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For thousands and thousands of years, approximately 10,500, humans have been selectively breeding animals. This process works by picking the most favorable member of a species and allowing it to breed, passing on its genes, which over time causes these genes or features to be displayed in extreme ways to increase output, efficiency or something else that makes the crop or animal more beneficial to farm. Of course, humans did not realise this was what we were doing and only more recently discovered the biological mechanisms behind this effect, which we dubbed artificial selection. A fantastic example of this would be the dairy cow, an animal we have been selectively breeding for roughly ten and a half thousand years, by choosing the cows that where the largest or produced the most milk to have offspring we unconsciously increased the efficiency at which they make milk, altered their biology to have more meat on it and domesticated it to either tolerate or enjoy human contact all for the sake of food production. Of course this does have effects that go beyond just the dairy cow and has had wider impacts on the environment and ecosystem.
Selective breeding cows originated in farmers catching and breeding a species known as the Auroch – which has been extinct for almost 400 years – and likely the first thing they were used for was meat rather than their milk at the time. Because of this it is likely only the calmest where actually farmable and tameable, as such those were the ones who actually had the offspring, passing on the more domesticated genes that made it easier for the farmers to work with and breed them thus passing on the genes of the calmer ones more often. This means that only those most suitable to either the environment they were farmed in or for the framers intended purpose in breeding them got to pass on their genes meaning the gene pool of the next generation will as such have a higher frequency of these preferred genes and make farming easier and more efficient as time passes. By no means is this a perfect system, as breeding a cow that has a few genes that you find favorable will always pass on those genes due to the randomness of the process of meiosis as well as the chance for mutation and hidden genes being present. Even though the traits it displays (its phenotype) may be favorable its genes (genotype) may not be. For example, if the parent with the favorable genotype was heterozygous with an unfavourable recessive gene and is bred with another individual that is also heterozygous with that unfavourable gene, there is a real chance its offspring will be homozygous for that recessive gene, meaning the phenotype you wanted from the original animal is not expressed. Examples in cows include genetic diseases like dwarfism or crooked tail syndrome, but really this process applies to any small inconvenience like maybe it does not breed as well or does not produce as much or as nutritious milk.
As time went on breeding for select traits became much easier to do on purpose as the traits you want as people discovered ways to find out an animal’s genotype. With this knowledge dairy and cattle farmers can breed two individuals that present a gene, either homozygous dominant or homozygous recessive, to guarantee that the offspring produced is “purebred” for that specific trait. The first way to discover an animal’s genotype used would be a test cross, which is where you cross one organism that expresses a trait with one you know is heterozygous and looking at the resulting offspring you can determine whether the first parent is dominant or heterozygous for the trait you want. Other, more recent methods include marker assisted selection or “MAS” which is used to mark a specific gene, usually one that’s difficult to measure/ observe, passed on very rarely (recessive) or aren’t expressed until later in life, in order to indirectly select for a genetic determinant of a trait of interest citation. The reason MAS is a more recent technique in selective breeding because very few traits in general have markers and it was first observed naturally – allegedly – in 1923 by a man named Sax K. when he observed an “association of a simply inherited genetic marker with a quantitative trait in plants when he observed segregation of seed size associated with segregation for a seed coat color marker in beans” Quote source. The steps for effectively using MAS include mapping the gene in questions location or “quantitative trait locus” (a.k.a QTL) by using various methods and then using this information for marker assisted selection, and linked or very close genes in the animals DNA are used to mark the desired traits presence. It is still possible for crossing over to occur between those two linked or very close genes so usually two or more markers are used to indicate its presence and reduce the margin of error for homologous recombination.
Since man first began domesticating the cow, its genetic diversity has decreased do to us breeding all the genes we didn’t want or need out of the species over time as a byproduct. Another biological implication is that, for the same reasons MAS works, breeding a gene out may be very difficult or impossible due to linked or very close genes. Linked genes are those found very close together on the same chromosome and have a recombination frequency of less than 50%, an example of this in cows might be milk and fat production Citation. A farmer may select a cow that is large, muscular or produces a lot of milk but those genes could also come with a negative side effect like fertility or immune system problems, which would likely outweigh the benefits of selecting for that gene. This is because they would either all die from the same disease due to their lack of variation, which may negatively, or positively, affect the ecosystem in some big way if all cows were wiped out suddenly.
