Complex sentence: The missing heritability is only missing if the reason for the heritability of a trait across many cultures/ecologies or geographic regions is the same, that is, only if the same alleles are effecting the observed heritability.
Split into simpler sentences: The missing heritability is only missing if the reason for the heritability of a trait across place and cultures is the same. That is, if the same alleles are effecting the observed heritability.