It contains coded information - the instructions to put an organism together and make it work

It is made up of two strands coiled together in the shape of double helix

Only 20 amino acids are used, but they make up thousands of different proteins

This, in turn, determines what type of cell it is (eg. a red blood cell, a skin cell)

Allows scientists to identify genes that are linked to different types of disease

Knowing which genes are linked to inherited genes could help us understand them better and help develop effective treatments

All modern humans are descended from a common ancestor who lived in Africa, now humans are all over the planet

The human genome is mostly identical in all individuals, but as different populations migrated from Africa, they gradually developed tiny differences

By investigating this, scientists can work out when new populations split off in a different direction and what route they took

DNA strands are polymers made up of lots of repeating units called nucleotides

The sugar and phosphate molecules in the nucleotides form a 'backbone' to the DNA strands

One of the four different bases - A, T, C or G - joins to each sugar

The order of the bases in a gene decides the order of amino acids in a protein

The amino acids are joined together to make various proteins, based on the order of the bases

They can switch genes on and off, so they control whether or not a gene is expressed (used to make a protein)

The DNA, however, is found in the nucleus and can't move out of it as it is too big

So the cell needs to get the code from the DNA to the ribosome by copying it

A molecule called mRNA is made by copying the code from DNA

mRNA acts as a messenger between the DNA and the ribosome carrying the code between them

The correct amino acids are brought to the ribosomes in the correct order by carrier molecules

When the protein chain is complete, it folds to form a unique shape

act as biological catalysts to speed up chemical reactions in the body

used to carry messages around the body (eg. insulin is a hormone released into the blood by the pancreas to regulate the blood sugar level)

are physically strong, eg. collagen is a structural protein that strengthens connective tissues (like ligaments and cartilage)

Most mutations have little/no effect on the protein and some will change it slightly so that its function/appearance is unaffected

The chance increases by exposure to certain substances or some types of radiation

If there is a mutation in the non-coding DNA, it can alter how genes are expressed

The sequence of bases codes for the sequence of amino acids, mutations can lead to changes in the protein that it codes for

Sometimes, the mutation will code for an altered protein with a change in its shape

eg. if the shape of an enzyme's active site is changed, its substrate will no longer bind to it

eg. structural proteins, eg. collagen, could lose their strength if their shape is changed, making them useless at providing structure and support

This changes the way the groups of three bases are 'read', which can change the amino acids that they code for

Can change more than one amino acid as they have a knock-on effect on the bases further in the sequence

Where a new base is inserted into the DNA sequence where it shouldn't be

This changes that the way that the base sequence is 'read' and has knock-on effects further down the sequence

Mother and father produce gametes be meiosis (eg. egg and sperm cells in animals)

Where genetic information from two organisms (a father and mother) is combined to produce offspring which are genetically different to either parent

The egg (from the mother) and the sperm cell (from the father) fuse together (fertilisation) to form a cell with the full number of chromosomes

Produces genetically identical cells (as there is only one parent)

Happens via mitosis - an ordinary cell makes a new cell by splitting in two

The new cell is a clone - it has exactly the same genetic information as the parent cell

There is no fusion of gametes, no mixing of chromosomes and no genetic variation

Instead of having two of each chromosome, a gamete just has one of each

Flowering plants can reproduce in this way. they have egg cells, but their version of sperm is pollen

|This is why the offspring inherits features from both parents - it's received a mixture of chromosomes from both

This mixture in genetic information produces variation in the offspring

In humans, it only happens in the reproductive organs (the ovaries in females, the testes in males)

In the second division, the chromosomes line up again in the center of the cell

The pairs are then pulled apart so each new cell only has one copy of each chromosome (some of the father's and some of the mother's go into each new cell)

In the first division, the chromosome pairs line up in the centre of the cell

You end up with 4 gametes, each with only a single set of chromosomes in it

After replication, the chromosomes arrange themselves into pairs

Each of the gametes is genetically different from the others because the chromosomes all get shuffled up during meiosis and each gamete only gets half of them, at random

Before the cell starts to divide, it duplicates its genetic information (forming two armed chromosomes - one arms of each chromosome is an exact copy of the other)

As the embryo develops, these cells then start to differentiate into the different types of specialised cell that make up an whole organism

Mitosis repeats many times to produce lots of new cells in an embryo

After two gametes have fused during fertilisation, the resulting new cell divides by mitosis to make a copy of itself

Therefore, it uses less energy than sexual reproduction as organisms don't need to find a mate

Many identical offspring can be produced in favourable conditions

As they are more likely to survive, they are more likely to breed successfully and pass the genes for the characteristic on. This is natural selection

