  We examine the effect of small amounts of magnetic substituents in the $A$
sites of the frustrated spinels MgCr$_2$O$_4$ and ZnCr$_2$O$_4$. Specifically
we look for the effects of spin and lattice disorder on structural changes
accompanying magnetic ordering in these compounds. Substitution of Co$^{2+}$ on
the non-magnetic Zn$^{2+}$ site in Zn$_{1-x}$Co$_{x}$Cr$_2$O$_4$ where
0\,$<$\,$x$\,$\leq$\,0.2 completely suppresses the spin-Jahn-Teller distortion
of ZnCr$_2$O$_4$ although these systems remain frustrated, and magnetic
ordering occurs at very low temperatures of $T$\,$<$\,20\,K. On the other hand,
the substitution of Jahn-Teller active Cu$^{2+}$ for Mg$^{2+}$ and Zn$^{2+}$ in
Mg$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ and Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ where
0\,$<$\,$x$\,$\leq$\,0.2 induce Jahn-Teller ordering at temperatures well above
the N\'eel temperatures of these solid solutions, and yet spin interactions
remain frustrated with long-range magnetic ordering occurring below 20\,K
without any further lattice distortion. The Jahn-Teller distorted solid
solutions Mg$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ and Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ adopt
the orthorhombic $Fddd$ structure of ferrimagnetic CuCr$_2$O$_4$. Total neutron
scattering studies of Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ suggest that there are
local $A$O$_4$ distortions in these Cu$^{2+}$-containing solid solutions at
room temperature and that these distortions become cooperative when average
structure distortions occur. Magnetism evolves from compensated
antiferromagnetism in MgCr$_2$O$_4$ and ZnCr$_2$O$_4$ to uncompensated
antiferromagnetism with substitution of magnetic cations on the non-magnetic
cation sites of these frustrated compounds.
