Variation

• Variation 


  • refers to the differences in traits between individuals of the same species.
• Continuous Variation


  • There are many intermediate forms (gradual difference)

  • It is caused by the additive effect of numerous genes.

  
• Discontinuous Variation


  • The differences are obvious and there are few or no intermediate forms.
  • controlled by single gene or a few genes

  
• Differences between discontinuous variation and continuous variation

  • Discontinuous :

  1. Involves a few clear-cut phenotypes
  2. Controlled by one or a few genes
  3. Genes do not show additive effect
  4. Relatively unaffected by environmental conditions

  • Continuous :

  1. Involves a range of phenotypes
  2. Controlled by many genes
  3. Genes show additive effect
  4. Greatly affected by environmental conditions

  
• Mutation


  • a change in the sequence of a gene or in the chromosome number
• Mutagens 


  Mutagens are agents in the environment that increase the rate of mutation in an organism.

  • Ionising radiation

  • Certain chemicals
• Meiosis


  • A source of genetic variation
  • Cell division (Nuclear division)
  • produces four haploid gametes are that are genetically unique from the parent and contain only half as much DNA (in human : 1 x 46 -> 4 x 23)
  • haploid gametes restore diploid number in zygote after fertilisation
• Mitosis


  • A source of genetic variation
  • Body Cells for growth and repair
  • produces two genetically identical daughter cells from a single parent cell
  • Typically get 2 cells (in human : 1 x 46 -> 2 x 46)
• Albinism


  • A recessive gene mutation

  • No pigments in the hair, skin and eyes

  • May be easily sunburnt since pigment plays protective role
• Sickle Cell Anaemia



  • A recessive gene mutation

  • A change in the sequence of bases

  1. Change in one amino acid
  2. HbS protein is synthesised instead of HbA
  3. Sickle-shaped red blood cell

  • Sickle-shaped red blood cells clump together and may block blood vessels
  • Normal red blood cells can live up to 120 days. But, sickle cells only live for about 10 to 20 days.
  • lower surface area to volume ratio, less oxygen binds to red blood cell

  
• Down Syndrome



  • A chromosome mutation

  • Three copies of chromosome 21