Selective breeding entails the breeding of organisms with desirable characteristics.
Scientists from the past harnessing the knowledge of genetics has resulted in many scientific breakthroughs and uses of this knowledge. Most notably, Gregor Mendel’s studies into Monohybrid and Dihybrid crossing and Charles Darwin’s study of evolution and natural selection meant that humans have learned to actively manipulate the phenotype of offspring by selective breeding in animals and plants.
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 that will benefit the organism and species in the long term.
This is usually done 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 dissimilar parents which usually produces offspring with more desirable qualities. Breeders continuously track which characteristics are possessed by each organism so when the breeding season comes once again, they can selectively breed the organisms to produce more favorable qualities in the offspring.
The offspring will become heterozygous, meaning the allele for each characteristic will possess one dominant and one recessive gene. 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 in the second instance.
This process of selecting parents is called artificial selection or selective breeding and poses no threat to nature from man manipulating the course of nature. It has allowed our species to increase 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.
However, while it is an advantage both to the species and to humans to produce these desirable qualities that may benefit the organisms in question, continuous inbreeding and selective breeding of particular genes runs the risk of losing some of the other genes from the gene pool altogether, which is irreversible. This is called inbreeding depression, where the exclusivity of the advantageous genes mean that some other less desirable genes are phased out. In the long term, it is more advantageous for organisms to remain heterozygous.
Genetic diversity means the gene pool of a species is prepared for a wide range of scenarios such as food shortage or an epidemic of disease. Some genes in some organisms may provide the organism with immunity against the disease or an ability to go long periods of time without food. If continuous inbreeding has occurred in a species, some of these genes may have been phased out due to the breeder wanting other more desirable genes to be present in their crop….
Genetic diversity in the long term is reduced, because many organisms end up with similar genomes due to breeding with each other constantly. In normal circumstances, this process would be random and would produce more variable offspring
With the above facts in hand, breeders need to produce more heterozygous offspring to ensure the long term welfare of the species they are breeding and their livelihood. The most important thing here is to preserve the genetic diversity of a species, and preferably keep the gene pool of a species as diverse as possible.
Humans have realized the above dangers, and instead of harnessing and exhausting nature reserves, we have learned to preserve their genetic information for their long term survival and our own well-being. One species becoming extinct can knock the balance of an ecosystem and have a detrimental knock-on effect.
With this in mind, humans have gene banks to preserve the genetic information in the case of extinction, and nurture species that are at dangerously low population levels.
Ironically, the human interference that has disrupted so many species can now provide a means of placing genes into organisms, therefore preparing them for the above hypothetical scenarios such as an epidemic of disease. Genetic engineering would provide the means of allowing organisms to suit their environment without the trial and error over time that comes from natural selection.
The next tutorial investigates genetic engineering and the distance it has come.
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