Some zebrafish genes appear to lack an ortholog in the human genome and 
researchers often call them "novel" genes. The origin of many so-called "novel" 
genes becomes apparent when considered in the context of genome duplication 
events that occurred during evolution of the phylum Chordata, including two 
rounds at about the origin of the subphylum Vertebrata (R1 and R2) and one round 
before the teleost radiation (R3). Ohnologs are paralogs stemming from such 
genome duplication events, and some zebrafish genes said to be "novel" are more 
appropriately interpreted as "ohnologs gone missing", cases in which ohnologs 
are preserved differentially in different evolutionary lineages. Here we 
consider ohnologs present in the zebrafish genome but absent from the human 
genome. Reasonable hypotheses are that lineage-specific loss of ohnologs can 
play a role in establishing lineage divergence and in the origin of 
developmental innovations. How does the evolution of ohnologs differ from the 
evolution of gene duplicates arising from other mechanisms, such as tandem 
duplication or retrotransposition? To what extent do different major vertebrate 
lineages or different teleost lineages differ in ohnolog content? What roles do 
differences in ohnolog content play in the origin of developmental mechanisms 
that differ among lineages? This review explores these questions.