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


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


  • Sperm and egg cells in animals
  • Pollen and egg cells in flowering plants
• Gametes are formed by meiosis, as they are non identical
• A normal cell has 46 chromosomes, in 23 pairs, one from each parent
• Each gamete has 23 chromosomes and they fuse in fertilisation
• The genetic information from each parent is mixed, producing variation in the offspring
• Asexual reproduction


  One parent with no gametes joining, using mitosis to form two identical cells
• Asexual reproduction leads to clones, which are genetically identical to each other and the parent
• Organisms that reproduce asexually

  • Bacteria
  • Some plants
  • Some animals
• Meiosis


  Cell makes copies of chromosomes, divides into two cells with half the chromosomes, then divides again to form four genetically different gametes
• Gametes with 23 chromosomes join at fertilisation to produce a cell with 46 chromosomes
• The cell divides by mitosis to produce many copies, forming an embryo which then undergoes differentiation
• Advantages of sexual reproduction

  • Produces variation in offspring
  • Allows selective breeding
• Advantages of asexual reproduction

  • Only one parent needed
  • Uses less energy and is faster
• Organisms using both sexual and asexual reproduction

  • Malarial parasites
  • Some fungi
  • Some plants
• DNA

  The genetic material in the nucleus of a cell, a chemical polymer with a double helix structure
• 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, improving understanding of genes linked to disease and human migration
• Nucleotide

  A small part of DNA, made up of a sugar, phosphate and one of four organic bases
• A and T bases, and C and G bases, form complementary pairs in the DNA double helix
• Protein synthesis

  DNA in nucleus transcribed to mRNA, mRNA transported to ribosomes, amino acids brought to ribosomes and joined to form protein
• Mutation

  A change in the sequence of bases in DNA, can be an insertion, deletion or substitution
• Most mutations do not alter the protein or only do so slightly, but some can have a serious effect on the protein structure and function
• Gamete

  An organism's reproductive cell, with half the normal number of chromosomes
• Chromosome

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

  A short section of DNA that codes for a protein, contributing to a characteristic
• Allele

  The different forms of a gene, humans have two alleles for each gene
• Dominant allele


  Only one is needed to be expressed and observed
• Recessive allele


  Two copies are needed to be expressed and observed
• Homozygous


  Both inherited alleles are the same
• Heterozygous


  One inherited allele is dominant, the other is recessive
• Genotype


  The combination of alleles an individual has
• Phenotype


  The physical characteristics that are observed in the individual
• 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