Fig. 1: Simplified chromosome cycle of dark-winged fungus gnats (Sciaridae), showing inheritance of the X chromosomes.
From: Maternal inheritance of primary sex ratios in the dark-winged fungus gnat Lycoriella ingenua
XXX zygotes form from the fusion of XX sperm and X eggs. Embryos that undergo elimination of one X chromosome develop into females (XX), those that lose two develop into males (X0). The first male meiotic division involves elimination of all paternally-inherited autosomes; the second division involves nondisjunction of the maternal X. As a result, males always produce sperm with two copies of the X.
