Principles of inheritance

• Inheritance


  The process by which characters are passed on from parent to progeny; it is the basis of heredity
• Variation


  The degree by which progeny differ from their parents
• Humans knew from as early as 8000-1000 B.C. that one of the causes of variation was hidden in sexual reproduction
• Humans exploited the variations that were naturally present in the wild populations of plants and animals to selectively breed and select for organisms that possessed desirable characters
• True-breeding line


  A line that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations
• Mendel's hybridisation experiments


  1. Crossed true-breeding pea plant varieties with contrasting traits

  2. Observed the inheritance patterns in the F1 and F2 generations
• Genes


  The units of inheritance that contain the information required to express a particular trait in an organism
• Alleles


  Slightly different forms of the same gene that code for a pair of contrasting traits
• Dominant trait


  The trait that is expressed in the F1 generation when a pair of dissimilar alleles are present
• Recessive trait


  The trait that is not expressed in the F1 generation when a pair of dissimilar alleles are present, but can be expressed in the F2 generation
• Homozygous


  Having identical alleles for a particular trait
• Heterozygous


  Having dissimilar alleles for a particular trait
• Monohybrid cross


  A cross between parents that differ in one character/trait
• Segregation of alleles


  The separation of alleles during gamete formation by meiosis, resulting in each gamete containing only one allele for a particular trait
• Punnett square is a graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross
• In a monohybrid cross between a true-breeding tall plant (TT) and a true-breeding dwarf plant (tt), the F1 generation is all tall (Tt), and the F2 generation shows a 3:1 ratio of tall to dwarf plants
• Monohybrid cross


  1. F1 plants of genotype Tt self-pollinated

  2. Gametes of genotype T and t produced in equal proportion

  3. Fertilisation results in zygotes of genotypes TT, Tt or tt
• 1/4 of random fertilisations lead to TT, 1/2 lead to Tt and 1/4 to tt
• Dominance


  The character T or 'tall' is expressed, the character t or 'dwarf' is not expressed
• In F2, 3/4 of the plants are tall (some TT, some Tt), 1/4 are dwarf (tt)
• Test cross


  Crossing the organism with the recessive parent to determine its genotype
• Test cross


  1. Organism with dominant phenotype crossed with recessive parent

  2. Progenies analysed to predict genotype of test organism
• Mendel proposed two laws: Law of Dominance and Law of Segregation
• Law of Dominance


  • Characters are controlled by discrete units called factors
  • Factors occur in pairs
  • In a dissimilar pair, one member dominates the other
• Law of Segregation


  • Alleles do not show blending
  • Both parental characters recovered in F2
  • Alleles segregate during gamete formation
• Incomplete dominance


  F1 phenotype is intermediate between the two parents
• Incomplete dominance


  • Flower colour in snapdragon (Antirrhinum)
• Co-dominance


  F1 phenotype resembles both parents
• Co-dominance


  • ABO blood groups in humans
• Multiple alleles can exist for a single character, but only two alleles are present in an individual
• Dihybrid cross


  Cross involving two pairs of contrasting traits
• In dihybrid cross, the phenotypes appear in a 9:3:3:1 ratio
• Law of Independent Assortment

  • The inheritance of one pair of characters is independent of the inheritance of another pair
• Cross between two plants

  1. Genotypes of parent plants shown in Figure 4.7

  2. RY and ry unite on fertilisation to produce F1 hybrid RrYy

  3. Mendel self-hybridised F1 plants

  4. 3/4 of F2 plants had yellow seeds, 1/4 had green seeds

  5. Yellow and green colour segregated in 3:1 ratio

  6. Round and wrinkled seed shape also segregated in 3:1 ratio
• Law of Independent Assortment


  When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters
• Dihybrid cross


  1. Phenotypes round, yellow; wrinkled, yellow; round, green; and wrinkled, green appeared in 9:3:3:1 ratio

  2. Ratio derived as combination of 3 round:1 wrinkled and 3 yellow:1 green
• Mendel published his work on inheritance of characters in 1865 but it remained unrecognised until 1900
• Reasons why Mendel's work was not accepted initially


  • Communication was not easy, his concept of genes was not accepted, his mathematical approach was unacceptable, he could not provide physical proof for factors
• Mendel's results on inheritance of characters were rediscovered by three scientists


  1900
• Advancements in microscopy led to discovery of chromosomes and their behaviour during cell division</b>