These sexes are confusing. So there are two female like ones and two male ones, and they need to exchange certain things between them in order to procreate.
Unlike many other hominids, Andorian sexual organs are not external. [
The Next Generation: The Devil's Heart (Pocket, 1993) by Carmen Carter.] Four partners are usually required for an Andorian marriage. [TNG episode:
Data's Day.]
The post-series DS9 Relaunch novels do give some brief explanations:
thaan = male analog
zhen = female analog/birth mother
chan = androgynous male analog
shen = androgynous female analog.
As the unique Andorian biology necessitates certain "expectations", it is considered unusual that an Andorian has not married by the age of twenty-three. The four Andorian genders can be signified by prefixes used with the surname, such as
zh' (birth mother) and
ch'. [
Deep Space Nine: Avatar duology (Pocket, 2001) by SD Perry.]
During the conception of an Andorian child, the
chan adds his gametes to those of the
shen, which have already been fertilized by the
thaan. The zygote is then implanted into the
zhen's pouch. [
Unity (Pocket, 2003) by SD Perry.]