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Exceptions to Mendelian Inheritance

Most traits are not inherited in a simple dominant/recessive fashion. In fact, most genes have more than two alleles, and most traits are influenced by more than one gene. It can be very confusing. We will not go into an in-depth discussion of all these exceptions, but we will list the most common variations in the dominant/recessive inheritance pattern: 
  • Multiple alleles: more than two alleles exist for a gene. The most common example of this is blood type in humans: the ABO system. Three different alleles exist, designated IA , IB and i.
  • Incomplete (or partial) dominance: the heterozygote displays a phenotype that is intermediate to the phenotypes of the two homozygotes. Flower color in snapdragons is a classic example of this. When two pure breeding plants are crossed, one that has red flowers and one that has white, all the F1 progeny have pink flowers. When the F1 plants are crossed, one quarter have red flowers, one half develop pink flowers, and one quarter have white flowers.
  • Codominance: the heterozygote exhibits both phenotypes of the homozygotes. We can go back to the ABO blood types for an example of codominance. The i allele is always recessive, so individuals who are IAi are blood type A, IBi  are blood type B, and ii are blood type O. Homozygotes IAIA and IBIB are A and B, respectively. However, when an individual inherits the alleles IAIB, that individual expresses both phenotypes, and is said to be blood type AB.
  • Gene interaction: two genes interact to affect the phenotype of one trait. A novel phenotype will be produced by this interaction. Polygenic traits are characteristics that are caused by many genes acting together. For example, skin color is a polygenic trait.
  • Epistasis: one gene interferes with the phenotypic expression of another gene. This differs from gene interaction in that no new phenotype is produced. A related phenomenon is pleiotropy. This refers to one gene affecting many traits. One example of this would be the genetic disorder Sickle Cell anemia. Individuals with this disease have a mutation in the gene encoding hemoglobin, but have many different organs and systems affected.

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