Lec 3

• Gene Segregation and Interaction

  1. Mendelian Genetics

  2. Law of Segregation (Monohybrid)

  3. Law of Independent Assortment (Dihybrid)
• Allelic Interactions

  • Complete dominance (3:1)
  • Modified ratios 9:3:3:1
  • Modified Ratios
  • Non-Allelic Interactions
  • Lethal Genes
  • Pseudoalleles
  • Incomplete dominance
  • Over-dominance
  • Co-dominance
  • Dominant
  • Recessive
  • Novel
• Phenotype


  Physical, physiological, biochemical, and behavioral traits of an individual determined by its genotype and its interaction with the environment
• Gene


  Inherited factor on the chromosome responsible for a trait
• Locus


  Location of a gene on a chromosome
• Genotype


  Genetic constitution of an individual
• Allele


  Alternative forms of a gene
• Dominant


  Allele exerting full effect despite the presence of another allele of the same gene
• Recessive


  Allele not expressed in the presence of another allele (dominant) of the same gene
• Homozygous


  Two copies of the same allele of a gene
• Heterozygous


  Two different alleles of the same gene
• Heterozygous alleles for a gene
• Hybridization


  Cross between two individuals with contrasting traits
• F1 or first filial generation is the first generation produced after mating between parents that are homozygous for different alleles
• F2 generation or second filial generation is the generation produced by self-fertilization or sib-mating of F1 individuals
• Backcrossing

  Cross of a heterozygote with one of its parents
• Pre-Mendelian Heredity was thought of as a blending process, a mixture of characteristics of both parents
• Mendel used self-pollinated plants, green peas (Pisum sativum)
• Mendel used pure lines or true breeding parents with contrasting traits, homozygous plants
• Mendel emasculated the female parent to prevent selfing and brushed pollen on the stigma of the female flower
• Genetic determinant (or a hereditary factor) for a specific character is passed from one generation to the next without blending with other genetic determinants
• Punnett Square


  Genotypic Ratio (Pp x Pp): 1PP : 2 Pp : 1pp, Phenotypic Ratio: 3 purple (P_) : 1 white (pp)
• Genotypes


  • p
  • P
  • PP
  • Pp
  • pp
• 22
• Genotypic Ratio (Pp x Pp):
• 23
• Phenotypic Ratio

  • 3 purple (P_) : 1 white (pp)
• Complete dominance


  • One dominant allele is enough to express the dominant trait
  • Homozygous dominant and heterozygote have the same phenotype
• 24
• 25
• Law of Segregation: Alleles in a gene pair separate cleanly from each other during meiosis
• Genotypes

  • AA
  • A
  • aa
  • a
  • Aa
• 27
• Genotypes

  • Aa
  • aa
• 28
• 29
• round vs. wrinkled seeds
• Mendel’s Observation (Fairbanks, D.J. and W.R. Anderson, 1999)
• Phenotypes of True Breeding Parents

  • Round seeds
  • Wrinkled seeds
  • Yellow seeds
  • Green seeds
  • Purple
  • White
  • Inflated pods
  • Constricted pods
  • Green pods
  • Yellow pods
  • Axial flowers
  • Terminal flowers
  • Tall plants
  • Dwarf plants
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