Essay on Inheritance: Allele and Variation

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Uniquely you…
The population of the Earth is more than 6 billion people, and no two individuals (apart from identical twins) are genetically the same. Why?
People are different because they inherit different characteristics (or traits) from their parents.
Like all babies, this child carries a unique set of genes; half from his mother and half from his father.
A person’s unique characteristics are caused by:
 the set of genes they inherited from their parents (nature)
 the environment in which they developed (nurture).
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Inherited and acquired characteristics
Some characteristics, such as eye colour and earlobe shape, are only determined by genes. These are called inherited characteristics.
Other types of characteristics, such as scars and hair length, are not inherited but depend on environmental factors.
These are called acquired characteristics. Differences in some characteristics are due to a combination of both inherited and environmental factors. In some cases, it can be difficult to say how much influence each factor has.
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Nature, nurture or both?

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Phenotype and genotype
The overall appearance of an organism depends on two things: 1. its genes (inherited characteristics)
2. the effects of the environment in which it lives.
All the observable characteristics of an organism are called its phenotype.
The full set of genes of an organism is called its genotype.
An organism’s phenotype therefore depends on its genotype plus environmental effects. phenotype = genotype + environmental effects

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Classifying variation
Characteristics can be classified in different ways. How would you categorize variation in eye colour? Could height be categorized in the same way?
 A feature that can be measured and given a value from a range of values shows continuous variation.  A feature that cannot be measured but is one of a few distinct options shows discontinuous variation.
Which type of variation are eye colour and height?
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What is continuous variation?
Characteristics that show continuous variation cannot easily be placed into individual categories. They vary over a continuous range of values.
Continuous variation is caused by the combined effects of genes and the environment.
Examples of continuous variation include height, mass, skin colour, intelligence and leaf area.
Are characteristics that show continuous variation fixed, or can they change?
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What is discontinuous variation?
Characteristics that show discontinuous variation can be placed into distinct categories – the organism either has a specific characteristic, or it does not.
Discontinuous variation is usually controlled by a small number of genes. The environment has little or no effect.
Examples of discontinuous variation include blood group and eye colour.
Are characteristics that show discontinuous variation fixed, or can they change?
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What type of variation?

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Environmental causes of variation
The effects of the environment in which an organism lives can cause significant variation between individuals.
Plants are affected by water, sunlight, temperature and the availability of nutrients.
When these factors are plentiful the plants thrive.

When these factors are scarce the plants wither.

Animals are similarly affected by water and nutrients.
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Observing variation in humans
As well as environmental factors such as climate and diet, humans are affected by education, culture and lifestyle.
Because these factors change our phenotype, the effects of many environmental factors can be clearly seen.
Scientists often use identical twins to study the effects of environmental factors.
Although the twins are genetically identical,
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