Mendilian genetics

• Sources of genetic variation

  • Random fertilisation
  • Mutations
  • Independent assortment of chromosomes
  • Crossing over and recombination
• Independent assortment of chromosomes

  Occurs during Metaphase 1
• Crossing over and recombination

  Occur during Prophase 1 of Meiosis
• Crossing over and recombination

  • Formation of a synaptonemal complex [synapsis]
  • Crossing over forms the chiasmata
  • Formation of recombinant chromatids
• Synapsis

  Homologous chromosomes pair up and are held together by the synaptonemal complex
• Determining recombinant frequency

  Use a test cross between a double homozygous recessive and double heterozygote
• Recombinant frequency equation

  Number of recombinants / Total number of offspring * 100
• Recombinant frequency of 50%

  Indicates the genes are on different chromosomes
• Recombinant frequency not 50%

  Indicates the genes are linked as they are on the same chromosome
• Arrangements for double heterozygotes

  • Coupling
  • Repulsion
• Coupling heterozygote

  The wild type is only found on one homologous chromosome
• Repulsion heterozygote

  The wild type is found on both homologous chromosomes
• High recombination frequency for linked genes

  Indicates the genes are far apart
• Low recombination frequency for linked genes

  Indicates the genes are close together
• 1% RF

  Indicates 1 centimorgan (cM) on a linkage map
• Multiple crossovers

  Reduces the distances in a linkage map. The RF cannot be > 50% due to multiple crossovers
• Causes of deviations from Mendelian ratios

  • Sex linkage
  • Incomplete dominance
  • Co-dominance
  • Pleiotropy
  • Polygenic inheritance
  • Epistasis
• Sex linkage

  When an allele is carried on the sex chromosome
• Incomplete dominance

  The dominant allele does not fully mask the recessive allele at the same locus, causing a new phenotype for the heterozygous offspring
• Co-dominance

  Each allele affects the phenotype separately, such as in the ABO blood group
• Pleiotropy

  A single gene affects multiple traits
• Polygenic inheritance

  A single trait is controlled by multiple genes
• Polygenic inheritance

  Provides continuous variation in the population
• Epistasis

  One gene masks the expression of another
• Epistasis ratio

  9:3:4 [HD:H:HR]