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					Extensions to Mendel: Complexities in Relating
            Genotype to Phenotype
              Outline of extensions to
                Mendel’s analysis
• Single-gene inheritance
   – In which pairs of alleles show deviations from complete dominance and
   – In which different forms of the gene are not limited to two alleles
   – Where one gene may determine more than one trait

• Multifactorial inheritance in which the phenotype
  arises from the interaction of one or more genes
  with the environment, chance, and each other
  Dominance is not always complete
• Crosses between true-breeding strains can
  produce hybrids with phenotypes different
  from both parents
  – Incomplete dominance
     • F1 hybrids that differ from both parents express an intermediate
       phenotype. Neither allele is dominant or recessive to the other
     • Phenotypic ratios are same as genotypic ratios
  – Codominance
     • F1hybrids express phenotype of both parents equally
     • Phenotypic ratios are same as genotypic ratios
Three Different Forms of Dominance
Incomplete Dominance for Flower Color in Snapdragon
Codominance of Spotted and Dotted Coat Pattern Alleles
Codominance of IA and IB Blood Group Alleles
   There Are Often More Than Two Alleles of a Gene

The ABO blood group system is determined by one gene with three alleles.
   There Are Often More Than Two Alleles of a Gene

Note that the ABO blood system shows both complete dominance and codominance.
Multiple Alleles Can be Grouped in a Dominance Series

        Dominance series for lentil bean coat color.
 Do variations on dominance relations
 negate Mendel’s law of segregation?
• Dominance relations affect only the
  relationship between genotype and
• Alleles still segregate randomly and unite
• Gene products control expression of
• Interpretation of phenotype/genotype
  relationship can be complex

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