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 Organisms that resemble one another and are able to interbreed to produce fertile offspring belong to same species. Offspring produced sexually inherit a mixture of characteristics from their parents, some similar to one parent or both. However, they will also have some unique characteristics not found in either parent. Members of a species are slightly different from each other. These differences are called variation which is defined as differences in physical structure, behavior and physiology among individuals of the same species.

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Variation of Humans

 

Although humans have identical physical shapes, organs and limbs, each individual shows variation from one another. Humans vary in terms of their physical characteristics such as height, weight, hair type and color. Physiologically, we are different in terms of the constituents in our blood like sugar and cholesterol levels and our resistance to disease and blood groups. Variation in humans can be classified as continuous or discontinuous variation.

Continuous and Discontinuous Variation

 

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Continuous Variation

 

Continuous Variation is the differences in trait which cannot be divided into distinct group show a wide range of values from one extreme to another. Height is an example of continuous variation where there is a wide range of values from 130 cm to 220 cm for students in a class. Body weight and skin color are other example of continuous variation among humans. Continuous variation can be determined by genetic factors but the same time it can be influenced and environmental factors such as the weather and nutrition.

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In studies of identical twins separated since birth, a twin living with a rich family and the other twin with poor family, the identical twins show different physical growth. The twin given better and more nutritious food grows big and tall while the other twin is retarded in growth. Similarly when two plants from the other twin is retarded in growth. Similarly when two plants from the same stock are planted in different soil, their growth is also different.

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Discontinuous Variation

Discontinuous variation is the differences in traits which are distinct. Individuals displaying discontinuous variation are grouped into two or more distinct groups, for example the human gender, (male and female) and the ability to roll one’s tongue or not being able to. Another example of a human trait where the individuals are grouped into more than two groups is blood groups. A human belongs to either blood group A, B, AB or O. Discontinuous variation is completely determined by genetics and not at all influenced by environmental factors. This type of variation does not change during the life of the individual and is very useful for studying genetics because they clearly show how characteristics are inherited.

Factors which Cause Variation

 

Asexual reproduction produces offspring which are identical to the parent but sexual reproduction produces offspring which are different from one another and also from the parents due to the mixing of genetic materials. Factors causing variation can be classified as genetic or environmental.

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Genetic Factors

Variation caused by genetic factors can be passed down from one generation to the next. Sexual reproduction which involves the formation of gametes by meiosis and the fertilization of gametes is one of the sources of variation. The crossing-over between chromatids from a homologous pair of chromosomes results in the exchange of genetic materials. Random distribution of chromosomes occurring in each gamete formed also result in an exchange of genetic materials. Finally fertilization occurs where there is random fusion between the male and female of genetic gametes. All these result in new combinations of genetic materials giving rise to enormous variation amongst the offspring. Another factor which can cause variation is either gene or chromosome mutation which can take place in somatic cells and gamete.

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Environmental Factors

Environmental factors that can cause variation are: a) soil texture b) temperature c) light intensity d) humidity e) concentration f) type of food and nutrients g) pressure. Two cloned plants having the genetic content can show variation in terms of height and the number of leaves and fruits because growth depends on factors such as soil fertility, amount of sunlight and amount of water.

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Variation in skin and hair color in humans can be caused by exposure to sunlight and high temperature. Some variation in characteristics results from the interaction between genetic and environmental factors. Hence the overall phenotype of an organism can be summarized as:

Phenotype = Genotype + Effects of the environment

Genetic and environmental factors can interact to influence the characteristics of plants and animals. For example, a bean seedling has the genes to develop the chlorophyll pigment but will not be able to unless there is presence of sunlight. A young mammal has the genes to develop strong bones but won’t be able to do so unless it has enough calcium in its diet. Children with genes for height may not grow tall if they consume an unbalanced diet.

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The Importance of Variation

Variation gives rise to variety, diversity and uniqueness in organisms. Variation enables organisms to adapt and survive in the changing environment. This allows the organisms to breed and pass these adaptations to the next generation. Variation helps to prevent the complete extinction of animals and plants species. Variation can be beneficial. For example, it gives rise to plants which are more resistant to bad weather conditions, diseases and pests.

