Variation

When two gametes (sex cells) combine to form a new diploid cell (zygote) at fertilisation, this results in genetic variation of the offspring

This is seen across all species which reproduce sexually as every living thing is genetically different, except for identical twins formed from the same zygote (so produced from the same sperm and the same egg) 

In humans, a normal body cell (produced as a result of mitosis) contains 23 different chromosomes with two copies of each one

This means there are a total of 46 chromosomes in each cell

This is called the diploid number

In the gametes (sex cells produced as a result of meiosis) the chromosome number is halved

There is only one copy of each chromosome in each cell, leading to a total chromosome number of 23

This is called the haploid number

Variation is simply the many differences within a species It can be caused in two main ways

It can be genetic – controlled entirely by genes Or it can be environmental – caused entirely by the environment in which the organism lives

Most often though, it is a mixture of both genes and environment, as very few things are entirely controlled by one or the other

This is often popularly called ‘nature vs nurture’

Characteristics of all species can be affected by environmental factors such as climate, diet, accidents, culture and lifestyle In this instance ‘environmental’ simply means ‘outside of the organism’ and so can include factors like climate, diet, culture, lifestyle and accidents during lifetime

Often features vary because of a combination of genetic and environmental causes

tall parents will pass genes to their children for height their children have the genetic potential to also be tall however if their diet is poor then they will not grow very well therefore their environment also has an impact on their height

Another way of looking at this is that although genes decide what characteristics we inherit, the surrounding environment will affect how these inherited characteristics develop

Mutations are random changes that occur in a gene or a chromosome

As a mutation is contained in the DNA, it can be passed on from one generation to the next, ie. mutations can be inherited

Mutations are random changes that occur in a gene or a chromosome This random change happens spontaneously and continuously in our cells

This can cause an alteration to the base pair sequence in the genetic code If the base pair sequence is different it could code for a different amino acid when translated in the ribosome

This could lead to a different protein being made that will not carry out its usual function or will work in a different way, causing the body or organs to behave differently

As a mutation is contained in the DNA, it can be passed on from one generation to the next

Most mutations have no effect on the phenotype The protein that a mutated gene produces may work just as well as the protein from the non – mutated gene These types of mutation will not affect the chances of survival of species Mutations rarely lead to the development of new phenotypes and if they do, most have a small effect on the organism

Occasionally, the new phenotype gives the individual a survival advantage over other members of the species, it can lead to rapid change

For example: A bird develops a mutation leading to a change in feather colours This makes it more attractive to birds of the opposite sex Which causes the bird to breed more frequently and have more chances of passing on the mutated phenotype to the next generation

Mutations can also lead to harmful changes that can have dramatic effects on the body – for example, cystic fibrosis in humans

Mutations happen spontaneously and continuously but their frequency can be increased by exposure to the following:

Gamma rays, x – rays and ultraviolet rays – all types of ionising radiation which can damage bonds and cause changes in base sequences

Certain types of chemicals – for example chemicals such as tar in tobacco

Increased rates of mutation can cause cells to become cancerous, which is why the above are linked to increased incidence of different types of cancer