Alternatives to Mendelian Inheritance

• Some alleles have multiple effects;
  e.g., dark feet and crossed eyes in Siamese cats arise from the same gene)
  
  
• Some genes alter the effects of other genes (e.g., a gene for no pigment may prevent expression of a gene for hair or eye color)
  vs. OPERONS
  
  
• Polygenes control characters without dominance, making continuous gradations in phenotype (e.g., skin color in humans)
  
  
• Mixed dominance plus alteration (eye color)
  3 genes 2 chromosomes: 15 19
  
  
• The environment can affect phenotype expression (e.g., fur color of Siamese cats: dark at low temperature, light at high temperatures)