Understanding Of Genetic Effects In Populations Biology Essay

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Discuss how populations evolve in order to explain how different breeds and ultimately different species may develop through artificial and/ or natural selection.

Critically Discuss the Importance of an Understanding of Genetic Effects in Populations

This assignment will be looking into the sheep genetics, looking into how breeds can develop through artificial and natural selection, and also how breeders are trying to preserve the gene pool of their chosen breed.

'The frequency of the colour genotype AaAaBbBbE+E+ of the Manx Loaghtan breed of sheep is 1.0'

Explain what this means, what the implications are for the breed, and how it may have arisen.

Genotype frequency is a frequency that a particular allele arises within a species, for example coding of hair colour, there may be a genetic frequency of 0.2, meaning for every organism, 0.2 have blonde hair. (Biology online, 2005)

Therefore as the Manx Loaghton Breed of sheep have a colour genotype of 1.0, they are all the same colour which has limited their gene pool. This could lead to serious implications for breeding these sheep, one of the problems with having such a limited gene pool is that the sheep all share the same genes, so they are more likely to all suffer with the same health problems or diseases.

This may have arisen because the Manx Loaghton Breed is a very rare breed of sheep, (Manx Loaghtan, 2013) so breeders would have a limited gene pool to begin with so may have bred with closely related sheep to increase the breed size, also as they are all the same colour this tells us that the sheep have been bred to provide the breeders with a certain breed of colour. Therefore this has limited the genetic diversity in the breed.

Discuss how populations evolve in order to explain how different breeds and ultimately different species may develop through artificial and/ or natural selection.

Evolution does not occur in individuals but in populations. A population is a breeding group of individuals of a species in a geographic area. A population evolves because the population contains a number of genes called a gene pool. When changes in the gene pool occur, a population evolves. There are many factors which can affect the gene pool like mutations, genetic drift, natural selection. (Cliffnotes (a), 2013)

Mutation is one of these factors, it's a random change in the populations gene pool. It changes the nature of the DNA in one or more of the chromosomes. Mutations cause new alleles to form, and therefore they cause variation in a population. Mutations can either be beneficial or harmful. Sometimes these mutations will help the animal survive, therefore with the help of natural selection these individuals survive while overs die. (Cliffnotes (a), 2013)

Genetic drift also has a role with genetics. Genetic drift occurs when a small group of individuals leaves a population and establishes a new one in a geographically isolated region. The new population will evolve into one which is different from the original species. Therefore causing different breeds/ species to evolve. (Cliffnotes (a), 2013)

Natural selection is an important influence of evolution, it occurs when an organism is subject to its environment. The fittest survive will contribute their genes to their offspring, which produces a population which is better adapted to the environment. The genes of less fit individuals are eventually lost. (Cliffnotes (a), 2013)

Artificial selection in the intentional reproduction of individuals in a population which have desirable traits. For example if you were breeding for wool or meat, you would look for the two parent sheep which produced either the best meat or wool production. The next generation will then consists of either better meat or wool producers. This also known as selective breeding, artificial selection is very similar to natural selection, apart from the fact that nature determines what individuals survive and reproduce. In both of these cases the outcome is the same, a population will change over time and certain traits become more common. (Annenberg Learner, 2013)

Artificial insemination

Part 2

The next part of the assignment will be looking into the prevention of scrapie and what breeders and the government are doing to try and prevent the disease.

What is the significance of this strategy in terms of preserving genetic variation? Comment in particular on breeds such as the Castlemilk Moorit, Boreray or White Face Dartmoor.

The National Scrapie Plan (NSP) in Great Britain was produced by Defra, it aims to reduce and remove scrapie from the national sheep flock. The NSP, tests sheep's genotyping which will help reduce the risk of scrapie in the sheep flocks. Before scrapie was controlled by selective culling and prompt removal of afterbirth was removed from lambing pens. Studies have known given farmers more knowledge about genetics. Scientists are now able to identify whether specific sheep breeds/ individuals are either resistant or susceptible to scrapie. By testing blood, tissue and semen samples containing the sheep's DNA. This test is known as the 'PrP genotyping' test and the results are known as 'PrP genotype' of a sheep. PrP is cannot tell if a sheep is infected with scrapie, however it will test for the sheep's susceptibility if they are exposed to the disease. There are five different alleles which have been identified in sheep which are; (DEFRA, 2006)



















In the early 20th century a breeder developed the Castlemilk Moorit, unfortunately the herd was culled when the breeder died. However a few ewes and two rams survived and produced two small groups, today all of the Castlemilk Moorits are descended from these two groups. (Rare Breeds Survival Trust, 2011) 606 individual Castlemilk Moorits were tested 3.1% showed they had the gene ARR, (The Scottish Government (c), 2005) this means that they are genetically most resistant to scrapie. (DEFRA (b), 2006) 0.4 % had the alleles ARH (The Scottish Government (c), 2005), which means that they are genetically resistant to scrapie, however they need to be selected carefully when breeding. (DEFRA (b), 2006) The other 96.5 % have the alleles ARQ (The Scottish Government (c), 2005), again they are genetically resistant to scrapie. (DEFRA (b), 2006) This shows that the Castlemilk Moorits are genetically resistant to scrapie, although breeding needs to be carefully planned to ensure future offspring so they do not have genes which are susceptible to scrapie.

