Selective Breeding

appaloosa, product of inbreeding:

Breeders of animals and plants in today's world are looking to produce organisms that will possess desirable characteristics, such as high crop yields, resistance to disease, high growth rate and many other phenotypical characteristics.

This result is usually accomplished by crossing two members of the same species which possess dominant alleles for particular genes, such as long life and quick metabolism in one organism crossed with another organism possessing genes for fast growth and high yield. Since both these organisms have dominant genes for these desirable characteristics, when they are crossed they will produce at least some offspring that will show ALL of these desirable characteristics. When such a cross occurs, the offspring is termed a hybrid, produced from two genetically sample hypothesis activitydissimilar parents which usually produces offspring with more desirable qualities.

The offspring are heterozygous, meaning the gene for each characteristic will possess one dominant and one recessive allele. Most professional breeders have a true breeding cross (i.e., AAbb with AAbb) so that they will produce a gene bank of these qualities that can be crossed with aaBB to produce heterozygous offspring. This way the dominant features are retained in the first breeding group and can be passed on to offspring. This process of selecting parents is called artificial selection or selective breeding.


Selective breeding programs that concentrate on a small subset of genes determining a limited group of desired characteristics often use a small number of individuals as the "founder population" for all the descendents. While considerable care is taken to produce individuals homozygous for the desired characteristics, there are many other genes that also end up being homozygous within the small group of inbred founders. Some of these genes have deleterious effects, and their adverse conditions tend to be disproportionately common in the members of populations derived from a small founder group. In larger populations, cross-breeding and natural hybridization result in heterozygotes in which a mutant, deleterious recessive allele is masked by a normal dominant allele.

Examples among dog breeds include canine cancer, heart disease, hip dysplasia, vision, and hearing anomalies. Ashkenazi Jews show a larger proportion of the incidence of Tay-Sachs disease and cystic fibrosis, among other conditions, than the general population. High rates of inbreeding or consanguineous marriage in certain Arab groups are thought to contribute to congenital malformations that are the second leading cause of infant mortality in Bahrain, Kuwait, Oman and Qatar and are the leading cause of infant mortality (40.3%) in the United Arab Emirates. The tracing of genealogies back to the groups of early humans that came out of Africa and then spread across the world also depends on the presence of particular mutations among all the members of the group and their descendents.

A Pact with the Devil

Artificial selection/selective breeding has resulted in an increase in the efficiency of the animals and plants we breed, such as increasing milk yield from cows by continuously breeding selected cows with one another to produce a hybrid.
Average yields of corn in the USA have increased from around 2.5 tons per hectare (t/ha) (40 bushels per acre) in 1900 to about 9.4 t/ha (150 bushels per acre) in 2001, an increase of 275% in this period of time. A wild jungle fowl might lay 30 eggs in a year, but today's hens can lay over 300 each. Producing polyploids has been a successful technique for many crops.

Unfortunately, many of the new breeds are highly dependent on human activity for the elevated levels of production. They require lots of fossil fuels, artificial fertilizers, irrigation, herbicides and pesticides. The impact on the environment is extraordinary. Furthermore, the treatment of some farm animals under industrial production greatly alters the quality of life for the domesticated species. The current population and its inexorable expansion rely on agricultural improvement to combat hunger and starvation. The gains that modern agriculture has achieved by using God's gift of genetics has come at a steep price.

Excerpted and adapted from: Selective Breeding, Ashkenazi Jewish Genetic Diseases, CTGA: the database for genetic disorders in Arab populations, Agriculture, Selective Breeding of Farm Animals, Ostrander, Elaine A. 2007. Genetics and the Shape of Dogs. American Scientist, 95(5):406-413. (see link at the bottom of the "Corn, Wheat, Goats, and Cows" page)
Click sample hypothesis test above for a sample test of hypotheses.