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In genetics dominance is defined as the relationship between alleles of one gene in which one allele is expressed over another at the same locus [1]. In heterozygotes the allele that is expressed is considered the dominant allele and the allele that isn't expressed is known as the recessive allele. Dominant alleles only need one copy to be expressed, while recessive alleles need two copies [2]. This is what causes heterozygous offspring to express the same phenotype as their homozygous dominant parent. This idea was one of Mendel's biggest contributions to the study of heredity [1].

As studies on inheritance continued they discovered that the relationship between genotype and phenotype wasn't as simple as the dominant and recessive outcomes discovered by Mendel. They determined that alleles can be completely dominant, partially dominant, codominant, and over dominant [1]. Complete dominance (as explained above) is when one allele is dominant over another, causing heterozygous offspring to express the same phenotype as their homozygous dominant parent. Partial dominance is when neither allele is dominant and the heterozygous offspring can express a range of phenotypes intermediate to the homozygous parents. Codominance occurs when the heterozygous offspring express both phenotypes of their parents. Lastly, overdominance occurs when the heterozygous offspring expresses a phenotype that is more extreme, or out of the range set by both of its parents [1]. These variations in dominance prove to scientists that the relationship between genotypes and phenotypes is complicated and important to understand.

References:

  1. Miko, I. (2008) Genetic dominance: genotype-phenotype relationships. Nature Education 1(1):140
  2. GeneticScience Learning Center (2014) Characteristics of Inheritance. LearnGenetics. Retrieved March 31, 2015 from: http://learn.genetics.utah.edu/content/inheritance/ 

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