  We present a comparative study of the high-spin ${11/2}^-$, ${10}^+$ and
${27/2}^-$ nuclear isomers, observed commonly in the full semi-magic $Z=50$
isotopic and $N=82$ isotonic chains, which are found to display nearly
identical systematics in excitation energy and half-lives. A constant and
particle number independent energy gap of $\sim$ 4 MeV between the $0^+$ ground
states and ${10}^+$ isomers and, the ${11/2}^-$ and ${27/2}^-$ isomers exists
before the mid-shell, which declines near the mid-shell, and then becomes
constant at $\sim$ 3 MeV after the mid-shell. Large scale shell model
calculations are carried out for the full chains of Sn-isotopes and $N=82$
isotones, which reproduce the observed energy systematics reasonably well. The
role of seniority, generalized seniority and alignment in the high-spin
isomeric states as well as in the states populating the high spin isomers has
been discussed. The empirical features suggest that the generalized seniority,
known to be valid in the low lying states of even-even nuclei before the
mid-shell, remains good up to the ${10}^+$ isomeric states; we further extend
this interpretation to the odd-A nuclei before the mid-shell, particularly the
${27/2}^-$ isomers. The seniority becomes good after the mid-shell. The change
in energy gap around the mid-shell, therefore, marks a transition from the
generalized seniority regime to the seniority regime. From the alignment
considerations, the high-spin isomers after the mid-shell emerge as maximally
aligned decoupled states. We also note that the similar features of the
isomeric half-lives in both the chains may be understood in terms of their
similar seniorities and configurations. An overview of the $0^+$ to ${10}^+$
yrast states in the full chain of even-even Sn-isotopes has also been
presented. We also make some predictions of possible new isomers based on the
empirical systematics.
