B6 - Inheritance, Variation and Evolution

B6 - Inheritance, Variation and Evolution

1. DNA

   • Chromosomes
     • Contains coded information/ instructions
     • Found in nucleus + mitochondria
       • Chromosomes normally in pairs
     • DNA is polymer - two strands, double helix
   • Genes
     • Small section of DNA found on a chromosome
     • Codes for particular sequence of amino acids - make specific proteins
     • Tell cells what order to put amino acids together
     • DNA determines what proteins the cell produces
       • Determines the type of cell
   • Genome
     • Entire set of genetic material in an organism
     • Understanding human genome
       • Allows scientists to identify genes that are linked to diseases
         • Could help understand better and develop effective treatments
           • Can trace migration of ancestors

2. Reproduction

   • Sexual reproduction
     • Genetic information from two organisms
       • Genetically different to both parents
     • Mother and father produces gametes by meiosis
       • In humans, gamete contains 23 chromosomes (half of normal cell)
       • Egg and sperm fuse together (fertilisation) to form cell with all chromosomes
         • Inherits features from both parents
           • Variation in offspring
     • Happens in flowering plants too
   • Asexual reproduction
     • Only one parent so genetically identical
       • Happens by mitosis
       • Clones of parent cell
       • Bacteria and some plants and animals

3. Meiosis

   • Happens in ovaries and testes
   • First duplicates genetic information
     • First division, chromosome pairs line up in centre of cell
       • Pairs are pulled apart so each new cell only has one copy of each chromosome
         • Second division, chromosomes line up again in centre of cell, arms are pulled apart
           • Four gametes with a single set of chromosomes in it, all genetically different

4. X and Y Chromosomes

   • XY - Male, XX - Female
   • 50/50 chance sperm has X or Y chromosome
     • 50% chance having girl, 50% having boy
     • Genetic diagrams show possible gamete combinations

5. Genetic diagrams

   • All genes exist in different forms called alleles
     • Two versions of every gene in body
       • If two alleles the same - homozygous, if different - heterozygous
         • One allele dominant (Capital), one recessive (lowercase)
           • Need two recessives to display recessive trait
             • Genotype - combination of alleles person has, phenotype - characteristics displayed

6. Inherited disorders

   • Cystic fibrosis - recessive allele
     • Cell membrane disorder, body produces thick sticky mucus in air passages and pancreas
     • People with only one copy of allele are okay - just carriers
       • For child to have disorder, parents must be carriers or affected
   • Polydactyly - dominant allele
     • Extra fingers or toes
     • Only one parent has to have defective allele
       • Can't be carrier because it is dominant
   • Embryo screening
     • IVF - embryo fertilised in lab, then implanted in womb
       • Possible to remove cell and analyse genes
         • Can detect some genetic diseases
         • Amniocentesis
     • For
       • Helps stop people suffering
         • Treating disorders is expensive for everyone
           • Laws stop it going too far
     • Against
       • Implies people with genetic problems are undesirable - could increase prejudice
         • Everyone may want to screen their embryos for desirable traits
           • Screening is expensive

7. Variation

   • Differences within a species
   • Genotype is all of genes and alleles organism has
     • Phenotype is characteristics it displays
       • Can be influenced by environment
   • Mutations are random changes in a gene
   • Mutations can make an individual more suited to environment

8. Evolution

   • All today's species evolved from simple life forms from 3 billion years ago
   • C.D. Survival of the Fittest
     • Mutations cause variation, variation makes some more suited, more likely to survive and reproduce to pass on genes to offspring
       • Characteristics became more common so species had evolved
   • Research into genetics, fossil record and antibiotic resistance supported Darwin's theory
   • Extinction because environment change, new predator, disease, competition or catastrophe

9. Selective Breeding

   • Selecting animals with desired characteristics to breed together
   • Get higher yield, resistance or good temperament
   • Drawback is that there is reduced gene pool
     • Inbreeding can cause health problems, can inherit defects or can be wiped out by one disease

10. Genetic Engineering

    • Useful gene cut from organism's genome by enzymes and put in a vector
      • Vector usually virus or plasmid
    • Pros
      • Can increase yield
      • Helps people in developing nations
    • Cons
      • Reduces farmland biodiversity
      • Some don't think they are safe
      • Transplanted genes could get into natural environment e.g. herbicide resistance creating 'superweed'

11. Fossils

    • Remains of organisms from millions of years ago
    • Gradual replacement by minerals
      • Teeth, bones etc. don't decay easily, replaced by minerals, form rock-like substance shaped like original hard part
    • Casts and impressions
      • Organism buried in soft material e.g. clay, later hardens and organism decays, leaving cast
    • Preservation in places where no decay happens
      • Amber + tar pits, no oxygen, decay microbes can't survive, glaciers, too cold, peat bogs, too acidic
    • Hard to prove how life began, early life was soft-bodied so decayed - incomplete fossil record, fossils destroyed by geological activity

12. Antibiotic-Resistant Bacteria

    • Random mutations mean some strains become more resistant, reproduce quickly, increasing population size, MRSA superbug hard to get rid of
    • Getting worse due to overuse and inappropriate use of antibiotics
    • If full course of antibiotics not taken, some bacteria remain and could mutate

13. Classification

    • Kingdom, Phylum, Class, Order, Family, Genus, Species
    • Named in Latin according to binomial system (genus species)
    • Evolutionary trees show common ancestors, more recent ancestor, more closely related the two species