A method is described for treating a specimen of semen containing sperm cells to increase the relative number of sperm cells of a desired sex type in a treated specimen to increase the potential for conceiving an offspring of the desired sex. A specimen of semen is treated after a predetermined time to increase the relative ability of a at least a portion of the semen to conceive an offspring of the desired sex. The treatment preferably comprises contacting the sperm cells with an agent that preferentially effects sperm cells of a selected sex type. In some preferred embodiments, the semen is separated into two components. A first component has a higher number of sperm of the desired sex type than sperm of a non desired sex type and a second component has a higher number of sperm of the non desired sex type relative to sperm of the desired sex type. In one embodiment, the separating step is performed after a predetermined percent of the sperm cells exhibit a punctate pattern, which is capable of determination by labeling the sperm cells with Koo antibody and determining the percent of cells labeled. In another embodiment, the separating step is performed after waiting for a time determined by locating a maximum in the curve obtained by plotting percent female cells determined by FISH against percent Koo positive cells, determining the time at which the maximum percent female cells occurs, and beginning the following step no earlier than about one hour before the time of the maximum percent female cells. In yet another embodiment, the separating step is performed after waiting for a period of time between about 2 hours and about 24 hours after collection of the ejaculate. Preferably, in the separating step, the sperm is contacted with a cell binding agent, permitting the sperm of the non desired sex type to preferentially bind to the cell binding agent, and the cell binding agent with preferentially bound sperm of the non desired sex type is separated from non bound sperm.
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