Sex Control

When considering a large population, approximately equal numbers of males and females are born in all common species of animals, although the proportion may vary from year to year in individual herds. In dairy animals, the female has a pair of similar sex chromosomes (called x chromosomes), whereas the male has a pair of unlike sex chromosomes (called X and Y chromosomes). The pairs of sex chromosomes separate when the germ cells are formed. Thus, the ovum or egg produced by female contains X chromosome, while the sperm of the bull are of two types, one-half containing the X chromosome and other half the Y chromosome. Because the egg and sperm are assumed to unite at random, half the progeny will be females (XX) and the other males (XY). Various studies have indicated that of each 100 dairy calves born, on average, 49 are heifer calves and 51 are bull calves. Obviously, some method of controlling the sex of offspring would have tremendous economic importance in the dairy field. Control of sex is currently possible. Predetermination of the sex of 6 to 12 day-old embryos and the separation of sperm cells containing X chromosomes from those containing Y chromosomes using flow cytometry is reality. The sexed semen is available in market for artificial insemination.

Sex-Sorting Technology:
Several attempts have been made to develop a method that efficiently separates semen into fractions containing higher concentrations of X- or Y-bearing sperm. These technologies include sex-specific antibodies, centrifugation, and flow cytometry. Of these attempts, the only method proven to be commercially viable is flow cytometry. This type of sorting was

first researched in the 1980s but yielded very low conception rates when sex-sorted semen was used fresh after pro­cessing. Work in the laboratory and field studies improved the results, and the first gender-selected calf using frozen semen was produced in 1999. Sexed semen became commercially available on a large scale in the United States in 2004.

The inefficiency of flow cytometry comes from its complexity and slow pace. The principle of this method relies on the fact that X-bearing (female) sperm contain 3.8 percent more DNA than Y-bearing (male) sperm. Before sorting, the sperm cells are stained with a fluorescent dye and then passed through the flow cytometer as drops of liquid containing a single

Schematic Flow Cytometry sort X bearing and Y bearing bovine sperm cells

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