The Product Rule: To determine the probability of two independent events occurring together (e.g., inheriting allele A and allele B), multiply their individual probabilities: .
The Sum Rule: To determine the probability of any one of two or more mutually exclusive events occurring (e.g., being either homozygous dominant or heterozygous), add their individual probabilities: .
Punnett Squares: These grids are used to predict the genotypic and phenotypic outcomes of a cross. For a monohybrid cross of heterozygotes (), the expected phenotypic ratio is , while a dihybrid cross () yields a ratio.
| Feature | Monohybrid Cross | Dihybrid Cross |
|---|---|---|
| Focus | Single trait/gene | Two independent traits/genes |
| Typical F2 Ratio | (Phenotypic) | (Phenotypic) |
| Gamete Combinations | 2 per parent | 4 per parent |
| Law Demonstrated | Law of Segregation | Law of Independent Assortment |
Identify the Parentals: Always check if the parents are "true-breeding" (homozygous). A cross between two different true-breeding parents () will always result in heterozygous offspring () in the F1 generation.
Recognize Ratios: If you see a ratio in the offspring, the parents were likely both heterozygous for that trait. If you see a ratio, one parent was likely heterozygous and the other was homozygous recessive.
Test Crosses: To determine the genotype of an individual with a dominant phenotype, cross it with a homozygous recessive individual. If any offspring show the recessive trait, the parent must have been heterozygous.
Dominance vs. Frequency: A common mistake is assuming that dominant alleles are more common in a population. Dominance only describes how alleles interact in an individual, not how frequently they appear in a group.
Gene vs. Allele: Students often use these terms interchangeably. Remember that a gene is the category (e.g., eye color), while an allele is the specific version (e.g., blue or brown).
Independent Assortment Limits: Independent assortment only occurs if genes are on different chromosomes. If genes are "linked" (close together on the same chromosome), they tend to be inherited together, violating Mendel's second law.
