Hypothetical example showing possible scenarios for tissue-specific expression of two duplicates. A black box depicts expression in a particular tissue, whereas a white box represents no expression in that particular tissue. Following duplication of a gene that is expressed in six different tissues, the two copies can (a) both remain expressed in all six tissues (redundancy), (b) diverge asymmetrically, where one gene is expressed in only a small subset of the tissues, while its duplicate remains expressed in the original six tissues, or (c) diverge symmetrically, where tissue-specific expression is complementarily lost between both duplicates. The absolute number of tissues in which a gene is expressed is six for both duplicates in (a) and for the second duplicate in (b), one for the first duplicate in (b) and three for both duplicates in (c). The total number of tissues in which the pair is expressed is 6 in all three cases. The relative number is the fraction of the previous two, and is 1 for the two genes in (a) and for the second duplicate in (b), 0.17 for the first duplicate in (b) and 0.5 for both duplicates in (c).