biology 12.3

• Incomplete dominance states that one allele is not completely dominant over another allele, the heterozygous phenotype lies

  somewhere between two homozygous phenotypes, resulting in a blending of phenotypes

  ♥ four o’clock flowers:

  RR (red) x rr (white) → Rr (pink)
• Codominance states that phenotypes produced by both alleles are clearly expressed

  
  

  ♥ chicken feathers:

  BB (black) x WW (white) → BW

  (mixture of black & white)
• Multiple alleles is when a gene with more than 2 alleles

  • an individual can only have 2 alleles, but population can have 3 or more alleles

  • rabbit coat color gene has 4 alleles

  C – full color allele

  (dominant to all other alleles)

  c^ch – chinchilla allele

  (dominant to ch & c alleles)

  c^h – himalayan allele

  (dominant to c allele)

  c – albino allele

  (recessive to all other alleles)
• Polygenic traits are controlled by 2 or more genes and they often have a bell-shaped distribution of phenotypes

  • skin color, eye color, and height in humans are polygenic traits
• environmental conditions can affect gene expression and influence genetically determined traits