Humans have 23 pairs of chromosomes, one half of each pair inherited from each parent. This suggests that the male-determining region on the Y chromosome should be relatively small. Levin and Foster Karyology of the Cyprinodontoid fishes of the Mexican family Goodeidae.
Matsuda et al. For example, while having an XY format, Xiphophorus nezahualcoyotl and X. Variable predation regimes predict the evolution of sexual dimorphism in a population of threespine stickleback.
For example, it was originally reported that the threespine stickleback Gasterosteus aculeatus had no heteromorphic sex chromosomes, but later studies using fluorescence in situ hybridization revealed that the X and the Y can be distinguished Ross and Peichel Y-A fusion; closely related species L.
This could be due to environmental factors such as seasons and temperature. It has long been appreciated that sex chromosomes might play a special role in speciation. For example, sucrose synthase showed the expected proportional expression in 2X and 4X tissues, but its expression was three and six times higher, respectively, in 1X and 3X tissues.