Mendel’s Laws

• Mendel's Laws of Inheritance

  Formulated by Mendel from his experiments
• Mendel's First Law of Inheritance

  Law (principle) of Segregation
• Mendel's First Law of Inheritance

  • Each gene is controlled by two factors (now known as homologous chromosomes)
  • When gametes form during meiosis, the two factors/alleles are separated and segregated
  • A gamete contains one of the two factors/alleles from each parent
• Mendel's Second Law of Inheritance

  Law of Dominance
• Mendel's Second Law of Inheritance

  • Certain alleles of a gene exist in either a dominant or a recessive form
  • If the pair of alleles are different (one dominant, one recessive) the phenotype will only show the dominant allele
• Mendel's Third Law of Inheritance

  Law (principle) of Independent Assortment
• Mendel's Third Law of Inheritance

  • Due to random arrangement of chromosomes at the equator during meiosis (gamete formation), any one of the two alleles of one characteristic can sort with any one of ANOTHER characteristic
  • The alleles of different genes move independently of each other into the gametes, and can therefore appear in the gametes in different combinations
• Genetic diagrams


  Important to start with a PAIR of alleles in the mother and another PAIR in the father because each individual inherits TWO alleles for a gene (one maternal and one paternal)
• Genetic diagrams

  • The pair of alleles may be identical (HOMOZYGOUS) or different (HETEROZYGOUS)
  • Alleles are represented by letters: CAPITALS for dominant alleles, lowercase for recessive alleles
  • P stands for the Parent generation