Fig. 1.From: Sex-limited chromosomes and non-reproductive traitsThere are three sex chromosome systems. Two systems exist in diploid organisms—XY (e.g. mammals), where male is the heterogametic sex (A), and ZW (e.g. birds), where female is the heterogametic sex (B). While in haploid organisms there is the UV system (e.g. some mosses), where the female gametophyte is U and the male is V (C)Back to article page