BiologyPage


		Exemplar for internal assessment resource Biology 3.7A for Achievement Standard 91607

See the commentary below for the significance of the red numbers	1	Mammal cloning is executed through a number of techniques; all involving the transfer of genetic material with the purpose of creating an individual that is genetically identical to the individual from which the genetic information is taken. In the case of Dolly the Sheep; a technique called the 'Roslin technique' was used. The Roslin technique, developed by researchers at the Roslin institute of technology in Scotland, involves 'somatic cell nuclear transfer'. This means that a somatic cell, which is a non- reproductive living cell, was taken from the udder of a six year old Finn Dorset breed sheep, and put into a solution of nutrients.
	2	Scientists then deprived the cell of most of these nutrients so that it would remain alive, but enter into the 'dormant' stage — where cell replication no longer occurs. When the cell has reached its dormant stage, it's nucleus is removed. The nucleus is then inserted into a 'enucleated oocyte (egg) cell'. This means that the nucleus from the udder cell is put into a reproductive cell (egg cell) of the donor sheep. This egg cell has had its nucleus removed. Once this process has been completed both cells (the somatic cell, and the egg cell that has been transplanted with the nucleus of the somatic cell), were shocked with an electric pulse. This released a surge of energy into the cells, which allowed them to fuse into one cell. This cell then divided and developed into an embryo. The embryo was then implanted into a surrogate mother (a sheep which would carry the artificial embryo without involving any of it's own genetic information). The embryo was then delivered in the normal way.
	4	The primary implication that became evident in Dolly the Sheep was the compromise to her individual survival. In the first instance the survival of cloned embryos such as Dolly has a very low rate. Dolly was the only embryo to survive to adulthood out of 277 trialled embryos. Dolly herself indicated many health problems, and only lived to the age of 6. This is young for sheep — who have an average lifespan of 12 years. Many of her vital organs displayed defects which were difficult to explain — including her liver, brain, and heart. She also displayed signs of premature aging before her death. These health issues are thought to be a result of a common cloning complication called 'telomere shortening'. A telomere is the compound structure at the end of a chromosome, which codes for many vital biological processes.
	5	Telomere shortening is an unexplained complication of cloning, which involves the telomeres of the chromosomes of sheep such as Dolly shrinking over time. At a certain point in Dolly these telomeres are thought to have reached a 'critical length'. This meant that they were so short that the cells in Dolly could no longer divide properly, and eventually not at all. At first this lead to tumours growing in her lungs as a result of incorrect cell division, and eventually it lead to her death. Dolly's early death which was result of telomere shortening is an indicator that the health or survival of cloned individuals is currently unlikely to be as good or as long as naturally created individuals.
	3	Selective breeding is an incredibly old method of manipulating the gene pool of a population through breeding specific individuals together to produce 'favourable' traits. In the case of producing easy-care sheep, the favourable genes scientists are looking for are bare heads and legs, bare bellies, and short tails. To be able to reliably produce these favourable traits; scientists must create offspring that display dominance in all of these traits. Displaying dominance simply means the traits are shown on the phenotype of the individual — e.g. the traits are physically observable. To do this; two members of the same species are crossed together (bred to produce offspring). These two members of the same species are selected for the dominant favourable traits that they show. This cross will produce some offspring that show dominance (phenotypically) in ALL the favourable traits.
	6	The other most prominent implication of cloning is that it will have a definite impact on the genetic biodiversity of populations. The biodiversity of a population refers to the amount of different genes present in the gene pool of the population. Cloning, by definition, creates genetically identical copies of a Certain species of — in this case study — sheep. If cloning were to be used on a widespread scale in farming — the biodiversity of the sheep population would be virtually nil. This has significant disadvantages to the survival of the population. Without biodiversity — the population as a whole increases in susceptibility to disease.
	7	An example of this can already be seen today in Cheetahs. The Cheetah population, which has very little biodiversity, is presently declining rapidly in numbers and struggling to fight off diseases as a population. This is because their genetic makeup is so similar. This similar genetic makeup means that while in a bio diverse population some individuals would be susceptible to a disease while others are immune; in the case of the Cheetah population if one is susceptible to a disease, they are all likely to be susceptible. This means that if such a disease should hit any population without genetic biodiversity; the population would not be able to resist the disease, and it would eventually die out.
