Actually, Male/female twins are the most common type in DZ twins (two eggs released simultaneously), which makes sense as the probability of a fetus being a male or a female is 50/50 (So the probability [which the statistics match perfectly] of two fetuses being female or male equals out to 25% FF, 25% MM and 50% FM/MF {as theres no way to differentiate between MF and FM}). BUT theres also MZ (one egg split) which only produces MM and FF twins (at, of course, a frequency of 50%MM and 50%FF) and occurs (in asian populations) at the same frequency as DZ; THEREFORE making MF/FM twins actually the least frequent, occurring only in 25% of twins, when compared to FF and MM (both, individually occurring in 37.5% of twins).