Step-by-Step Reasoning Framework

Step 1: Identify the Cross Type

• How many traits are involved? → Monohybrid (1), Dihybrid (2), or more
• Are the parents homozygous or heterozygous?
• Is this a P cross, F1 self-cross (F2), or test cross?

Step 2: Identify the Dominance Pattern

• Are all F1 offspring showing one parental phenotype? → Complete dominance
• Is F1 intermediate/blended? → Incomplete dominance (use 1:2:1 in F2)
• Are two distinct phenotypes in F1? → Co-dominance (use 1:2:1 in F2)
• Are there more than 2 alleles in the problem? → Multiple allelism (ABO-type analysis)

Step 3: Check for Gene Interaction (for dihybrid problems)

• Is the F2 ratio standard 9:3:3:1? → No gene interaction, independent assortment
• Is the ratio modified? → Identify which classes are merged:
  • 9:7 → Complementary (9 with both dominants vs. 7 without)
  • 9:3:4 → Recessive epistasis (4 = aa__ merged)
  • 12:3:1 → Dominant epistasis (12 = all with epistatic dominant)
  • 15:1 → Duplicate (15 = all with at least one dominant of either gene)
  • 13:3 → Inhibitory (13 = all with inhibitor or double recessive)

Step 4: Set Up Punnett Square

• For monohybrid: 2×2 square
• For dihybrid: 4×4 square (16 cells)
• List gametes of each parent on axes
• Fill systematically

Step 5: Calculate Probabilities

• Count matching cells for the desired genotype or phenotype
• Express as fraction (e.g., 3/16) or percentage (18.75%)
• Multiply by total offspring for expected number

Step 6: Apply to ABO/Complex Genetics

• Write all possible genotypes for each blood group
• Identify which gametes each parent can produce
• Determine which offspring genotypes are possible

Quick Reference: Is This an Exception to Mendel?