  A modified generalized Chaplygin gas (MGCG) is considered as the unified dark
matter-dark energy revisited. The character of MGCG is endued with the dual
role, which behaves as matter at early times and as an quiessence dark energy
at late times. The equation of state for MGCG is
$p=-\alpha\rho/(1+\alpha)-\vartheta(z)\rho^{-\alpha}/(1+\alpha) $, where
$\vartheta(z)=-[\rho_{0c}(1+z)^{3}]^{(1+\alpha)}(1-\Omega_{0B})^{\alpha}\{\alpha\Omega_{0DM}+
\Omega_{0DE}[\omega_{DE}+\alpha(1+\omega_{DE})](1+z)^{3\omega_{DE}(1+\alpha)}\}$.
Some cosmological quantities, such as the densities of different components of
the universe $\Omega_{i}$ ($i$ respectively denotes baryons, dark matter and
dark energy) and the deceleration parameter $q$, are obtained. The present
deceleration parameter $q_{0}$, the transition redshift $z_{T}$ and the
redshift $z_{eq}$, which describes the epoch when the densities in dark matter
and dark energy are equal, are also calculated. To distinguish MGCG from
others, we then apply the Statefinder diagnostic. Later on, the parameters
($\alpha$ and $\omega_{DE}$) of MGCG are constrained by combination of the
sound speed $c^{2}_{s}$, the age of the universe $t_{0}$, the growth factor $m$
and the bias parameter $b$. It yields
$\alpha=-3.07^{+5.66}_{-4.98}\times10^{-2}$ and
$\omega_{DE}=-1.05^{+0.06}_{-0.11}$. Through the analysis of the growth of
density perturbations for MGCG, it is found that the energy will transfer from
dark matter to dark energy which reach equal at $z_{eq}\sim 0.48$ and the
density fluctuations start deviating from the linear behavior at $z\sim 0.25$
caused by the dominance of dark energy.
