Inheritance, Variation and Evolution

• Meiosis


  The formation of four non-identical cells from one cell
• Mitosis


  The formation of two identical cells from one cell
• Sexual reproduction


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

  2. Sperm and egg cells in animals

  3. Pollen and egg cells in flowering plants
• Gametes are formed by meiosis, as they are non identical
• Normal cell


  Has 46 chromosomes, two sets of 23 chromosomes (one from each parent)
• Gamete


  Has 23 chromosomes, fuses in fertilisation
• The 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
• Meiosis


  1. Cell makes copies of its chromosomes

  2. Cell divides into two cells, each with half the amount of chromosomes

  3. Cell divides again producing four cells, each with a quarter the amount of chromosomes

  4. These cells are called gametes and they are all genetically different from each other
• Gametes with 23 chromosomes join at fertilisation to produce a cell with 46 chromosomes, the normal number
• This cell divides by mitosis to produce many copies, and an embryo forms
• Advantages of sexual reproduction

  • Produces variation in offspring
  • 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
  • In favorable conditions lots of identical offspring can be produced
• Organisms that use both sexual and asexual reproduction

  • Malarial parasites
  • Some fungi
  • Some plants
• DNA


  The genetic material in the nucleus of a cell, composed of a chemical called DNA
• Gene


  A small section of DNA on a chromosome that codes for a specific protein
• Genome


  All the genes coding for all of the proteins within an organism
• The whole human genome has now been studied and this has improved our understanding of the genes linked to different types of disease, the treatment of inherited disorders and has helped in tracing human migration patterns from the past
• DNA structure


  • DNA is a polymer made up of two strands which wrap around each other like a rope in a double helix structure
  • Between the two strands are the four nitrogenous bases lined up in single rows that form complementary pairs
• Protein synthesis


  1. DNA contains the genetic code for making a protein

  2. The two DNA strands pull apart and mRNA nucleotides match to their complementary base on the strand

  3. The mRNA strand is created as a template of the original DNA

  4. The mRNA moves to the ribosomes where the bases are read in threes to code for an amino acid

  5. The corresponding amino acids are brought to the ribosomes and connected to form a protein

  6. The protein folds to form a unique 3D structure
• Mutation


  • Changes the sequences of bases in DNA, either by insertion, deletion or substitution
  • This can change the type/sequence of amino acids which affects the way the protein folds and functions
• Most mutations do not alter the protein or only do so slightly, but some can have a serious effect
• Variation between organisms arises from both the coding DNA that determines proteins and their activity, and the non-coding DNA that determines which genes are expressed
• Gamete


  An organism's reproductive cell (egg in female and sperm in males), which has half the number of chromosomes (23)
• Chromosome


  A structure found in the nucleus which is made up of a long strand of DNA
• Gene


  A short section of DNA that codes for a protein, and therefore contribute to a characteristic
• Alleles


  The different forms of the gene - humans have two alleles for each gene as they inherit one from each parent
• Dominant allele


  Only one (out of the two alleles) is needed for it to be expressed and for the corresponding phenotype to be observed
• Recessive allele


  Two copies are needed for it to be expressed and for the corresponding the phenotype to be observed
• Homozygous


  When both inherited alleles are the same (i.e. two dominant alleles or two recessive alleles)
• Heterozygous


  When one of the inherited alleles is dominant and the other is recessive
• Genotype


  The combination of alleles an individual has, e.g. Aa
• Phenotype


  The physical characteristics that are observed in the individual, e.g. eye colour
• Family trees show the inheritance of different phenotypes over generations in the same family
• A single gene cross looks at the probability of the offspring of two parents having certain genotypes and phenotypes, using a Punnett square diagram
• Single gene cross


  1. Look at the probability of the offspring of two parents having certain genotypes and phenotypes

  2. Use the alleles the two parents have for a gene and a Punnett square diagram

  3. Be able to draw and use a Punnett square diagram
• Uppercase letters


  Represent dominant characteristics
• Lowercase letters


  Represent recessive characteristics
• Genetic disorders are caused by inheriting certain alleles