Intragenic line
It is very difficult to allot the feature of an individual to a Gène in particular because, very often, a character is determined by several genes which depend on the whole of genes of the individual (“genetic bottom”). And one cannot compare an individual with another because them Génome is not the same one.
In a intragenic line all the individuals share same genetic inheritance, which makes it possible to easily compare them between them. Thus when a Gène is replaced by another and that one can observe a new characteristic, it is thus possible to allot the modification to this new Gène. Without forgetting however that there can be interaction of several genes to give different characters, and that in certain cases there can even be a Réversion genetic mutation.