Re: At what point in development would a YY embryo be aborted?

Firstly, we need to restrict the scope of the question to mammals, since sex determination varies tremendously throughout Animalia (even within Vertebrata). For mammals, the X chromosome carries multiple essential genes not found on the Y chromosome, so that an embryo without an X chromosome, i.e. with a YY or Y0 karyotype, would not be expected to last long. In fact, I found an older article on MEDLINE that used diploid sperm injected into enucleated oocytes to study "diandric diploid embryos" in mice. According to their numbers, the YY embryos never implanted, suggesting that they did not complete cleavage - during early cleavage, most cellular activity is controled by residual maternal factors, so most phenotypes won't appear until late cleavage. After consulting several references, it appears that most (if not all) people with Turner's Syndrome (X0 karyotype) exhibit sex-linked traits common to their mothers, suggesting that X-less oocytes either can't develop, or are lost prior to fertilization. This makes the natural occurence of YY zygotes all but impossible.