From the detailed discussions in the Gemara – mainly in Massekhet Yevamot– it appears that an aylonit suffers from a genetic defect that does not allow her to have children. This is a different categorization than an akara – a barren woman – whose physical and sexual development is ordinarily normal, but cannot have children because of some other deficiency or impediment. From those descriptions it appears that an aylonit can be recognized by certain unique physical traits, including a lack of secondary sex characteristics, like pubic hairs. Furthermore, it appears from the Gemara that there are different types of aylonit, ranging from women who have an overabundance of male hormones to those who suffer from Turner syndrome, where only one X chromosome is present and fully functioning. Approximately 98% of all fetuses with Turner syndrome spontaneously abort; the incidence of Turner syndrome in live female births is believed to be about 1 in 2500.
Within Jewish law there are many discussions about the status of an aylonit, mainly because of the lack of secondary female sex characteristics and because they develop at a relatively advanced age. Thus we find questions about when an aylonit is considered to have reached the age of adulthood, which halakha ordinarily defines as physical maturity.
From our Gemara, the exception of aylonit appears to be based on the fact that according to the Tanna Kamma a man is not permitted to marry an aylonit since she will not be able to bear him children. The Talmud Yerushalmi, however, suggests that the source for this law is the passage (Bamidbar 5:28) that promises that a sota who is tested by the “bitter water” and found innocent will become pregnant – a promise that applies only to women who can become pregnant.