Genetic change and variation

• Haploid


  Having one set of chromosomes (n)
• Diploid


  Having two sets of chromosomes (2n)
• Homologous chromosomes


  a pair of chromosomes, one inherited from each parent, each carrying the same genes at the same locus.
• Locus


  position of a gene on the chromosome
• Sister chromatid


  one of two copies of a replicated chromosome produced during cell division.
• Centromere


  the area where sister chromatids are joined together.
• Mutation


  a permanent change in the DNA base sequence.
• Gametic mutation


  mutation that occurs in the sex cells and can be passed on to the offspring during fertilisation.
• Somatic mutation


  mutation that occurs in the DNA of body cells and only affects the individual (not passed on).
• Meiosis


  Cell division to produce 4 daughter cells from one parent cell. Each cell has half the number of chromosomes, so are haploid. The daughter cells are called gametes.
• Gametes


  sex cells: Sperm, egg, ova and pollen
• Mitosis


  Cell division for growth and repair.
• Stages of Meiosis

  1. Chromosomes replicate
  2. Nuclear membrane breaks apart
  3. Homologous chromosomes pair and cross over
  4. Independent assortment
  5. Chromosomes are pulled apart
  6. Cell divides
  7. Independent assortment
  8. Chromosomes are pulled apart
  9. Cell divides
  10. 4 daughter cells are made (gametes)
• Crossing over


  the exchange of segments of DNA between homologous chromosomes. Results in each gamete having a different allele combination.
• Independent Assortment


  the random way homologous pairs line up along the equator of during meiosis. Results in each gamete having a mix of maternal and paternal chromosomes.
• Segregation


  homologous pairs separate so that each gamete carries one allele per gene. Results in each gamete having a different combination of alleles.
• Recombinants


  offspring or chromosomes with new allele combinations due to crossing over.
• Co-dominance


  alleles of a gene in heterozygotes are both independently and fully expressed
• Incomplete dominance 


  an allele for a specific trait is not completely dominant over the other, resulting in heterozygotes having an intermediate phenotype.
• Lethal alleles


  allele combinations that code for a phenotype that causes the death of an individual
• Multiple alleles 


  more than two alleles for a gene at a locus
• Gene pool


  the total alleles available to a population
• Genotype


  The combination of alleles that an individual possesses
• Segregation


  occurs when sister chromatids separate during meiosis II, so that each gamete receives only one copy of a gene
• Allele


  an alternative form of a gene
• Recombination


  The rearrangement of alleles due to crossing over during meiosis
• Complete dominance


  When the dominant allele is always expressed
• Phenotype


  the physical expression of our genes