The Mishna on today’s daf introduces the category of an aylonit. An aylonit is a woman whose physical makeup will not allow her to have children (the term stems from the word ayil – a male ram – alluding to her lack of femininity). An aylonit does not become a yevama because the point of yibum, as clearly stated in the Torah, is to bear children in order to keep the dead brother “alive” (see 25:6, which states that the first child to be born will be named for the late brother).
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 akarah – a barren woman – whose physical and sexual development is ordinarily normal, but who 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.