Characteristics

No two individuals are identical.

Different species can be identified because they have different features, or characteristics.

These can be used to classify organisms into groups.

Even individuals of the same species show variation in their characteristics. For example, hair colour, or whether they have freckles or not.

People in the same family share similar characteristics. Some characteristics are inherited from the mother and others from the father.

This information is passed on from parents in the sex cells (gametes).

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Genetic Variation

Information about an individual's characteristics is carried in the form of genes.

Genes are located inside the nuclei of cells on structures called chromosomes.

Variation due to an individual's genes is called genetic variation.

For example, the shape of a pear, the colour of a rose, whether you are male or female and whether you can roll your tongue or not, are all examples of genetic variation.

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Environmental Variation

Individuals also show variation, which is due to conditions they have developed in.

For example, one sunflower seed may grow into a taller plant than another from the same packet of seeds if it is grown in a brighter, warmer place or given more water and minerals.

An individual may decide to colour their hair or pierce their ears.

Someone may have a scar or lose part of a finger as a result of an accident.

All these are examples of environmental variation.

Usually, an organism's characteristics are a result of both genetic and environmental causes.

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It gives rise to great variation in offspring because:

• Each parent produces gametes by meiosis, so no two gametes are identical
• Male and female gametes fuse together at fertilisation
• One of each pair of alleles comes from each parent to produce an individual that has a mixture of genetic information from the two parents
• The alleles in a pair vary, giving rise to different characteristics

Asexual reproduction involves one parent only.

New cells are produced by mitosis from the parent cell.

All the offspring are genetically identical to the parent, there is very little variation.

Genetically identical organisms are called clones.

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Mutations

A mutation is a sudden change in a gene or chromosome.

Mutations occur randomly, but the chance of one happening is increased by:

• Exposure to ionising radiations including ultraviolet light, X-rays and radioactive radiation. The greater the dose of radiation, the greater the chance of a mutation
• Certain chemicals, for example, chemicals in cigarette smoke

Most mutations are harmful. 

If they happen in body cells, they may cause cells to grow and divide in an uncontrolled way and invade other parts of the body. This is cancer.

If they happen in the reproductive cells, the mutation can be passed on to the offspring.

The offspring may develop abnormally or die at an early stage in development.

Some mutations are neutral in their effects.

In rare cases, a mutation may increase the chances of survival, so they are more likely to be passed onto other generations.

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Mendel

Gregor Mendel was an Austrian monk who discovered that certain characteristics of pea plants were passed on in a predictable way. He observed the characteristics flower colour, seed shape and height.

From his results, he stated that characteristics are carried on particles which cannot combine.

He worked out that there were two particles for each characteristic, but only one was passed on to the next generation by each parent.

For example, when red and white flowered plants were cross-pollinated, the offspring would only have red or white flowers, never pink. We now call the particles 'genes'.

The importance of Mendel's work was not recognised until after this death because it had not been well-publicised.

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