We have examined by transmission electron microscopy In0.48Al0.52As layers grown by molecular beam epitaxy on (100) InP substrates. The characterization of the nature of the crystal defects (dislocations and stacking faults) observed in the layers, and the study of their asymmetric distribution in the [011] directions point towards the differentiation of two mechanisms for defect nucleation and therefore towards two models explaining the observed anisotropy depending on the growth temperature (T-g). Then, for low T-g (300 degrees C) we have observed a {1 $($) over bar$$ 11}-microtwinned structure whose origin has been explained taking into account the limited kinetics of III atoms during the growth and the differences in the propagation speed of the surface steps during the molecular beam epitaxy growth. For growth temperatures in the 440-530 degrees C range, the defect densities decrease monotonically, being threading dislocations and stacking faults on the (1 $($) over bar$$ 11) and (11 $$($) over bar 1) planes the prevalent type of defects. The asymmetry of fault densities between the {1 $($) over bar$$ 11} and {1 ($) over bar 1($) over bar 1} planes have been discussed on the basis of the different mobilities of the partial dislocations bordering the faults.