genetics.

• sexual reproduction.


  the joining of two sex cells or gametes during fertilisation by two parents.
• asexual reproduction.


  only involves one parent so there is no joining of sex cells during fertilisation.
• meiosis.


  produces haploid non-identical sex cells.

  fuse to form a diploid fertilised egg during fertilisation.

  produces sperm and egg cells.
• meiosis diagram:


  meiosis diagram:

  
• DNA.


  sugar phosphate backbone.

  bases: A - T and C - G.

  double helix structure held by weak hydrogen bonds.

  carries the genetic code for a living organism.
• chromosomes.


  long threads of DNA, that is made up of many genes.
• genes.


  small section of DNA in a chromosome.

  codes for specific substances.
• genome.


  all of an organisms DNA.
• mutation.


  change of a gene or a chromosome and happens continually.

  can be inherited.
• natural selection.


  organisms that are better adapted to an environment will survive and reproduce.

  called ' survival of the fittest. '
• alleles.


  different forms of the same gene.
• homozygous.


  two alleles for the characteristic are identical.

  for example: BB or bb.
• heterozygous.


  two alleles for a particular characteristic are different.

  for example: Bb.
• dominant.


  the relationship between two versions of a gene.

  represented by capital letters.

  for example: B ( dominant. )
• recessive.


  a gene that can be masked by a dominant gene.

  represented by lower case letters.

  for example, b ( recessive. )
• punnet square example:


  punnet square example:

  
• selective breeding.


  humans breed plants and animals for particular genetic characteristics.
• main steps in selective breeding:


  main steps in selective breeding:

  
• diagram of selected breeding:

  diagram of selected breeding:

  
• desired characteristics in plants:

  disease resistant.

  wheat plants that produce lots of grain.

  large flowers.
• desired characteristics in animals:

  animals that produce lots of meat or milk.

  chickens that lay large eggs.
• benefits of selective breeding:


  producing more or better quality food.

  animals can be selected that cannot cause harm.
• risk of selective breeding:


  lack of genetic variation can lead to an attack by specific insects or disease.