  We propose a schematic model of nucleons moving in spin--orbit partner
levels, $j=l\pm{\sfrac12}$, to explain Gamow--Teller and two-nucleon transfer
data in $N=Z$ nuclei above $^{40}$Ca. Use of the $LS$ coupling scheme provides
a more transparent approach to interpret the structure and reaction data.We
apply the model to the analysis of charge-exchange,
$^{42}$Ca($^3$He,t)$^{42}$Sc, and np-transfer, $^{40}$Ca($^3$He,p)$^{42}$Sc,
reactions data to define the elementary modes of excitation in terms of both
isovector and isoscalar pairs, whose properties can be determined by adjusting
the parameters of the model (spin--orbit splitting, isovector pairing strength
and quadrupole matrix element) to the available data. The overall agreement
with experiment suggests that the approach captures the main physics
ingredients and provides the basis for a boson approximation that can be
extended to heavier nuclei. Our analysis also reveals that the SU(4)-symmetry
limit is not realized in $^{42}$Sc.
