GCSE AQA Biology 2 Genetics

Stem cells
1. Differentiation: Process by which a cell changes to become specialised for its job
  1. In most animal cells, ability to differentiate is lost v. early but lots of plant cells don't ever lose the ability
2. Some cells are undifferentiated & can develop into different types of cell depending on instructions given - stem cells
  1. Found in early human embyos (can turn into any type of cell) & in adult bone marrow (can turn into certain ones - less versatile)
3. Medicine already uses adult stem cell e.g. people with blood diseases (sickle cell anaemia) can be treated with bone marrow transplant (turn into blood cells to replace faulty ones)
4. Scientists can extract embryonic stem cells & use them to replace faulty cells
  Nota:
  - Could make beating heart muscle cells for people with heart disease, insulin-producing cells for people with diabetes, nerve cells for people with paralysis etc
5. To get cultures of one specific type of cell, researchers try to control differentiation by changing environment - needs more research
6. Stem cell research
  1. For
    1. Curing patients who already exist & are suffering is more important than embryo rights
    2. Embryos used in research are usually unwanted ones from fertility clinics - would be destroyed anyway
  2. Against
    1. Human embryos shouldn't be used as each one is potential life
    2. Should find other sources of stem cells
  3. Banned in some countries, but is allowed in UK if it follows strict guidelines
X and Y chromosomes
1. 22 matched pairs of chromosomes in each human body cell - 23rd pair are labelled XX or XY (chromosomes which decide gender)
2. All men have X and Y chromosome - Y chromosome causes male characteristics
3. All women have 2 X chromosomes - XX combination allows female characteristics to develop
4. When making sperm and eggs, chromosomes are split apart in first meiosis division - 50% chance sperm gets X/Y, 100% chance egg gets X
5. Probability of getting boy/girl can be shown with a genetic diagram:
The work of Mendel
1. Gregor Mendel - Austrian monk
2. Noted how characteristics in plants were passed on from one generation to the next
3. Results published in 1866 & eventually became foundation of modern genetics
4. Reached 3 important conclusions:
  1. Characteristics in plants are determined by 'hereditary units'
  2. Hereditary units are passed on from both parents, one from each parent
  3. Hereditary units can be dominant or recessive - if individual has both dominant & recessive unit, dominant will be expressed
5. We now know 'hereditary units' are genes - in Mendel's time nobody knew anything about genes or DNA so significance of his work was not realised until after his death
Inheriting characteristics
1. Alleles: different versions of the same gene
  1. In genetic diagrams, letters usually represent alleles
2. Homozygous: organism has 2 alleles for a particular gene that are the same
3. Heterozygous: organism has 2 alleles for a particular gene that are differnt
4. If 2 alleles are different, only one can determine characteristic - the dominant one
5. For organisms to display recessive characteristic, both its alleles must be recessive
6. Genotype: what alleles are present
7. Phenotype: characteristic shown
Genetic disorders
1. Cystic fibrosis
  1. Genetic disorder of cell membranes - results in body producing lots of thick sticky mucus in air passages & pancreas
  2. Caused by a recessive allele - carried by about 1 person in 25
  3. For child to have disorder, both parents must be carriers or sufferers
2. Polydactyly
  1. Genetic disorder where baby is born with extra fingers or toes - not life-threatening
  2. Caused by dominant allele - can be inherited if one parent carries gene (all carriers are sufferers)
Embryo screening
1. During IVF, embryos are fertilised in lab & implanted into mother's womb - more than one egg is fertilised so there's better chance of success
  1. Before implantation it is possible to remove a cell from each embryo to analyse its genes
    1. Many genetic disorders could be detected in this way
      1. Embryos with 'good' alleles would be implanted, one with 'bad' alleles would be destroyed
2. For
  1. Will help to stop people suffering
  2. There are laws to stop it going too far
  3. During IVF, most embryos are destroyed anyway - screening just allows selected one to be healthy
  4. Treating disorders is very expensive
3. Against
  1. Everyone might want to screen so they can pick most 'desirable' embryo
  2. Rejected embryos are destroyed - potential human life
  3. Implies people with genetic disorders are 'undesirable' - could increase prejudice
  4. Screening is expensive

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GCSE AQA Biology 2 Genetics

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