Law of Independent Assortment

• Two-factor crosses- The inheritance of two different

  characters within the same groups of individuals
• Dihybrid- a mating experiment

  between two organisms that are

  identically hybrid for two traits
• Two possible hypotheses:

  • Linked assortment- Two genes always stay associated with each other. The alleles of the same gene are not linked

  • Independent assortment- Two different genes randomly segregate into haploid cells
• Non-parentals- combination of traits were not found in the true-breeding of the parental generation. Contradicts the linkage model.
• Mendel’s results from many two-factor crosses rejected the linkage

  hypothesis of assortment
• “Two different genes will randomly assort their alleles during the formation

  of haploid cells”
• Genetic recombination-

  When an offspring receives a combination of alleles that differs from those in the parental generation
• In this two-factor cross,

  make a Punnett square

  containing 16 boxes.

  • The phenotypes of the

  resulting offspring are

  predicted to occur in a

  ratio of 9:3:3:1.
• In crosses involving three or more

  genes, the construction of a single

  large Punnett square becomes very

  unwieldy.

  Trihybrid- between two individuals that

  are heterozygous for three different

  traits. Ratio 27:9:9:9:3:3:3:1
• 2 methods

  • Multiplication method- we can simply use the product rule and

  multiply these three combinations together

  • Forked-line method- the genetic proportions are determined by

  multiplying together the probabilities of each phenotype