Natural and selective breeding
How it works - Selective breeding is when a breeder breeds animals with known alleles to try and make a chosen allele the phenotype, or to remove an unwanted allele from the gene pool. Sheep with fast growing fur would be a desirable trait, because the farmers would earn more money from wool, meaning the breeder would try to only breed his sheep with fast growing fur so as the offspring would also have a fast growing fur allele. The three techniques of selective breeding are out-crossing, line-breeding and in-breeding. Out-crossing is when the animals have no relation at all, which is best for adding or removing a variation when the variety for a gene is high. This is because there is a larger chance of different alleles. Line-breeding is when you breed cousins/aunts/uncles, but never immediate relatives. This is for mixed effects of out-crossing and in-breeding. In-breeding is when the offspring is made between direct relatives, such as sibling, parent or child. Normally breeders use this technique to make an animal pre-potent , and to rule out a certain allele from the gene pool. Selective breeding works best if animals being bred have homozygous dominant alleles, for if the alleles are heterozygous then recessive alleles can appear in later generations. This process is often done with animals or plants to improve qualities, such as long lived or fast growing.
There are advantages and disadvantages of selective and natural breeding. This paragraph will discuss the disadvantages of selective breeding, an example being no control over alleles of animals. A large problem of natural selection is everything chance because you have no control over traits and conditions. Lets use giraffes as a example. Imagine a population of giraffes there has tall giraffes, who are more prone to disease and die faster, and shorter giraffes who have a longer life expectancy and are faster. So in that community the short giraffes eat the shorter trees and the tall giraffes eat the tall trees, but imagine an event happens to wipe out the short trees. Now the shorter and more evolved giraffes cannot eat because their food source is gone. However the taller and less evolved giraffes live. You then end up with a population of less evolved giraffes that do not live as long. Another disadvantage is that if a recessive allele with great advantages, such as cows producing lots of milk, happens to be ruled out by a dominant gene of not as much milk then it is harder to get it back. This is why selective breeding is used, so as animal species can rule out unhelpful alleles and replace them with the superior variation. This next section will be the advantages selective breeding, and the main reasons for its use. Selective breeding is used when you want to breed a animal for one main purpose, such as a livestock sheep or a domestic dog. Traits that a farmer might try to selectively breed for a livestock cow would be grows fast, and is fat with lots of meat. But a dog breeder breeding domestic dogs to sell might want them to be fluffy and have very high immunity to disease. People use selective breeding because it can make animal/plant produce taste nicer, and it can make there be more animal/plant produce, meaning the breeder earns more for better quality. It can make organisms be more resistant to disease, but if they all possess the same genes a disease can be deadly to all.
Selective breeding has problems however, such as how it decreases the level of variety in a species. Another issue is that when animals are in-bred their chances of disease increase, because the genes are similar and might all have low resistance to a single disease. Also there is no easy way to determine what allele a organism passes down, and especially if a recessive allele is wanted as a phenotype it can be difficult. The main issue that will be studied is the decreased variety of a species. Loss in the variation of a population occurs because in a population normally the animals are all bred for the same purpose, meaning often they are born from the same parents and receive the same alleles. This also means that within a population in-breeding and line-breeding would be very common making the gene pool very similar for many alleles. Luckily the species would stay varied having different populations bred for different purposes. A large reason low variety is a problem is because if the offspring of the selectively bred animal wasn't domestic they wouldn't cope as well in their natural habitat because the natural traits would have been replaced with selected traits. Another more important example of why a low variety range is a problem is because if they all had a high disease resistance for most known diseases, but then a new disease arrived that could infect these animals, then all animals would be infected because they all carried a low disease resistance for this disease. Although this is a rare occasion it can happen, making selective breeding dangerous for a population of a farmers livestock.
There are advantages and disadvantages of selective and natural breeding. This paragraph will discuss the disadvantages of selective breeding, an example being no control over alleles of animals. A large problem of natural selection is everything chance because you have no control over traits and conditions. Lets use giraffes as a example. Imagine a population of giraffes there has tall giraffes, who are more prone to disease and die faster, and shorter giraffes who have a longer life expectancy and are faster. So in that community the short giraffes eat the shorter trees and the tall giraffes eat the tall trees, but imagine an event happens to wipe out the short trees. Now the shorter and more evolved giraffes cannot eat because their food source is gone. However the taller and less evolved giraffes live. You then end up with a population of less evolved giraffes that do not live as long. Another disadvantage is that if a recessive allele with great advantages, such as cows producing lots of milk, happens to be ruled out by a dominant gene of not as much milk then it is harder to get it back. This is why selective breeding is used, so as animal species can rule out unhelpful alleles and replace them with the superior variation. This next section will be the advantages selective breeding, and the main reasons for its use. Selective breeding is used when you want to breed a animal for one main purpose, such as a livestock sheep or a domestic dog. Traits that a farmer might try to selectively breed for a livestock cow would be grows fast, and is fat with lots of meat. But a dog breeder breeding domestic dogs to sell might want them to be fluffy and have very high immunity to disease. People use selective breeding because it can make animal/plant produce taste nicer, and it can make there be more animal/plant produce, meaning the breeder earns more for better quality. It can make organisms be more resistant to disease, but if they all possess the same genes a disease can be deadly to all.
Selective breeding has problems however, such as how it decreases the level of variety in a species. Another issue is that when animals are in-bred their chances of disease increase, because the genes are similar and might all have low resistance to a single disease. Also there is no easy way to determine what allele a organism passes down, and especially if a recessive allele is wanted as a phenotype it can be difficult. The main issue that will be studied is the decreased variety of a species. Loss in the variation of a population occurs because in a population normally the animals are all bred for the same purpose, meaning often they are born from the same parents and receive the same alleles. This also means that within a population in-breeding and line-breeding would be very common making the gene pool very similar for many alleles. Luckily the species would stay varied having different populations bred for different purposes. A large reason low variety is a problem is because if the offspring of the selectively bred animal wasn't domestic they wouldn't cope as well in their natural habitat because the natural traits would have been replaced with selected traits. Another more important example of why a low variety range is a problem is because if they all had a high disease resistance for most known diseases, but then a new disease arrived that could infect these animals, then all animals would be infected because they all carried a low disease resistance for this disease. Although this is a rare occasion it can happen, making selective breeding dangerous for a population of a farmers livestock.
The picture to the right side is a good example of selective breeding. Imagine that this breeder sells animal hide. If the breeder had many white cows with low worth hide and many red cows with a medium worth. But if a red hide with white spots was of high worth the breeder could selectively breed the white cows with the red cows. But now imagine that white hide was a recessive gene then this technique would be most effective if the red cow had a known genotype of RR. This is an extremely basic example of selective breeding.
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