Haldane's rule, which states that the heterogametic sex (XY or ZW females) fares more poorly in interspecific hybrids, is generally attributed to absence of one of the two species' X/Z chromosomes. However, Haldane's rule is also observed in mouse placentas despite paternal X silencing. This pattern could reflect Y chromosomes having evolved to promote growth due to maternal-paternal conflict. If so, balanced sex investment arises from a complex intra- and intergenomic duel.
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