HL 1 Biology

10.2 Dihybrid Cross

<<Advanced Genetics>>

Mendel’s dihybrid cross Experiment

  • Dihybrid cross: the inheritance of two genes is investigated together
  • crossed pure-breeding peas of round  yellow seeds with wrinkled green seeds
  • F1 all round yellow seeds (dominant alleles)
  • Ratio from his Punnett grid was 9:3:3:1
  • conclusion: genes are inherited independently of each other, allowing new combinations of characters to be formed freely

Independent Assortment:  theory that alleles of two genes pass into gametes without influencing each other

  • if genotype SsYy
  • 1/4 chance of gamete containing 2 specific allele
  • 1/2 chance of gamete containing each allele

Grasshopper observation

  • In meiosis: shown homologous chromosomes pair up and then separate to opposite poles
      • Direction chromosome pair moves depends on the way pair is facing (RANDOM)
      • one way pair is facing does not affect the direction in which other pairs are facing = independent orientation
  • If heterozygous for a gene: one chromosomes in pair will carry one  allele of the gene and the other chromosome will carry the other allele
  • orientation of pair chromosome determines which allele moves to which pole
  • 50% chance allele moving to particular pole
  • 25% chance 2 alleles coming together (due to random orientation)

*Mendel’s law of independent assortments have exceptions: gene linkage (behavior of chromosomes in meiosis)


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