The Boreray sheep is another rare breed, which descended from the domestic sheep, which were kept by St. Kildans. The flock was killed; however a flock of domestic sheep was kept on the island of Boreray and became feral creating the Boreray sheep. (Soay Sheep Society, 2011) 104 individual Boreray sheep were tested 2.4 % had the ARR gene (The Scottish Government (c), 2005), 0.5 % had the AHQ (The Scottish Government (c), 2005) again these individuals are genetically resistant to scrapie, however some individuals with this allele are highly susceptible to scrapie, so further testing should be put in place to analyse the individual. (DEFRA (b), 2006) 68.8 % had ARQ alleles and the other 28.4 % had the VRQ alleles, (The Scottish Government (c), 2005) which means they are all susceptible to scrapie and should not be used for breeding. Therefore farmers/ breeders keeping these sheep should only breed the individuals with ARR and ARH alleles, but monitoring ARH individuals closely when breeding. (DEFRA (b), 2006)

The White Face Dartmoor is another rare breed sheep and is one of the most ancient breeds in Britain. (Whiteface Dartmoor Sheep Breeders Association, 2013) 327 individuals were tested 17.9% had ARR alleles, 0.3 % had AHQ alleles, 0.2 % had ARH alleles, 62.1 % had ARQ alleles and 19.6 % had VRQ alleles. (The Scottish Government (c), 2005) Overall there is a mixture of individuals which are susceptible and resistant to scrapie and screening of these individuals should be taken into consideration before farmers/ breeders can produce new generations. (DEFRA (b), 2006)

What different breeding systems are used in sheep breeding?

There are several breeding systems which include Pure-breeding, Out-breeding, Inbreeding, Linebreeding and Crossbreeding. Natural and Artificial selection can be used in any of the breeding systems, which has been mentioned on page 3.


Pure-breeding is the mating of rams and ewes of the same breed or type. (Sheep 101 (d), 2011)


Is within pure-breeding, out-breeding is the mating of two animals which have no close relationships with at least 4 to 6 generations. Out-breeding is advised for most purebred sheep breeders. (Sheep 101 (d), 2011)


Inbreeding is when two closely related animals are mated, this can include sire to daughter, son to dam and brother to sister. (Sheep 101 (d), 2011)


Linebreeding is similar to inbreeding, however is less intense. It is usually used to keep the offspring related to a highly prized ancestor. This mating's will occur within half siblings and cousins. (Sheep 101 (d), 2011)


Crossbreeding is the mating of ewes and rams which are both of different breeds. (Sheep 101 (d), 2011)

How do different breeding systems affect genetic variation?

Pure-breeding and out-breeding will limit the genetic variation in a breed, especially in rare breeds were the gene pool is very small to start with, more diseases like scrapie can be found in many purebred sheep. Whereas sheep that are crossbreed will have more genetic diversity as they will have a larger gene pool. Another factor which will affect genetic variation is the ewes and rams themselves, especially if the animal is prized or produces good offspring they are going to be used to produce more generations, again limiting the gene pool.

How might different breeding systems be used in rare-breed sheep breeding to preserve genetic variation while still maintaining breed integrity?

To preserve breeds, breeders will only mate ram and ewes of the same breed, however this can cause problems as gene pools can become smaller if breeders are isolated with only one herd, this is where artificial insemination can play an important role and can help preserve genetic diversity. Using artificial insemination means that males that are unrelated to females in that group can be introduced to increase variation in that breed. Another benefit for these males is that semen will be collected and sent to various farmers, without having to move the male to different parts of the country, to achieve natural breeding. While using rams and ewes from different herds and countries provides the population with variation, while allowing only pure-bred sheep to breed and not introducing another breed which will provide the bred with different traits will maintain breed integrity. (The Free Dictionary, 2013)

What are the advantages and disadvantages of inbreeding with particular reference to scrapie genotyping and to preserving genetic variation in rare breed sheep?


It maintains desired traits within a breed

Can breed an ewe and ram which have ARR, to produce offspring which are resistant to the scrapie disease.

Breeders will keep a pure blood line


Undesired traits

Possibility of genetic defects


Recessive genes which causes diseases like scrapie

What are the frequencies of the ARR/ ARR, ARR/ VRQ, AND VRQ/ VRQ genotypes in the Wensleydale breed?



90.6 %/ 90.6%

Sheep that are genetically most resistant to scrapie. (DEFRA (b), 2006)



90.6 %/ 9.4 %

Sheep that are genetically susceptible to scrapie and should not be used for breeding. (DEFRA (b), 2006)



9.4%/ 9.4%

Sheep that are highly susceptible to scrapie and should not be used for breeding. (DEFRA (b), 2006)

Why is an understanding of genetic effects in populations important in animal breeding and conservation?

It is important to understand that certain breeds have genetic problems and to ensure that these animals are not bred so that these diseases are not passed onto the following generations. However many breeds have been shown to have serious health problems but are still bred due to high demand from keepers/ owners. A good example of this is Pugs and Cavalier King Charles, these dogs are high demand and because of this they're being inbreed and line breed to ensure traits are carried forward. However many of these traits are hereditary problems, but are still bred. Another example is the King Cheetah, a mutant gene causes the change of colour in this particular species, although because of this gene they would never survive in the wild. While breeding in the wild, natural selection has taken the gene out of the gene pool to increase the cheetahs survival. (The New York Times, 2013)