Gene loss following whole genome duplication (WGD) is often biased,with one subgenome retaining more ancestral genes and the other sustaining more gene deletions.While bias toward the greater expression of gene copies on one subgenome can explain bias in gene loss,this raises the question to what drives differences in gene expression levels between subgenomes.Differences in chromatin modifications and epigenetic markers between subgenomes in several model species are now being identified,providing an explanation for bias in gene expression between subgenomes.WGDs can be classified into duplications with higher,biased gene loss and bias in gene expression between subgenomes versus those with lower,unbiased rates of gene loss and an absence of detectable bias between subgenomes;however,the originally proposed link between these two classes and whether WGD results from an allo-or autopolyploid event is inconsistent with recent data from the allopolyploid Capsella bursa-pastoris.The gene balance hypothesis can explain bias in the functional categories of genes retained following WGD,the difference in gene loss rates between unbiased and biased WGDs,and how plant genomes have avoided being overrun with genes encoding dose-sensitive subunits of multiprotein complexes.Comparisons of gene expression pattes between retained transcription factor pairs in maize suggest the high degree of retention for WGD-derived pairs of transcription factors may instead be explained by the older duplication-degeneration-complementation model.