	8	A major implication towards the genetic biodiversity of a population that is selectively bred for over multiple generations is the flow-on effect of constantly breeding for the same genes. This flow-on effect is called inbreeding depression. Inbreeding depression is an observed effect constant selective breeding has on a population. Because scientists are constantly breeding for the same traits in this breed of sheep — they are likely to 'weed out' many genes — genes that are not being consciously selected for. This will cause the gene pool of the population to significantly decrease. Scientists go to efforts to avoid inbreeding depression — however many genes that are removed from populations such as the sheep populations go un-noticed, due to their apparent insignificance.
	9 & 10	Inbreeding depression will therefore also effect the survival of the sheep population. The changing and sometimes reducing of a gene pool happens in populations naturally through normal breeding patterns — however when it occurs naturally it is in response to environmental factors changing, rather than a deliberate act by a scientist. In selective breeding Some traits may be selected that are actually vital to the survival of the population — which would not happen naturally
	11	For example a gene that does not contribute toward a favourable trait in the eyes of the scientists such as a short tail may be selected against and eventually removed from the gene pool of a population. This gene, however, may have been providing immunity against a certain disease. In this case immunity against this disease would be lost. This would mean more individuals within a population would be likely to die from this disease, and the survival of the population would then be adversely affected.
	12	Both cloning and selective breeding have easily observed advantages. In the case of Dolly the sheep — the largely successful cloning of a mammal has significant economic advantages. Animal cloning is a much more efficient way of reproducing animals — which is a definite advantage for the production of livestock for farming. More livestock will be able to be produced in a shorter frame — producing an economic advantage. A similar thing can be said for selective breeding - In the case of the production of easy care sheep; the manipulation of genetic material for the production of easy—care sheep will in this case produce sheep which have less health complications. This will mean that care of the sheep will be more cost and time effective.
	13	The disadvantages, however, are extensive for both forms of genetic manipulation; especially cloning. Both forms of genetic manipulation cause the gene pools of populations (in this case study the sheep population) to shrink considerably. This has adverse effects on the survival of the population. These effects are more notable in cloning, however, because cloning produces genetically identical sheep. This narrows the gene population more than selecting for certain genes and allowing others to vary naturally as in selective breeding, Cloning also produces significantly more risks in terms of the survival of individuals. Although Selective breeding, as talked about, may remove genes that give an individual immunity to certain diseases; complications arising from telomere shortening in mammal cloning reduce the life span and health of cloned sheep much more significantly and definitely. Telomere shortening produces life-threatening health complications that are at this stage not properly explained; therefore not preventable. For this reason cloning has many more potential disadvantages than selective breeding.
		NOTE: the exemplar above is the work of a student that just achieves at the excellence level. It is by no means perfect. Some facts are not quite accurate and it is by no means comprehensive. This just one part of one of two exemplars that can be accessed from http://www.nzqa.govt.nz/qualifications-standards/qualifications/ncea/subjects/biology/annotated-exemplars/ Below is the explanation of how the student achieves excellence Grade Boundary: Low Excellence 1. At Excellence the standard requires the student to demonstrate comprehensive understanding of human manipulations of genetic transfer (EN 3) and its biological implications (EN 2). This involves linking biological ideas about human manipulations of genetic transfer and its biological implications, which may involve the impact on (EN4): ecosystems genetic biodiversity health or survival of individuals survival of populations evolution of populations. The student provides evidence that just meets the criteria for Excellence by describing the human manipulation of cloning (1), explaining how the genetic manipulation occurs (2), and describing selective breeding (3). The biological implications and their impact or consequences relating to each of these manipulations have also been presented: Cloning on the health and survival of the individual (4), supported by a good biological explanation (5) and genetic biodiversity (6) including consequences, e.g. susceptibility to disease and supported with an example (7) Selective breeding on genetic biodiversity and the related consequences, e.g. inbreeding depression (8) and the health and survival of the individual (9). The student has also linked two implications and their consequences (10) to consider the differences between selective breeding and natural selection (11). The student has made comparisons between the benefits and cost effectiveness (economical implication) of using both manipulations (12) and linked them to decreased biodiversity and survival of populations (13). The student could more convincingly secure this grade at Excellence by providing more supporting evidence using biological terms at Level 8 of the curriculum.