B6 Inheritance, Variation and Evolution

• Gamete

  Sex cell, sperm or egg
• Haploid

  1 set of chromosomes
• Zygote

  The cell resulting from a sperm and egg fusion
• Diploid

  2 set of chromosomes
• Asexual reproduction

  One parent makes clones - no fusion
• Asexual reproduction

  • Involves one parent and no gamete
  • The offspring are genetically identical to the parents - they are clones
  • Advantages are that it is faster, so quicker to increase the population
  • The disadvantage is that there is a lack of genetic variation, so if the environment changes, the population may not have features to survive
• Mitosis

  1. Interphase - growth, doubling of organelles (e.g. mitochondria), DNA replicates
  2. Mitosis - chromosomes line up on the equator, chromosomes are pulled apart to opposite poles, 2 nuclei forms
  3. Cytokinesis - cell divides into 2 genetically identical cells
• Advantages of sexual reproduction

  • produces variation in the offspring
  • if the environment changes, variation gives a survival advantage by natural selection
  • natural selection can be speeded up by humans in selective breeding to increase food production
• Some organisms reproduce asexually & sexually

  • Malarial parasites reproduce asexually in the human host, but sexually in the mosquito
  • Many fungi reproduce asexually by spores but also reproduce sexually to give variation
  • Many plants produce seeds sexually, but also reproduce asexually by runners such as strawberry plants, or bulb division such as daffodils
• Comparison of Mitosis and Meiosis

  • Mitosis is a form of nuclear division resulting in 2 daughter cells that are genetically identical. Mitosis is used for replace or repairing of tissues, for growth or asexual reproduction
  • Meiosis is a form of nuclear division resulting in 4 daughter cells that are not genetically identical - they are haploid
  • Mitosis involves 1 division, meiosis involves 2
  • Mitosis cells are genetically identical, meiosis are not
  • Mitosis is used for growth, repair and replacement of tissue, meiosis is used to make gametes
• Gene


  Small section of DNA on a chromosome, recipe of a protein
• Genotype


  Genetic makeup e.g. Bb
• Phenotype


  Physical appearance e.g. brown eyes
• Homozygous


  2 copies of the same allele e.g. BB bb
• Heterozygous

  2 different alleles for the same gene e.g. Bb
• Recessive


  The allele that is not expressed in the heterozygote
• Dominant


  The allele that is expressed in the heterozygote
• Alleles

  Versions of the same gene, for example, the gene for eye colour would have blue, green and brown
• Genome

  The entire genetic material of that organism
• Mutation

  The change to the DNA
• XX 


  The genotype for a female
• XY 


  The genotype for a male
• Polydactyl 


  Having extra fingers or toes caused by a dominant allele
• Cystic fibrosis


  (a disorder of cell membranes) is caused by a recessive allele
• Preimplantation Genetic diagnosis (PGD)

  • Pros: Child is very unlikely to have the condition, less medical costs of raising an ill child, no emotional decisions regarding abortion, less carriers in population, frozen embryos can be used to have another child, occurs before pregnancy
  • Cons: Very expensive screening, could damage embryo, embryo has no ‘voice’ - ethical concern
• Prenatal Genetic Diagnosis (PND)

  • Pros: Can give parents the decision to abort or not, cheaper than PGD
  • Cons: Chance for a miscarriage, errors - healthy baby aborted, embryo has no ‘voice’ - ethical concern
• Causes of variation

  1. Genes
  2. Environment
  3. Genes and environment: genes give you the potential, the environment allows the realisation of their potential
• Continuous Variation - line graph

  • No distinct categories
  • Controlled by lots of genes
  • Strongly influenced by environment
  • e.g. height
• Discontinuous Variation - bar chart

  • Distinct categories
  • Controlled by one gene
  • Unaffected by environment
  • e.g. eye colour
• Species


  A species is a group of organisms that are genetically similar, biochemically and anatomically similar, can breed together successfully and get fertile offspring
• Evolution by Natural Selection

  1. There is variation in a population (e.g…..) due to mutation
  2. Environmental pressure (e.g….)
  3. The organism with advantageous features (e.g…) will survive, reproduce and pass on their genes to the next generation
  4. The organism with disadvantageous features (e.g…) will die, not reproduce and will not pass their genes onto the next generation
• Selective Breeding

  1. Select the individuals that have the desired characteristics (e.g. bigger muscle mass)
  2. Breed together
  3. Select the offspring that have inherited all the desired characteristics (e.g…)
  4. Breed together
  5. Repeat for several generations
• Genetic engineering

  1. The gene for the desired characteristics is isolated (“cut out”) of the chromosome using an enzyme (restriction endonuclease)
  2. A plasmid is cut with the same enzyme and the human gene inserted into it (makes recombinant DNA)
  3. This is then put into a new organism (e.g. bacteria) (becomes a GM organism)
  4. The new organism is allowed to replicate
• Ethics of genetic technologies

  • One huge potential benefit of genetic engineering is to cure human disorders
  • Gene therapy
  • Benefits of Genetic engineering in agriculture: improved growth rates of plants and animals, increased food value of crips as GM crops usually have bigger yields than ordinary crops, crops can be designed to grow well in dry, hot or cold environments, crops can be engineered to produce plants that make their own pesticide or are resistant to herbicides used to control weeds
• • Genetic engineering is still a very new science and no one can be sure what the long term effects might be:
  • Concerns about the effect of eating GM food on human health
  • Genes from GM plants and animals might spread into the wildlife of the countryside
  • Fears of human engineering
• Fossils

  Fossils are the ‘remains’ of organisms from millions of years ago which are found in rocks
• Geologists can tell us the age of a fossil by studying the breakdown of radioactive isotopes in the rock
• Fossils can form:

  • Lack of decay and when one condition for decay is absent: lack of O2, too cold, wrong pH (too acidic), e.g. in ice, amber, bogs
  • Mineral replacement: an organism dies, it gets buried under a rock or in mud, this prevents exposure to oxygen, organism does not decay, minerals replace the hard parts of the organism
  • Trace fossils: traces of organisms left behind e.g. footprints , covered and preserved by mud/ash which takes shape of the print and preserves it
• What can we learn from fossils

  • How organisms changed over time (evidence for evolution)
  • Linking extinct organisms to current ones
  • Learn what organisms lived, how they lived millions of years ago
• Extinction

  • No individuals left of that species alive
  • Causes of extinction by living organisms: new disease (Tasmanian devil), new predator (Caine toad), new competitor (grey squirrel)
  • Non-living causes: changes in environment (temperature, sea levels)