Lesson 3 Gen Bio

• Traits that do not follow Mendel's assumptions


  Inherited through modifications to Mendel's classic ratios (gene interaction)
• Describe modifications to Mendel's classic ratios (gene interaction)


  1. Describe the modes of inheritance that deviate from Mendelian ratios

  2. Analyze problems involving non-Mendelian modes of inheritance
• Assumptions of Mendel


  • Alleles of a gene separate during gamete formation (Law of Segregation)
  • Alleles from different loci assort independently (Law of Independent Assortment)
• Assumptions of Mendel


  • In heterozygous individuals, the dominant allele completely masks the expression of the recessive allele (Principle of Dominance)
  • AA or Aa aa
• 4 Extensions of Mendelian Inheritance


  • The expression of the alleles does not follow complete or simple dominance
  • May have greater number of allelic variations for a single gene
  • May have greater number of phenotypic variations for a single trait
  • Gives offspring ratios that are different from the 3:1 and 9:3:3:1 of Mendel
• 3 Basic Extensions of Mendelian Inheritance


  • Incomplete Dominance
  • Codominance
  • Multiple Alleles
• Incomplete Dominance


  • Also called partial dominance
  • Two-allele system
  • Blending of homozygous phenotypes
  • At most three phenotypic classes
  • Heterozygote with the intermediate phenotype
• Flower Color in Mirabilis

  • C^R = red flowers
  • C^W = white flowers
• Genotypes and Phenotypes in Mirabilis

  • C^R C^R = red
  • C^R C^W = pink
  • C^W C^W = white
• Coat Color in Horses


  • chestnut
  • palomino
  • cremello
• Alleles in Coat Color in Horses

  • C^B = brown fur
  • C^W = white fur
• Genotypes and Phenotypes in Coat Color in Horses

  • C^B C^B = chestnut
  • C^B C^W = palomino
  • C^W C^W = cremello
• Cross 1: Flower Color in Mirabilis


  1. P generation: red x white

  2. F1 generation: pink x pink
• Genotypes and Phenotypes in Mirabilis Cross

  • C^R C^R = red
  • C^R C^W = pink
  • C^W C^W = white
• Cross 2: Coat Color in Horses

  1. P generation: chestnut x cremello

  2. F1 generation: palomino x palomino
• Genotypes and Phenotypes in Coat Color in Horses Cross

  • C^B C^B = chestnut
  • C^B C^W = palomino
  • C^W C^W = cremello
• Codominance

  • Simultaneous expression of two alleles
  • Two-allele system
  • No blending of homozygous phenotypes
  • At most three phenotypic classes
  • Heterozygote shows both alleles
• Coat Color in Cattle

  • F^R = red coat
  • F^W = white coat
• Genotypes and Phenotypes in Coat Color in Cattle

  • F^R F^R = red
  • F^R F^W = roan
  • F^W F^W = white
• Cross 1: Coat Color in Cattle

  1. P generation: red x white

  2. F1 generation: roan x roan
• Multiple Alleles

  Refer to a non-Mendelian inheritance pattern that involves more than just typical two alleles that code for a certain characteristic
• Blood Type

  • I^A dominant
  • I^B dominant
  • i recessive
• Solve for the probability of obtaining the provided offspring in column B, given the cross in column A.

  1. pink flowers x white flowers - pink

  2. palomino x palomino - palomino

  3. type N x type MN - type N
• Mendelian inheritance assumes that each unit factor or gene has alleles that are inherited through complete dominance. Thus, monohybrid and dihybrid crosses give characteristic ratios of 3:1 and 9:3:3:1, respectively.
• Extensions of Mendelian inheritance or non-Mendelian inheritance

  • Consists of modes of inheritance that do not follow all of the assumptions of Mendel
  • Incomplete dominance involves the blending of phenotypes in heterozygous individuals. Examples include flower color in Mirabilis and fur color in horses.
  • Codominance involves the simultaneous expression of two different alleles in heterozygotes. Examples include the MN blood group in humans and the coat color in cattle.
  • Multiple allelism involves characteristics with at least different alleles. An example is the inheritance of ABO blood type in humans.