Humans have different features, such as eye colour, hair colour or blood groups. These are called CHARACTERISTICS. The characteristics of all human are similar because we belong to the same species.
The differences we find between members of the same species are called VARIATION.

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There are two types of variation:
§ CONTINUOUS VARIATION: when the characteristic varies in small amounts between individuals. For example, height is a continuous variation.
§ DISCONTINUOUS VARIATION: when there are no one in-beteween. For example, tongue rolling: some would be able to do it and others would not; there would be no one in-between.
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external image familia.jpgMost of the characteristics that you have are passed on to you from your parents through genes. GENES are small sections of DNA which control a particular characteristic.Within a family there may be variation. This type of variation is caused by genes or GENETIC FACTORS.
Some characteristics are due to factors around us. This type of variation is caused by ENVIRONMENTAL FACTORS. Most characteristics depend partly on genetic factors and partly on environmental factors.


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§ The human body contains billions of cells.
§ Each cell has a specific function controlled by its NUCLEUS.
§ The nucleus contains long, threadlike strands called CHROMOSOMES.
§ There are 46 chromosomes in each human cell.
§ Chromosomes carry pieces of information called GENES.
§ Chromosomes and genes are made of DNA (large molecule of deoxyribonucleic acid) which is arranged like a DOUBLE HELIX.
§ Each “rung” on the ladder consists of a pair of BASES.
§ The genetic instructions are carried in the sequenced DNA bases: make up each chromosome.
§ These determine the inherited characteristics of an individual. external image 01-coll-dna-knoll-l.jpg
§ This information is in the form of a code: controls the order in which amino acids are assembled to produce PROTEINS.
§ Proteins are used for - growth of cells
- as enzymes: control all the chemical reactions in the body

§ Each human cell contains about 2m of DNA.

All human cells contain 23 pairs of chromosomes except gametes (sperm and ovum). These pair of chromosomes are called HOMOLOGOUS PAIRS. The fertilised egg cell then contains 23 pairs of chromosomes, one of each from each parent.

Short sections of the DNA in a chromosome make up each gene, which consists of thousands of bases. Genes contrl the developmentof inherited characteristics in individuals. Different genes control hair colour, eye colour, nose shape... as well as the production on hormones such as insulin. external image suicide-genes-l.jpg
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GENETICS is the branch of science which studies genes.
Each chromosome in a homologous pair carries the same genes. A gene always codes for one characteristic but it can have different forms. The alternative forms that the gene can take are called ALLELES.external image f6.jpg
Some alleles are always seen in the appearance of an individual. These are called DOMINANTS ALLELES. Other alleles are present but their effect is not seen. They are called RECESSIVE ALLELES.

Occasionally mistakes happen when cells divide. Information in chromosomes may be lost, changed or added to. This change in the DNA is called a MUTATION. Some chemicals and radiationcan damage DNA causing mutations. Exposure to radiation may result in the cells dividing very quickly over and over again causing a lump called a tumour or cancer. Sometimes DNA changes happen in a sperm or egg cell and involve whole chromosomes.
In the case of Down’s syndrome pairs of chromosomes do not separate properly; they have an extra chromosome (47 rather than 46).external image S%C3%ADndrome-de-Down.jpgexternal image ni%C3%B1o-sindrome-de-down.jpg


All living things are made up of genes. Genes are section of DNA. The DNA code is very similar in many organisms so that one organism can understand the genes of an other. This makes GENETIC ENGINEERING possible.

Genetic engineering involves scientists adding genetic material to, taking genetic material away from, or transferring genetic material between organisms.
Genes can be cut out of DNA of one organism using enzymes and then added into the DNA of a bacterium. The added genes is then copied by the bacterium many times.

§ To produce human hormones. For example insulin is a hormone tha lowers the concentration of sugar in the blood. People that suffers from diabetes don’t produce enough insulin. But with the genetic engineering the gene for healthy human insulin can be inserted into bacteria. The bacteria then produced lots of human insulin. This is now widely available as humulin (human insulin).
§ Cystic fibrosis is a genetic disease that causes thick mucus to be produced in the lungs and gut which makes breathing difficult and, as the mucus is a good breeding ground for microbes, cystic fibrosis sufferers tend to get lung infections easily. This disease is caused by a faulty gene. Scientists have developeda nasal spray that contains the normal gene, and when it is inhaled it causes the cell in the lungs to make normal mucus but does not cure the disease.

Cloning is a method of reproduction that results in genetically identical offspring being produced.
external image 20070402131005-oveja-dolly3.jpgexternal image dolly-the-sheep.jpgIn 1997, the first adult mammal was cloned, Dolly the sheep. Dolly, was the first successful clone out of 227 attempts. A skin cell waas removed form a fully grown sheep. An egg from another sheep was taken and its nucleus removed to leave just the cytoplasm of the cell. The nucleus of the skin cell was put into the empty egg cell and the whole egg cell was implanted in a third sheep. It grew into Dolly. Dolly is a clone of the sheep that donated the skin cell. This means that the two sheep are genetically identical

- ORGAN TRANSPLANTSexternal image transplante.jpgToday, some people with damaged organs can have them replaced by TRANSPLANTATION. Medicines have been developed to prevent the immune system rejecting the new organ and techniques for matching tissue have also been developed so that the implanted organ matches the rest of the body tissue as closely as possible.
XENOTRANSPLANTATION is a process which uses animal organs. Scientists ahve been able to modify donor animals genetically in a way that enables them to carry a small piece of human DNA. Xenotransplants can also be used on human with damaged skin.

Genetic modification is a method used where individual genes can be copied and transferred to another living organism.
Plants are modified so that they can be resistant to disease, to improve their nutritional value or their ability to survive in different conditions. For example, maize has been modified so that it can make a protein from the bacterium. This protein kills an insect which is a major threat to the maize. Soya has also been modified to make it tolerant to herbicides. This means that the crop only needs to be sprayed once with herbicide and it kills everything except the soya.
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Genetic engineering has many positive uses. It can be used to help produce cures for disease and can help in food production. However, there may be many unknown side-effects. The action of removing genes from one organism and putting them into another one could have unexpected effects. But the issue of genetic engineering raises many ethical and moral question.