Gender Preselection in Farm Animals

Lawrence A. Johnson, research physiologist, Animal Science Institute, Reproduction Laboratory, Beltsville Agricultural Research Center, Agricultural Research Service.

Every living being has a set of paired chromosomes, which carry all the genetic material necessary to maintain life and also to propagate new life.

All but one pair of chromosomes are called autosomes and carry genes for all the characteristics of the body, such as skin, hair and eye color, mature size, and body characteristics. The remaining pair are called sex chromosomes. They carry the genetic material that specifies gender. One sex chromosome is called X, the other Y.

A sperm from the male or an egg from the female contains one of each Pair of autosomes; in addition, in mammals the egg always contains an X chromosome, while the sperm always carries either an X or Y chromosome.

When a sperm and egg unite and the sperm carries the Y chromosome, the offspring is male (XY); however, if the sperm carries an X chromosome when it unites with the egg, the resulting offspring is female (XX).

In most animals, including humans, the ratio of males to females is 50:50. Because the determination of sex, or gender, takes place when a sperm fertilizes an egg, preselection of gender by selecting the sperm that fertilize eggs must be done before the sperm are used for insemination.

In all bird species, including turkeys and chickens, the female determines the sex of offspring. In birds, the sex chromosomes are exactly the same in all sperm, so poultry sperm cannot be manipulated to preselect the sex of offspring.

Purpose of Gender Preselection

Gender of animal offspring is important to livestock producers. Selection of gender by separating X-bearing sperm from Y-bearing sperm before semen is used in artificial insemination could give farmers the choice of sex of offspring.

Because the dairy farmer has little use for most bull calves, the use of sexed semen to produce only females would make milk production more efficient. Swine farmers would produce pork more efficiently if they were able to market only female swine because females grow faster than males.

In beef cattle and sheep breeds, the male grows at a faster rate than the female and hence is preferred for meat production.

In addition, the ability to specify male or female offspring should shorten the time required for genetic improvements, since desirable traits are often associated with one or the other parent. Planning the sex of cattle offspring is already practiced on a limited basis. This procedure consists of removing embryos from the cow, identifying their potential gender, and re-implanting only those of the desired gender. However, an ability to separate sperm into male-producing and female-producing groups before they are used for artificial insemination could enhance the overall value of offspring produced by embryo transfer.

History of Gender preselection

Interest in controlling the sex of offspring dates back at least to Hippocrates (460-377 B.C.). In the 20th century, particularly since 1950, many attempts have been made to determine differences between Y-bearing sperm and X-bearing sperm, particularly with regard to cattle. Most techniques that have been tested have been aimed at distinguishing subtle physical differences, such as swimming ability, size, shape, density, and weight of the sperm. Any such physical differences, however, are small, and the methods used to separate X- from Y-bearing sperm are not sufficiently precise to detect the differences.

Nevertheless, some entrepreneurs have tried to capitalize on the interest in controlling sex by selling so-called "sexed" semen. As one might expect, they fail to follow up to determine the outcome of live births from that supposedly "sexed" semen.

In short, no valid practical method exists today for separating a sample of livestock semen into X-bearing or Y-bearing sperm, and regardless of the claims, no practical method exists for even enriching a sample of livestock semen for either X- or Y-bearing sperm.

Sexing Semen by DNA Content

The only established and measurable difference between X and Y sperm that is known and has been proved to be scientifically valid is their difference in deoxyribonucleic acid (DNA) content. The X chromosome is larger and contains slightly more DNA than does the Y chromosome. The difference in total DNA between X-bearing sperm and Y-bearing sperm is 3.4 Percent in boar, 3.8 percent in bull, and 4.2 percent in ram sperm.

Flow cytometers are advanced cell sorters that use lasers to excite fluorescent dye to separate X and Y chromosome-bearing sperm.

The amount of DNA in a sperm cell, as in most body cells, is stable. Therefore, the DNA content of individual sperm can be monitored and used to differentiate X- and Y-bearing sperm.