Critically, however, freemartins are a form of microchimerism. They literally have cells with Y chromosomes in their bodies, received from the male twin through

Critically, however, freemartins are a form of microchimerism. They literally have cells with Y chromosomes in their bodies, received from the male twin through the shared chorion. You're correct that there is a shared mechanism - they receive testosterone and anti-Müllerian hormone through this link, which are primarily resonsible for masculinizing the freemartin. In contrast, this mechanism is thought to be only testosterone exposure in utero. A better comparison, then, might be intra-uterine position in mice [1]. This is similarly a pure testosterone-exposure mechanism (no microchimerism). Briefly, mouse pups are arranged in linear fashion within the uterine horn, and so a given (non-end) female pup has 0, 1, or 2 males adjacent to her. There are clear physiological and behavioral differences in these 0M, 1M and 2M mice, corresponding to the level of testosterone exposure. [1]: https://en.wikipedia.org/wiki/Prenatal_testosterone_transfer#Mice https://en.wikipedia.org/wiki/Prenatal_testosterone_transfer...