Ch7s2

• Types of dominance


  • Complete dominance
  • Incomplete dominance
  • Codominance
• Complete dominance


  A relationship in which in the heterozygous condition one allele is completely dominant over the other
• Complete dominance


  • Heterozygous plants and homozygous dominant plants are indistinguishable in phenotype
  • Both pea plants PP and Pp for flower color have purple flowers
• Identifying complete dominance


  1. See a dominant allele hide the effect of the recessive allele in heterozygous conditions

  2. See the phenotype of heterozygous is the same as the homozygous dominant

  3. See only two traits for one characteristic, like in a pea plant
• Rules of complete dominance

  • Use capital letters for dominant traits
  • Use same letters as dominant, but with a small letter for recessive traits
• Complete dominance


  • Black X White

  BB X bb

  Bb (100% black)
• Incomplete dominance


  A relationship where the phenotype of a heterozygote is an intermediate between the phenotypes of two contrasting homozygous traits. Neither allele is completely dominant over the other allele.
• Identifying incomplete dominance


  1. See intermediate phenotypes like pink, gray, short tail, short horn, wavy hair

  2. See 3 traits for one characteristic

  3. See two alleles blend affects each other in heterozygous conditions

  4. Phenotypic and genotypic ratios are always the same (equal)
• Rules of incomplete dominance


  Use capital letters and different letters for each trait
• Incomplete dominance


  • Black X White

  BB X WW

  BW (gray)
• Codominance


  A relationship where both alleles for a gene are expressed in heterozygous offspring. Neither allele is dominant or recessive, nor do the alleles blend in the phenotype.
• Identifying codominance


  See both alleles showed their effects in heterozygous conditions

  See the AB blood type in the ABO blood groups, and MN blood type in the MN blood groups

  See words like spotted, dotted, both phenotypes appeared in heterozygous offspring
• Rules of codominance


  Use capital letters and different letters for each trait