The roles of fA and fB swap if fA≥fB. From this we can see that in order for a high positive correlation to be possible we need to have fA≈fB, and for a high negative correlation we require fA and fB to both be large.1 A correlation in either direction will suffice for the marker to be a good surrogate. Thus, we can conclude that in situations where one of the SNPs is rare (either the marker or the causal SNP), the ability to detect associations will be impaired unless the other SNP is also rare and highly correlated.
