While the cause for the disparate numbers is unknown, it may arise because of an XO sex determination mechanism, where males (2n=17) lack the sex chromosome and therefore have one less chromosome than the female (2n=18).[27] [27] Radish (Raphanus sativus) 18 [21] Carrot (Daucus carota) 18 The genus Daucus includes around 25 species.