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Animal Breeding

Sex Selection

It would be extremely useful if a breeder were able to predetermine the sex of each embryo produced, because in many cases one sex is preferred. For instance, in a herd of dairy cows or a flock of laying hens, females are the only commercially useful sex. When the owner of a dairy herd has inseminated a cow at some expense, this issue becomes more crucial. In some cases, an animal is being bred specifically for use as breeding stock; in this case, it is far more useful to produce a male that can be bred with multiple females than a female, which can only produce a limited number of offspring.

Whether or not an animal is male or female is determined by its sex chromosomes, which are called X and Y chromosomes. An animal with two X chromosomes will develop into a female, while an animal with one X and one Y chromosome will become a male. In mammals, the sex of the offspring is almost always determined by the male parent, because the female can only donate an X chromosome, and it is the presence or absence of the Y chromosome that causes maleness (this is not true in, for instance, birds; in that case it is the female who has two different sex chromosomes). The problem in sex selection is to separate the Y-carrying sperm from the X-carrying sperm. Thus far, attempts to do so have been largely unsuccessful or too expensive for commercial application, but the economic advantages make this an area of intense research, and it is quite probable that an efficient and cost-effective method will soon be developed.



Babiuk, Lorne A., and John J. Phillips, eds. Animal Biotechnology. New York: Pergamon Press, 1989.

Dawkins, Richard. The Selfish Gene. New York: Oxford University Press, 1989.

Hill, William G. and Trudy F.C. Mackay, eds. Evolution and Animal Breeding. Wallingford, UK: CAB International, 1989.


Blasco, A. "The Bayesian Controversy in Animal Breeding." Journal of Animal Science 79, no.8 (2002): 2023-2046.

Sarah A. de Forest



—Reproduction from two male parents.

Artificial insemination

—The artificial introduction of semen from a male with desirable traits into females of the species to produce pregnancy.


—An embryo at that stage of development in which the cells have differentiated to form embryonic and extra-embryonic tissue. The blastocyst resembles a sphere with the extra-embryonic tissue making up the surface of the sphere and the future embryonic tissue appearing as a cluster of cells inside the sphere.


—The embryonic cell during the first cellular divisions, before differentiation has occurred.


—An animal (or embryo) formed from two distinct cellular lineages that do not mingle in the cells of the animal, so that some cells contain genetic information from one lineage and some from the other.


—The production of multiple genetically identical embryos or zygotes.

Crossing over

—In meiosis, a process in which adjacent chromosomes exchange pieces of genetic information.


—Nucleus or cell containing two copies of each chromosome, generated by fusion of two haploid nuclei.

Extra-embryonic tissue

—That part of the developmental tissue that does not form the embryo.


—The unborn or unhatched animal during the latter stages of development.


—Half a diploid set of chromosomes; the genetic information from one parent.

Hybrid vigor

—The quality of increased health and fertility (superior to either parent) usually produced when two different genetic strains are crossed.


—Reproduction from two female parents.


—The organ to which a fetus is attached by the umbilical cord in the womb. It provides nutrients for the fetus.


—Embryonic cells able to produce an entire fetus.


—Cells or species that have undergone genetic engineering.


—Having only one genetic parent.

Additional topics

Science EncyclopediaScience & Philosophy: Ambiguity - Ambiguity to Anticolonialism in Middle East - Ottoman Empire And The Mandate SystemAnimal Breeding - The Genetic Basis Of Animal Breeding, Economic Considerations, Modern Methods In Biotechnology, Artificial Insemination