Sex and reproduction: a theoretical analysis

Sex in some organisms shows a continuous spectrum. Sometimes sexual differences in an individual’s organs are contradictory and determined by physiological, pathological and environmental factors, leading to debate over the definitions of sex in individuals. This study indicates that the essence of sex during reproduction distinguishes between reproductive subjects and that the unit of reproduction is not the individual but the reproductive cell. To properly supplement the existing definitions of sex, a definition of sex in reproduction in which only reproductive cells have sexes was herein addressed, focusing on material contributions to reproduction and the formation of zygotes by reproductive cells. Equations to describe sexes are proposed, as well as equations to make this only reproductive-cell sex definition compatible with the individual sex definition based on the quantitative proportions of different reproductive cells. The explanation of sex determination, sex differences and sex competition based on the definition of only reproductive-cell sex in reproduction is discussed.