Variation

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Differences Between Individuals

The presence of differences between individuals is called variation. No two individuals are exactly alike, however similar they may look. Identical twins start as one cell that divides and then separates into two cells. Each of these two cells then develops into a separate person. While the two original cells had the same genetic information, the subsequent replication of DNA and cell divisions may have introduced changes to the DNA. Also, slight environmental differences in the womb or after brith can mean that the two individuals show physical differences.

Variation within species

There are differences between members of population of a species - height, eye colour, hair colour.

Variation Between Species

Continous Variation

Where there are two extremes and a full range of intermediate values between those extremes. Most individuals are close the the mean value. The number of individuals at the extremes is low. Examples of continous variation include:

height in humans

length of leaves on an oak tree

length of stalk of a toadstool

Discontinuous Variation

Where there are two or more distinct categories with no intermediate values. The members of a species may be evenly distributed betweem the different forms, or there may be more of one type than the other.

Examples of discontinuous varitaion within species include:

human blood groups

some bacteria have flagella

What causes variation?

Inherited or genetic variation

The genes we inherit from our parents provide information that is used to define our characteristics. The combination of allels that we inherit is not the same as tha in any other living thing. We may share many alleles with other members of our species and share genes with many other species. However there is never a complete match.

Human cells contain approximately 25000 genes, and many of these may have more than one allele.

Environmental causes of variation

An overfed pet will become obese.

Peoples skin tan.

Trees may grow sideways if in a windy environment.

Not all our genes are active at any one time.

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