mom is true breeding for all the traits you are interested in. Otherwise, her offspring will show two phenotypes for every trait that she is not true breeding.
There are two basic models for selfing a plant such as cannabis the first one being where the plant is homozygous for the trait in question. Let's assume again that pineapple flavour is controlled by the recessive gene pp. If we self the plant we fill get the following S1 cross.
S1 cross = pp x pp = pp + pp + pp + pp or 100% pineapple flavoured female offspring. But no matching males
The other likely possibility is that special individual heterozygous dominant for the pineapple flavour. In this case P will indicate for pineapple flavour and the S1 cross will be:
S1 cross = Pp x Pp = PP + Pp + Pp + pp, our familiar 1:2:1 mendelian ratio.
In this second example only 75% of the offspring will have pineapple flavour and the frequency of the P gene will only be 50%, a far cry from 100% or true breeding. From here on, this isn't much different from a half sib cross involving regular inbreeding or backcrossing. It will take a few generations to achieve something close to true breeding, but as with backcrossing, as long as we use the P1 mom in the crosses (selfing in this case), we will never achieve a true breeding population.