Incomplete Dominance, Codominance, and Multiple Allelism

• Mendelian inheritance

  • The inheritance of traits, according to Mendel, primarily follows the law of dominance.
  • The law of dominance states that when a pure line trait, known as a homozygous dominant trait, is crossed with a recessive trait, all the resulting offspring will express the dominant trait.
• Non-Mendelian Genetics


  Non-Mendelian inheritance is a complex pattern of inheritance that does not follow the laws of heredity by Gregor Mendel. It refers to a pattern of inheritance in which traits do not segregate in accordance with the laws of Mendel. These are traits that do not follow the law of dominance.
• • Human eye color is not governed by Mendelian patterns. In truth, one parent with brown eyes and the other with blue eyes may have children with different eye colors.
  • In this case, the children may have brown eyes, green eyes, or blue eyes.
• Non-Mendelian Laws

  • The Law of Incomplete Dominance
  • Multiple alleles (Multiple Allelism)
  • Sex-linked traits
• Incomplete Dominance

  • Incomplete dominance is a pattern of inheritance describing the formation of a trait that is in between the phenotypes of the parents.
  • There is an absence of the expression of the complete dominant allele. It is very common in the expression of flower color in most plants.
  • For example, when a plant with a red flower (RR) is crossed with a plant with a white flower (rr), the resulting offspring will have pink heterozygous flowers (Rr). Then, when these plants are crossed, red, pink, and white flowers are yielded
• Writing Genotypes for Incomplete Dominance


  Writing the genotypes for alleles under incomplete dominance has the same rules as how the genotypes for alleles under Mendelian genetics are written. The only difference is that the phenotype of a heterozygous trait under incomplete dominance does not follow the dominant trait as stated by Mendelian genetics but rather a trait in between the dominant and recessive traits
• Inheritance under incomplete dominance will have a “mixing of traits” as there is an absence of the expression of the complete dominant allele.
• Another example of a trait that shows incomplete dominance is a person's hair texture. Try to observe the hair texture of the parents of people with wavy hair. It is likely that one parent has curly hair, while the other has straight or somewhat wavy hair.
• Codominance

  • Codominance is a non-Mendelian type of dominance where the alleles of a gene pair in a heterozygote are fully expressed
  • It will result in an offspring with a phenotype that is neither dominant nor recessive
• An excellent example of codominance can be seen in the feather pattern of chickens, with the colors of the offspring's feathers being a checkered mix of the colors of the parents'.
• Codominance is quite common when the coat color of animals is concerned. For example, the coat colors of dogs and cows are usually a mix of the coat colors of their parents.
• Multiple Alleles

  • In some traits, a certain gene can have more than a pair alleles that controls the expression of traits.
  • This is evident in the patterns of inheritance in human blood type
• Multiple Alleles and Codominance in Blood Types

  • Both A and B are dominant alleles over O.  
  • Blood type O can be expressed by homozygous recessive, OO.  
  • Blood type A can have a homozygous dominant AA or heterozygous dominant AO.
  • Blood type B can have homozygous dominant BB or heterozygous dominant BO.
  • Blood type AB has codominant AB alleles because both are equally expressed in the phenotype of the individual with a heterozygous gene.
• • Blood type is controlled by two alleles, and each allele can affect what the blood type of a person is.
  • The alleles for A and B are codominant. The allele for O, on the other hand, is recessive.
• Non-Mendelian Inheritance


  is a complex pattern of inheritance that does not follow the modes of inheritance proposed by Gregor Mendel
• Incomplete dominance


  is a pattern of inheritance characterized by the formation of a trait that is in between the phenotypes of the parents.
• Codominance


  is a non-Mendelian type of dominance where the alleles of a gene pair in a heterozygote are fully expressed
• The expression of some traits may be governed by multiple alleles. An example of this is seen in human blood types