GCSE AQA Biology 2 Genetics

GCSE AQA Biology 2 Genetics

1. Stem cells

   • Differentiation: Process by which a cell changes to become specialised for its job
     • In most animal cells, ability to differentiate is lost v. early but lots of plant cells don't ever lose the ability
   • Some cells are undifferentiated & can develop into different types of cell depending on instructions given - stem cells
     • Found in early human embyos (can turn into any type of cell) & in adult bone marrow (can turn into certain ones - less versatile)
   • 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)
   • Scientists can extract embryonic stem cells & use them to replace faulty cells
     • 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
   • To get cultures of one specific type of cell, researchers try to control differentiation by changing environment - needs more research
   • Stem cell research
     • For
       • Curing patients who already exist & are suffering is more important than embryo rights
       • Embryos used in research are usually unwanted ones from fertility clinics - would be destroyed anyway
     • Against
       • Human embryos shouldn't be used as each one is potential life
       • Should find other sources of stem cells
     • Banned in some countries, but is allowed in UK if it follows strict guidelines

2. X and Y chromosomes

   • 22 matched pairs of chromosomes in each human body cell - 23rd pair are labelled XX or XY (chromosomes which decide gender)
   • All men have X and Y chromosome - Y chromosome causes male characteristics
   • All women have 2 X chromosomes - XX combination allows female characteristics to develop
   • When making sperm and eggs, chromosomes are split apart in first meiosis division - 50% chance sperm gets X/Y, 100% chance egg gets X
   • Probability of getting boy/girl can be shown with a genetic diagram:

3. The work of Mendel

   • Gregor Mendel - Austrian monk
   • Noted how characteristics in plants were passed on from one generation to the next
   • Results published in 1866 & eventually became foundation of modern genetics
   • Reached 3 important conclusions:
     • Characteristics in plants are determined by 'hereditary units'
     • Hereditary units are passed on from both parents, one from each parent
     • Hereditary units can be dominant or recessive - if individual has both dominant & recessive unit, dominant will be expressed
   • 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

4. Inheriting characteristics

   • Alleles: different versions of the same gene
     • In genetic diagrams, letters usually represent alleles
   • Homozygous: organism has 2 alleles for a particular gene that are the same
   • Heterozygous: organism has 2 alleles for a particular gene that are differnt
   • If 2 alleles are different, only one can determine characteristic - the dominant one
   • For organisms to display recessive characteristic, both its alleles must be recessive
   • Genotype: what alleles are present
   • Phenotype: characteristic shown

5. Genetic disorders

   • Cystic fibrosis
     • Genetic disorder of cell membranes - results in body producing lots of thick sticky mucus in air passages & pancreas
     • Caused by a recessive allele - carried by about 1 person in 25
     • For child to have disorder, both parents must be carriers or sufferers
   • Polydactyly
     • Genetic disorder where baby is born with extra fingers or toes - not life-threatening
     • Caused by dominant allele - can be inherited if one parent carries gene (all carriers are sufferers)

6. Embryo screening

   • 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
     • Before implantation it is possible to remove a cell from each embryo to analyse its genes
       • Many genetic disorders could be detected in this way
         • Embryos with 'good' alleles would be implanted, one with 'bad' alleles would be destroyed
   • For
     • Will help to stop people suffering
     • There are laws to stop it going too far
     • During IVF, most embryos are destroyed anyway - screening just allows selected one to be healthy
     • Treating disorders is very expensive
   • Against
     • Everyone might want to screen so they can pick most 'desirable' embryo
     • Rejected embryos are destroyed - potential human life
     • Implies people with genetic disorders are 'undesirable' - could increase prejudice
     • Screening is expensive

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