Doctors at a fertility center near Washington report that they can substantially stack the odds that a couple using artificial insemination can have a baby of the sex they choose. The method, developed by the Genetics & IVF Institute in Fairfax, Virginia, involves sorting sperm by the amount of DNA they contain and then using them for insemination. It capitalizes on the fact tha there is only one difference between sperm that carry the Y chromosome, which produces males, and those with an X-chromosome, which produces females: sperm with a Y-chromosome have about 2.8 percent less genetic material.