A change in the environment may kill some individuals, it's likely that variation will have led to some of the offspring being able to survive in the new environment. They have a survival advantage

This is where individuals with the desired characteristic are bred to produce offspring that also have that characteristic

This means that we can increase food production (eg. by breeding animals that produce a lot of meat)

When a mosquito carrying the parasite bites a human, the parasite can be transferred to the human

Asexually-produced spores form fungi that are genetically identical to the parent fungus

They release spores which can become new fungi when they land in a suitable place

Sexually-produced spores introduce variation and are often produced in response to an unfavourable change in the environment, increasing the chance that the population will survive the change

Loads of species of plant produce seeds sexually, but can reproduce asexually

They produce 'runners' which are stems that grow horizontally on the surface of the soil away from the plant

At various points along the runner, a new strawberry plant forms that is identical to the original plant

The other 22 are matched pairs that just control characteristics

A similar thing happens when making eggs but the original cell has two X-chromosomes, so all the eggs have one X-chromosome

There's a 50% chance each sperm cell gets an X-chromosome, and a 50% chance it gets a Y-chromosome

Genetic diagrams are just models used to show all the possible genetic outcomes when you cross together different genes or chromosomes

You can draw a genetic diagram to find the probability of getting a boy or a girl

What genes you inherit control what characteristics you develop

Some characteristics are controlled by a single gene (eg. mouse fur colour and red-green colour blindness in humans)

Most characteristics are controlled by several genes interacting

You have 2 alleles of every gene in your body - one on each chromosome in a pair

If the two alleles are different, only one can determine what characteristic is present

The other allele is recessive (represented by a lower case letter)

For an organism to display a recessive characteristic, both allels must be recessive (eg. cc)

For an organism to display a dominant characteristic, it can either be CC or Cc as the dominant allele overrules the recessive one

For a child to have the disorder, both parents must be carriers or have the disease

Results in the body producing lots of thick mucus in the air passages and in the pancreas

It can be inherited if just one parent carries the defective allele

It's a genetic disorder where a baby's born with extra fingers or toes

The parent that carries the allele will also have the disease as it is dominant - there are no carriers

There are laws to stop it going to far (eg. parents can't select the sex of their baby)

It implies that people with genetic problems are 'undesirable' - this could increase prejudice

There may come a point where everyone wants to screen their embryos so they can pick the most 'desirable' (eg. a blue-eyed, blond-haired, intelligent boy)

Embryos produced by IVF would be destroyed if they had 'bad' alleles

It's also possible to get DNA from an embryo in the womb and test that for disorders

Screening could lead to a decision to terminate the pregnancy for embryos in the womb

Many genetic disorders can be detected in the way (eg. cystic fibrosis)

Before being implanted, it's possible to remove a cell from each embryo and analyse its genes

During IVF (in vitro fertilisation), embryos are fertilised in a lab, and then implanted into the mother's womb

Mendel found out that the height characteristic was determined by separately inherited "hereditary units" passed on from each parent

Noted how characteristics in plants were passed on from one generation to the next

Trained in mathematics and natural history at the University of Vienna

He bred pea plants together to see the correlation with the offspring

The ratio of tall and dwarf plants in the offspring showed that the units for tall plants, T, was dominant over the unit for dwarf plants, t.

1) Characteristics in plants are determined by "hereditary units"

2) Hereditary units are passed on to offspring unchanged from both parents, one unit from each parent

They didn't have the background knowledge to properly understand his findings - they din't know about genes, DNA and chromosomes

They were able to observe how they behaved during cell division

Based on this, it was proposed that the "units" were found on the chromosomes

This allowed scientists to find out exactly how genes work

All plants/animals have characteristics that are in some way similar to their parents

These genes are passed on in gametes, from which the offspring develop

Most animals (and some plants) get some genes from the mother and some from the father

This combining of genes causes genetic variation (sexual reproduction)

eg. a plant grown in plenty of sunlight would be luscious and green, but the same plant grown in darkness would grow tall and spindly and have yellow leaves

The environment, inc. the conditions that organisms live and grow in, cause differences between members of the same species

They are changes to the sequence of bases in DNA which can lead to changes in the protein a gene codes for

Although very rare, mutations can result in a new phenotype being seen in a species

If the environment changes, and the new phenotype makes an individual more suited to the new environment, it can become more common through the species - natural selection

Genes are the codes inside your cells that control how you're made

In animals these include: eye colour, blood group and inherited disorders (eg. haemophilia or cystic fibrosis)