Make the link (in thinking) with the gene for Barring which is a temporary inhibitor of pigment production :
B/B = (maschio) barrato in tonalità CHIARA because B/B give 2 signals to inhibit the pigment production
B/b+ = (maschio) barrato in tonalità SCURA because B/b+ give only 1 signal to inhibit the pigment production
now :
as Bl/bl+ give 1 signal to dilute the black pigment
and lav/lav give a different signal to dilute the black pigment
so :
Bl/bl+ and lav/lav together giving 2 different signals to dilute the black pigment resulting in a tonalità troppo CHIARA = slavature