The inheritance of traits are controlled by factors called genes. Each gene is present in more than one form called alleles or allelomorphs. Each pair of alleles will have a dominant allele and a recessive allele. In the presence of the dominant gene, the recessive gene will never express itself. Only if both the alleles are recessive, the recessive trait will be exhibited.
For example - In case of pea plants, the plant will be tall if one or both genes for tallness i.e. TT or Tt are present. However the plant will be dwarf only if both the recessive genes i.e. tt are present.
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