Genetic diversity

• Mutation


  Any change to the quantity or the base sequence of the DNA of an organism.
• Gene mutation 


  Any change to one or more nucleotide bases, or a change in the sequence of the bases in DNA.
• Substitution of bases 


  Type of gene mutation in which a nucleotide in a DNA molecule is replaced by another nucleotide that has a different base.
• Deletion of bases

  A gene mutation by deletion arises when a nucleotide is lost from the normal DNA sequence.
• Chromosome mutations


  Changes in the structure or number of whole chromosomes
• Forms of chromosome mutation

  Changes in whole sets of chromosomes occur when organisms have three or more sets of chromosomes rather than two. This condition is called polyploid and occurs mostly in plants.

  Changes in the number of individual chromosomes. sometimes individual homologous pairs of chromosomes fail to separate in meiosis. This is known as non-disjunction and usually results in gamete having one more or one fewer chromosome.
• Mitosis 


  Produces two identical daughter cells with the same number of chromosomes as the parent cell and as each other.
• Meiosis

  Usually produces four daughter cells, each with half the number of chromosomes as the parent cell.
• The process of meiosis

  1. In the first division homologous chromosomes pair up and their chromatids wrap around each other. Equivalent proportions of these chromatids may be exchanged in a process called crossing over. By the end of this division the homologous pairs have separated, with one chromosome from each pair going into one of the two daughter cells.
  2. In the second division the chromatids move apart. At the end of meiosis 2, four daughter cells have been formed. In humans they each contain 23 chromosomes.
• How does meiosis produce genetic variation?

  • Independent segregation of homologous chromosomes
  • New combinations of maternal and paternal alleles by crossing over
• Gene


  A length of DNA that codes for a polypeptide
• Locus 


  The position of a gene on a chromosome of DNA molecule
• Allele


  One of the different forms of a particular gene
• Homologous chromosome 


  A pair of chromosomes, one maternal and one paternal, that have the same gene loci
• Independent segregation 


  Homologous chromosomes line up alongside each other. When they arrange themselves in this line they do so at random. One of each pair will pass to each daughter cell. The combination of chromosomes of maternal and paternal origin that go into the daughter cell at meiosis 1 is a matter of chance.
• Genetic recombination by crossing over

  • The chromatids of each pair become twisted around one another
  • During this twisting process tensions are created and portions of the chromatids break off
  • These broken portions might then rejoin with the chromatids of its homologous partner
  • Usually it is the equivalent portions of homologous chromosomes that are exchanged
  • In this way new genetic combinations of maternal and paternal alleles are produced
• Crossing over

  The chromatids cross over one another many times and so the process is known as crossing over
• Recombination


  The broken-off portions of chromatid recombine with another chromatid.
• Genetic diversity 


  The total number of different alleles in a population
• Directional selection

  Selection may favour individuals that vary in one direction from the mean of the population. This changes the characteristics of the population.
• Stabilising selection


  Selection may favour average individuals and it preserves the characteristics of a population.
• Natural selection results in species that are better adapted to their environment. These adaptions could be:

  • Anatomical
  • Physiological
  • Behavioural