Cloning is also a more recent technique that would allow for the selection of specific individuals in order to preserve a specific genotype that the farmer finds very efficient or convenient without having to go through the risk/ reward system of breeding them normally and would as such have a higher success rate as the scientists involved are in complete control of the process. Additionally it is fairly easy to add extra genes to the species or individual that they may not naturally possess, an example being cows that can donate their plasma to humans Citation. This technique is called transgenesis, where genes from one species are placed into the genome of another species or the use of gene editing through various other means Citation. The reason for these cows is a lack of blood donors to provide plasma to critically injured patients, and because the farmer does not use the cow’s blood for anything as it is, it’s something of an extra product they can sell or give away. This works by inserting the gene responsible for making plasma for humans and replacing it with the one the cow uses, preferably the plasma gene from a type AB person as that would make then a universal plasma donor, which allows the cow to make “human” plasma which it can donate to humans without any negative impact on the cows lifespan or the farmer but with a large impact for patients suffering heavy blood loss and trauma throughout all the hospitals around the world as well as helping populations fight disease Citation. To make these cows scientists take the ABO gene from humans that is responsible for plasma generation and type in the humans body and replace it with the equivalent gene in a cow cell which is then inserted into an “empty” embryo and electrocuted, a very similar process to cloning just with the added transgenesis or gene editing. Depending on what gene is transferred and the individual performing the procedure as well as what type of donor cell is used (options include bone marrow and blood cells) the success rate tends to be around 5% or less, and if it does succeed there is no guarantee the individual is viable and can reproduce to pass on those genes CItation. If and when it does succeed however, because the individual possessed those genes from birth or rather from the original cell it was produced from, it can pass on those genes to its offspring meaning the process does not have to be repeated if you want to keep that specific gene for such a wide use like plasma generation, which is what makes the <5% success rate and costs worth it in the long run in my opinion, especially since gene editing techniques like CRISPR-Cas9 are making the process of transgenesis cheaper and simpler than ever before. We use this technology because humans have an ever increasing population and despite increasing medical science we still need plasma donation to help fight disease and heal trauma patients which would allow for better survival of the human species.
A biological implication of this is that one day these gene edited cows might end up out-breeding normal cows, not because they are better at anything in particular but because humans are artificially selecting for them out of convenience. Another implication may be that we only breed donor gene cows with other donor gene cows in order to make sure they are “purebred” for that gene in particular and because we have already been selectively breeding cows for thousands of years and because it is unlikely we will decide to make a plasma donating clone of ever cow, the cows gene pool will be even further reduced which means that ironically while they are helping us fight disease they will be more susceptible to be wiped out by a single disease themselves. Additionally as not much research has been done into the long term effects that changing the plasma of these cows causes it may affect another species like bacteria that’s meant to be in the cow’s blood and digestive system, or blood sucking parasites that target cows which may influence how other species in the chain survive and interact with each other, not just the cow.
For a species to survive long-term it must have variation in its gene pool or high genetic diversity otherwise they will all fail equally at adapting to a new situation or surviving a new pathogen. Because of this, despite the many advantages of selective breeding and gene editing cows there are for humans the cows themselves may suffer from it and put the whole species at risk. WIth selective breeding there is no return for a species like the cow that has been artificially selected by humans for so many thousands of years, however with transgenesis the original unedited animal will exist for at least a good long while before the gene edited one either replaces it or outnumbers it, so if the edited version of the species does end up in trouble the original species may be fine or vica versa and can be bred with each other to increase diversity to some degree. If a species is made so specialised by gene editing however and after so many years there may either be no original species to breed them with due to them being obsolete, as cows cannot survive without humans realistically, or have become so genetically different from the original species they can no longer produce offspring in a dire situation which may have further knock-on effects to the ecosystem and food chain due to such a small amount of genetic variation of the species. This could result in extinction or something very close, or a shortage of food as well as, in this case, a source of plasma and possibly other unforeseen effects on the environment and other species.
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