  We present updated chemical evolution models of two dwarf spheroidal galaxies
(Sculptor and Carina) and the first detailed chemical evolution models of two
ultra-faint dwarfs (Hercules and Bo\"otes I). Our results suggest that the
dwarf spheroidals evolve with a low efficiency of star formation, confirming
previous results, and the ultra-faint dwarfs with an even lower one. Under
these assumptions, we can reproduce the stellar metallicity distribution
function, the $[\alpha/Fe]$ vs. $[Fe/H]$ abundance patterns and the total
stellar and gas masses observed at the present time in these objects. In
particular, for the ultra-faint dwarfs we assume a strong initial burst of star
formation, with the mass of the system being already in place at early times.
On the other hand, for the classical dwarf spheroidals the agreement with the
data is found by assuming the star formation histories suggested by the
Color-Magnitude diagrams and a longer time-scale of formation via gas infall.
We find that all these galaxies should experience galactic winds, starting in
all cases before $1$ Gyr from the beginning of their evolution. From comparison
with Galaxy data, we conclude that it is unlikely that the ultra-faint dwarfs
have been the building blocks of the whole Galactic halo, although more data
are necessary before drawing firm conclusions.
