GCSE review

• Most phenotypic features are the result of multiple genes acting together, and not single genes. For example, as many as 16 genes are thought to be responsible for human eye colour.
• Genetic variation


  Different characteristics can arise as a result of both random mutation and sexual reproduction. Random mutation occurs in gametes to produce offspring with 'brand new' phenotypic characteristics, whereas sexual reproduction causes the offspring to have a new combination of characteristics from both its mother, and its father.
• Environmental variation


  Characteristics can also be caused by an organism's environment, but these changes are generally not heritable (there is no change in the DNA of the organism). For example, a child who does not receive adequate nutrition will not grow to their full height, however this has no effect on their potential height as determined by their genetics.
• Sexual reproduction


  1. Joining of male and female gametes, each containing genetic information from the mother or father

  2. Gametes are formed by meiosis, as they are non identical

  3. A normal cell has 46 chromosomes, with two sets of 23 chromosomes (one from each parent)

  4. Genetic information from each parent is mixed, producing variation in the offspring
• Asexual reproduction


  1. One parent with no gametes joining

  2. Happens using the process of mitosis, where two identical cells are formed from one cell

  3. No mixing of genetic information

  4. Leads to clones, which are genetically identical to each other and the parent
• Organisms that reproduce asexually


  • Bacteria
  • Some plants
  • Some animals
• Advantages of sexual reproduction


  • Produces variation in offspring
  • Decreases chance of whole species becoming extinct
  • Allows selective breeding
• Advantages of asexual reproduction


  • Only one parent is needed
  • Uses less energy and is faster as organisms do not need to find a mate
• DNA


  Chemical that contains genetic material, made up of nucleotides with a sugar, phosphate and one of four organic bases (A, C, G, T)
• Double helix


  Structure of DNA, with two strands twisted around each other
• Gene


  Short section of DNA that codes for many amino acids, which are joined together to make a specific protein
• Genome


  All the genetic information (DNA) of a single organism
• Protein synthesis


  DNA contains genetic code, mRNA is made as a template, mRNA moves to ribosomes, amino acids are brought by tRNA and joined to form a polypeptide, which then folds into a 3D protein structure
• Genetic variants


  Small changes in the order of bases that make up a strand of DNA, can affect the structure of proteins
• Genotype


  The genes present in the DNA of an individual
• Phenotype


  The visible effects of those genes (e.g the proteins that they code for)
• Mutations


  Changes the sequences of bases in DNA, can insert, delete or substitute bases
• Most mutations do not alter the protein or only do so slightly, but some can have a serious effect and change the shape of the protein
• Monohybrid inheritance


  Using a Punnett square to determine the probability of offspring having certain genotypes and phenotypes based on the alleles of the parents
• Dominant and recessive alleles

  Uppercase letters represent dominant characteristics, lowercase letters represent recessive characteristics
• Family pedigrees


  • Used to show how a condition is passed down through generations
• Codominance and multiple alleles


  Two dominant alleles can be expressed together, and there can be three or more alleles at the same locus
• ABO blood group


  • Example of codominance and multiple alleles
• Bacteria reproduce at a fast rate, and mutations during reproduction can result in new genes like antibiotic resistance
• Exposure to antibiotics


  Creates a selection pressure, allowing antibiotic resistant bacteria to survive and reproduce
• Bacterial diseases spread rapidly because people are not immune to these new resistant bacteria and there is no treatment for it
• Selective breeding


  Humans choose which organisms to breed in order to produce offspring with a certain desirable characteristic
• Problems with selective breeding

  • Can lead to inbreeding and reduction of the gene pool, increasing chance of genetic defects
• Genetic engineering


  Modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic
• Stages of genetic engineering

  Genes are cut out using restriction enzymes

  2. The vector (e.g. virus or plasmid) is cut using the same enzymes

  3. Ligase enzyme is used to attach the gene to the vector, creating a recombinant gene product