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Variation may also be harmful. For example, variation in bacteria results in them being more resistant to antibiotics. This is bad in the field of medicine because it makes it more difficult for doctors to treat their patients. Variation in some animals provides camouflage and protects them from their predators. One such example is shown in the Biston betularia or peppered moths which exists in two varieties.

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The tree trunks around Britain before 1850 were light in color and so were the Biston moths which are active during the night but rest on tree trunks during the day. Their light color blended with the environment, thus making them less easily preyed upon by birds. The few dark colored Biston moths quickly seen and eaten by birds. After 1850, when Britain was more industrialized, more factories were set up, thus polluting the environment with soot. The soot covered tree and the light colored moth were easily seen and eaten. In less than 75 years, most of the moths became dark colored to blend in with environment.

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Family Tree

In a family, there may be traits which occur in the children and not in the parents but are found in the grandparents. To show the inheritance of a trait in a family, a family tree or a pedigree analysis can be drawn. An example of family tree: both parents have curly hair. They have three children; a son and a daughter with straight hair and another daughter with curly hair. Their son marries a woman with curly hair and they have a son and a daughter with curly hair.  

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Explanation of the above family tree: curly hair is due to a dominant gene ‘S’ and straight hair due to a recessive gene‘s’. Genotypes of (a) both parents are Ss, (b) their son (I1) is ss, (c) first daughter (I2) is SS or Ss, (d) second daughter (I3) is ss, granddaughter (II1) is Ss and (e) grandson (II2) is Ss.

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The Need to Adhere to a code of Ethics in Genetic Research

 

Scientist have used selective breeding technology and genetic engineering to produce better quality stocks, to improve the yields of crop, to produce various types of genetically modified food as well as medicine to treat and cure diseases. These have helped in increasing productivity and indirectly bring about profits. Thus genetic research is useful to mankind if it is used correctly.

Misuse of Knowledge in Genetics can Endanger Life

 

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An imbalance of genes in the gene pool may cause certain talents, traits or abilities to flourish or diminish greatly in the population, if human cloning is allowed to be practiced freely. Genetic research should be used to save lives, to conserve the inheritance of humans and not to destroy them. The misuse of genetic knowledge as in the production of deadly viruses and bacteria for use as biological weapons would result in the death of all living things and humans as well as the destruction of natural resources.

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Harmful animals produced by genetic engineering can threaten human lives and animals. All these would eventually affect the natural equilibrium and ecosystems on earth. Cloning of stem cells may be unethical because after cloning the embryos are discarded. In the field of medicine, the genetic knowledge of fetuses before birth may be misused, for example the number of abortion cases may increase on the pretext of not wanting babies with deformities and abnormalities. This may give rise to social problems.

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The Importance of Establishing and Adhering to Ethics and Morals in Scientific Research

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Scientist should be sincere and their research should contribute to the well-being of the world and not solely for personal glory, gains and profits. To prevent the misuse of genetic knowledge in scientific research, there must be a universal code of ethics and moral values to be honored and followed. The security and safety of all living things on earth should be given utmost importance. The code of ethics should state: a) no human cloning should be allowed b) humans should only be used as subjects in genetic research for medical and health purposes c) there must be control in the production of genetically modified food and if the food is found harmful, it should be banned and destroyed.

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All scientific researches, successful or otherwise, must be documented and made known to the public for the benefit of mankind. There should be an international body or authority to monitor and enforce rules and regulations scientific research.

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Info Gallery    

 

Variation can also be due to internal environment factors such as the type and level of hormones. Such a variation is seen in the plumage of male and female birds, for example, in the scarlet robin, Petroica multicolor, only the males have rose-red breasts which are absent in the females.

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In 1993, Jerry Hall, the director of the in vitro fertilization laboratory at George Washington University was the first to successfully clone human embryos.