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Genes, Chromosomes and DNA

Size order from biggest to smallest: Cell, Nucleus, Chromosomes, DNA, Gene

Genes control the development of different characteristics. They exist in different versions. 

A gene is a code for making a certain protein. 

Structual proteins: Skin, hair, blood, and the cytoplasm in our cells. e.g. collagen is found in tendons, bones and cartilage.

Functional proteins: Enzymes help with digestion by breaking down food molecules - amylase is a digestive enzyme that breaks down starch.

Organism's genotype describes the genes it's got. The characteristics that and organism displays are called a phenotype.

Some characteristics, e.g. dimples, are only controlled by genes. Others, e.g. scars, are only caused by environmental factors.

Skin colour is a characteristic which is caused by both factors.

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Genes, Variation, Inheritance & Sex Chromosomes

In body cells all chromosomes come in pairs. 1 chromosome in a pair has come from each parent.

An egg cell and a sperm cell each have 23 single chromosomes. When the egg is fertilised the chromosomes join to make 23 pairs of chromosomes.

Alleles are different versions of the same gene. Alleles can be dominant or recessive. 

Homozygous = two alleles the same for that particular gene.                                                             Hetrozygous = two different alleles for that particular gene.

There are 23 pairs of chromosomes in every human body cell. The 23rd pair are labled XY. These are the sex chromosomes.

All men have an X and a Y chromosome: XY                                                                                     The Y chromosome causes the male characteristics.

All women have two Y chromosomes: YY                                                                                          The lack of a Y chromosome causes the female characteristics. 

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Genetic Disorders

Some disorders are inherited - one or both parent carry the faulty allele. For example, Cystic Fibrosis and Huntington's Disease.

Cystic Fibrosis

CF is a genetic disorder of the cell membranes caused by a fauty recessive allele.                         Symotoms include: Thick stick mucus in the air passages, gut and pancreas. Breathing difficulty. Chest infections. Difficulty in digesting food.                                                                                      CF is rescessive so they must have both copies of the recessive allele.

Huntington's Disease

HD is caused by the dominant allele.                                                                                  Symptoms include: tremors, clumsiness, memory loss, mood swings, poor concentration. There is no cure.                                                                                                                                      HD is dominant so they only need 1 copy of the allele to inherit the disease.

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Genetic Testing

Genetic testing can be used on Embryos, Children and Adults.

Embryos: Using IVF doctors can test the embryos to check for genetic disorders. This is known as Pre-Implantation Genetic testing. The healthy ones are then implanted into the mother's womb. (Doctors can also test the fluid around the fetus in the womb).

Children & Adults: They can be checked to see if they have the certain alleles. Couples may do this to see if there children are likely to inherit a particular genetic disorder.

Issues                                                                                                                                               There are often errors e.g. false negative/false positive. People may make desisions based on inncorrect information.                                                                                                              Medical records are also like to cause descrimination against people. For example people are less likely to get certain types of insurance becasue companies arent willing to give insurence to people with the wrong alleles.

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Clones are genetically identical organisms.

Natural Clones                                                                                                                                            Plants often use asexual reproduction. There is only 1 parent and the offspring is genetically identical. Bacteria simply divide in two, they can divide very quickly. Some plants produce horizontal stems called runners that move out from the base of the plant, e.g. strawberry plant.   Identical twins are also clones, this is when the embryo splits into two, and two separate, genetically identical clones are born.

Scientific Clones                                                                                                                                   1) The nucleus of an egg cell is removed.                                                                                              2) A nucleus from and adult donor cell is inserted in its place.                                                         3) The cell is then stimulated so that is starts dividing as if it was a normal embryo.                             4) The embryo produced is genetically identical to the donor cell.

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Stem Cells

Stem Cells are unspecialised. They can be developed into different types of cells depending on what instructions they're given. There are 2 main types:

Embryonic Stem Cells: Unspecialised cells found in early embryos. The stem cells are removed from the embryo, which is then destroyed. These can be developed into potentially any cell.

Adult Stem Cells: Unspecialised cells found in adult animals. These are mainly used to maintain and repair old and damaged tissues and can specialise into many cell types. These can be safely removed from adult patients, e.g. extracting their bone marrow - no embryos are destroyed.

Treating Illness                                                                                                                               1) Medicine  already uses adult stem cells to cure disease. Some blood diseases, e.g. sickle cell anaemia, can be treated by bone marrow transplants, these stem cells are turned ito new blood cells.                                                                                                                                                   2) Embryonic stem cells could be used to replace faulty cells in sick people - you could make heart muscle cells (heart disease), insulin-producing cells (diabetes), never cells (paralysed by spinal injuries) etc. All these treatments are still being researched.

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