- All organisms develop following a set of instructions that are coded inside the cell inside the nucleus.
- These control how the organism develops and functions.
- The basic unit for the instructions is called the gene (section of DNA that describe how to make proteins).
- Genes occur in very long DNA molecules called chromosomes.
- Chromosomes are made of DNA molecules - each DNA molecule consists of two strands which form a double helix.
- Some characteristics are coded for by a number of genes that work together e.g. eye colour.
- There is a number of genes coding for the different pigments in the iris, mainly on chromosome 15 in humans.
- It is easiest to follow what is happening with the inheritance of gene characteristics by drawing genetic diagrams. Family trees can be used to trace the inheritance of a characteristic and to work out who must have been carrying a faulty allele.
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- This is the differences between individuals of the same species.
- Due to genes (the different characteristics that an individual inherits) or environmental factors (how the environment changes an individual)
- Usually due to a combination of genes and the environment.
Genotype is the term describing the genetic makeup of an organism (combination of alleles).
Phenotype describes the observable characteristics the organism has
- Disorders are usually caused by a recessive gene but occasionally the faulty gene is dominant, meaning that only one allele is needed for the disorder to be present.
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- All body cells contain pairs of chromosomes-the genes on each chromosome is in the same place in each one which means that body cells usually have two copies of each gene.
- These copies of each gene can be different versions, called alleles.
- Alleles are dominant or recessive.
- Dominant allele controls the development of a characteristic even if it is only present on one chromosome in a pair.
- Recessive allele controls the development of a characteristic only if the dominant allele is not present (need both alleles to be recessive to get that characteristic).
- Allele of a gene is usually represented by a letter.
- If it's dominant it is a capital letter.
- If it's recessive it is a lower-case letter.
- If you carried one of each allele, you would not express the recessive gene but you would still be a carrier for this gene.
- When the two alleles are the same, it is called homozygous.
- When the two alleles are different, it is called heterozygous.
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- The seex cells (eggs produced by the ovaries in females and sperm produced by the testes in males) only carry one copy of each chromosome.
- Humans have 23 pairs of chromosomes in their body cells (46).
- Sex cells contain half the amount (23 single chromosomes).
- When fertilisation takes place (the nuclei of the sperm and the egg fuse) , the total number of chromsomes doubles as they pair up again.
- The pairing up of the chromosomes is random so the offspring differs from the parents and any brothers or sisters.
- This leads to variation.
- The child will share similarities with the parents depending on which characteristics come from the father and the mother and which ones are dominant and recessive.
- In females the seex chromosomes are the same-both X chromosomes.
- In males there is an X chromosome and a Y chromosome, the Y chromosome is much shorter than the X chromosome.
- 50% chance of offspring being female, 50% chance of offspring being male.
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- Sex of an embryo is determined by a gene on the Y chromosome.
- If the gene is not present (two X chromosomes) the embryo will develop into a female and ovaries will grow.
- If the gene is present (both an X and Y chromosome is present) then testes will begin to develop.
- Six weeks after fertilisation, the undifferentiated gonads start producing a hormone called androgen - specialised receptors in the developing embryo detect the androgen which stimulates the male reproductive organs to grow.
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- Caused by the presence of a single dominant faulty allele.
- If a parent carries the dominant allele, the child has a 50% chance of carrying it too.
- Genetic disorder that affects the central nervous system.
- Caused by a dominant faulty allele on chromosome 4.
- The allele that causes this damages the nerve cells in certain areas of the brain.
- This leads to gradual physical, mental and emotional changes.
- Symptoms of this normally develop in adulthood, meaning sufferers may have already had children and passed on the gene.
- Symptoms include late onset, a tremor, clumsiness, memory loss, an inhability to concentrate and mood changes.
- Everyone that inherits the allele will develop the disease (due to it being dominant).
- Only one parent needs to pass on the allele for child to inherit.
- If one of the parents has it, there is a 50% chance (Hh), which can be shown in a punnet square.
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- Caused by the presence of a faulty recessive gene.
- Both recessive alleles are needed for the disease to develop.
- UK's most common life threatening genetic disease.
- Affects the cell membranes, causing a thick mucus to be produced in the lungs, gut and pancreas.
- Symptoms are difficulty in breathing, an increased number of chest infections and difficulty in digesting food.
- No cure but scientists have identified the faulty recessive allele and are looking at ways to repair or replace it.
- If an individual is a carrier they will not have characteristics as it is recessive but they can pass this allele on to their children.
- Both parents need this allele for child to get disease.
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- Tests adults, children and embryos for a faulty allele if there is family history of a genetic disorder.
- If the test is positive, individual decides if they want to risk passing it on to their children or not.
- Can also determine whether an adult or a child can be prescribed a particular dug without suffering from serious side effects.
- IVF can be used to make sure that only healthy embryos are implanted.
- Procedure for embryo selection is called pre-implantation genetic diagnosis (PGD), each cell is tested and then a healthy embryo is implanted.
- Risks of this are inaccurate results (false positive or false negative), ethical considerations and decreased chance of the embryo surviving once it has been implanted.
- Two ways of testing a developing fetus-amniocentesis testing and chorionic villus testing.
- Both carry risks such as miscarriage and infection.
- Amniocentesis is carried out at 14-16 weeks of pregnancy, a needle is inserted into the uterus and a small sample of amniotic fluid is extracted, then if the test is positive for the disease the parents could terminate.
- Chorionic villus takes place at 8-10 weeks, a special catheter is inserted through the vagna and cervix until it reaches the placenta, part of the placenta (chorionic villi, finger-like protrusions) is removed for testing, then parents choose if they want to terminate.
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Asexual Reproduction and Clones
- When a cell grows and divides into two, it is a form of reproduction but as it does not involve sex, it is called asexual reproduction.
- All bacteria reproduce asexually, along with many plants and animals.
- When an organism has exactly the same genetic information as another individual it is called a clone.
- Only differences in clones are the environmental factors.
- Plants such as strawberries produce shoots called runners which eventually break off and become new strawberry plants which are clones of the original.
- Other plants grow bulbs which grow into genetically identical plants when planted.
- No two organisms occupy the same space in the universe so the environment will always be different for individuals, even if they are clones.
- Clones of animals occur naturally when the embryo splits into two in the early stages of fertilisation which leads to the creation of identical twins.
- Identical triplets can also happen but it is very rare, this occurs when the fertilised egg splits into two and one of the new cells splits into two again.
- Clones can happen artificially by taking the nucleus from an adult body cell and transferring it into an empty, unfertilised egg cell. The most famous example of this was Dolly the sheep but cloning has been successful with a wide range of organisms.
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- Cloning depends on cells that have the potential to become any cell type in the body (stem cells).
- Adult stem cells are unspecialised cells that can develop into many, but not all, types of cell, these are found in the bone marrow
- Embryonic stem cells are unspecialised cells that can develop into any type of cell, including more embryonic stem cells.
- Both types of cell can be used to treat some injuries or illnesses (e.g. skin can be grown to treat serious burns or sight can be restored to people who are blind due to a damaged cornea)
- Majority of cells in the embryo start to become specialised after the zygote has divided four times to reach the 16 cell stage.
- Certain genes are switched on and off.
- Leads to production of proteins specific to the specialised cell type.
- Specialised cells can only divide to produce the same type of specialised cell.
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