Inheritance, Variation, Evolution

• Sexual reproduction


  Joining (fusion) of male and female gametes
• Sexual reproduction


  • Mixing of genetic information which leads to variety in the offspring
  • Meiosis is involved
• Asexual reproduction


  • Only one parent and no fusion of gametes
  • No mixing of genetic information
  • Only mitosis is involved
• Cell division to form gametes


  1. Copies of genetic information are made

  2. Cell divides twice to form four gametes, each with a single set of chromosomes

  3. All gametes are genetically different from each other
• Fertilisation


  1. Gametes join to restore the normal number of chromosomes

  2. The new cell divides by mitosis

  3. The number of cells increases

  4. Cells differentiate as the embryo develops
• Advantages of sexual reproduction

  • Produces variation in the offspring
  • Variation gives a survival advantage by natural selection
  • Natural selection can be speeded up by selective breeding
• Advantages of asexual reproduction

  • Only one parent needed
  • More time and energy efficient
  • Faster than sexual reproduction
  • Many identical offspring can be produced when conditions are favourable
• Some organisms reproduce by both sexual and asexual methods depending on the circumstances
• DNA


  • The genetic material in the nucleus of a cell
  • A polymer made up of two strands forming a double helix
  • Contained in structures called chromosomes
• Gene


  • A small section of DNA on a chromosome
  • Codes for a particular sequence of amino acids to make a specific protein
• Genome


  The entire genetic material of an organism
• DNA bases


  A, C, G and T
• Genetic code


  • A sequence of three bases codes for a particular amino acid
  • The order of bases controls the order in which amino acids are assembled to produce a particular protein
• DNA polymer


  • Made up of alternating sugar and phosphate sections
  • Attached to each sugar is one of the four bases
• Complementary DNA strands


  A is always linked to T, and C is always linked to G
• Protein synthesis


  1. Proteins are synthesised on ribosomes

  2. Carrier molecules bring specific amino acids to add to the growing protein chain in the correct order

  3. The protein chain folds up to form a unique shape
• Mutations


  • Most do not alter the protein or only alter it slightly
  • A few mutations code for an altered protein with a different shape
• Not all parts of DNA code for proteins
• Alleles


  Different forms of a gene
• Genotype


  The alleles present
• Phenotype


  The characteristics that can be expressed
• Dominant allele


  Always expressed, even if only one copy is present
• Recessive allele


  Only expressed if two copies are present
• Homozygous


  The two alleles present are the same
• Heterozygous


  The two alleles present are different
• Most characteristics are a result of multiple genes interacting, rather than a single gene
• Inherited disorders


  • Polydactyly (having extra fingers or toes) caused by a dominant allele
  • Cystic fibrosis caused by a recessive allele
• Human body cells


  Contain 23 pairs of chromosomes, 22 pairs control characteristics and one pair determines sex
• Variation


  • Differences in the characteristics of individuals in a population
  • May be due to differences in genes, environment, or a combination
• Mutations occur continuously
• Rarely a mutation will lead to a new phenotype that is suited to an environmental change, leading to a relatively rapid change in the species
• Theory of evolution by natural selection

  All species of living things have evolved from simple life forms that first developed more than three billion years ago
• Speciation


  If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species
• Selective breeding


  1. Choosing parents with the desired characteristic from a mixed population

  2. Breeding them together

  3. Continuing over many generations until all the offspring show the desired characteristic
• Characteristics selected for in selective breeding

  • Usefulness (e.g. disease resistance, increased meat/milk production)
  • Appearance (e.g. gentle nature, large/unusual flowers)
• Selective breeding can lead to 'inbreeding' where some breeds are particularly prone to disease or inherited defects
• Genetic engineering


  Genes from the chromosomes of humans and other organisms can be 'cut out' and transferred to cells of other organisms
• Genetically modified (GM) crops

  • Resistant to insect attack or herbicides
  • Generally show increased yields
• Concerns about GM crops include the effect on populations of wild flowers and insects, and the effects on human health
• Genetic engineering process


  1. Enzymes are used to isolate the required gene

  2. The gene is inserted into a vector, usually a bacterial plasmid or a virus

  3. The vector is used to insert the gene into the required cells

  4. Genes are transferred to the cells of animals, plants or microorganisms at an early stage in their development