Mendel's Laws
High-Yield Summary
- Mendel's pea-plant crosses showed traits are inherited as discrete units (genes), not blended. Generations: P (original parents) → F1 (their offspring) → F2 (F1 × F1 offspring).
- Law of segregation: each individual carries two alleles per gene (one per parent); during meiosis, the alleles separate so each gamete carries only one. Explains why a recessive trait can vanish in F1 but reappear in F2.
- Monohybrid cross (Pp × Pp): 1:2:1 genotypic ratio (1 PP : 2 Pp : 1 pp) → 3:1 phenotypic ratio under complete dominance, since PP and Pp look identical.
- Law of independent assortment: alleles of different genes assort independently into gametes IF on different chromosomes (or far apart on the same one). Rooted in random homologous chromosome alignment at metaphase I.
- Dihybrid cross (YyRr × YyRr): each parent makes 4 gamete types (YR, Yr, yR, yr) → 16-box Punnett square → 9:3:3:1 phenotypic ratio = product of two independent 3:1 ratios.
- Independent assortment breaks down for genes closely linked on the same chromosome (genetic linkage) — a significant deviation from 9:3:3:1 signals linkage.
Dihybrid Cross Ratio
(3:1) × (3:1) = 9:3:3:1
- 9 = both dominant traits (e.g., yellow, round)
- 3 = dominant/recessive combo (e.g., yellow, wrinkled)
- 3 = recessive/dominant combo (e.g., green, round)
- 1 = both recessive traits (e.g., green, wrinkled)
- Each gene independently produces its own 3:1 ratio; independent assortment means the combined ratio is the product of the two.
Key Terms
- P / F1 / F2 generation
- P = original parental cross; F1 = their direct offspring; F2 = offspring of crossing F1 individuals together.
- Monohybrid cross
- A cross tracking inheritance of a single gene with two alleles.
- Dihybrid cross
- A cross tracking two genes simultaneously; dihybrids are heterozygous for both.
- Genetic linkage
- Genes located close together on the same chromosome tend to be inherited together, breaking independent assortment.
Why the Ratios Come Out the Way They Do
- 1Segregation: each heterozygous parent contributes one of two equally likely alleles to each gamete
- 2Combining two independent 50/50 draws (Pp × Pp) → 1:2:1 genotypic split
- 3Complete dominance collapses 1:2:1 genotypic into 3:1 phenotypic (PP and Pp look identical)
- 4Independent assortment: two genes each give their own 3:1 ratio; combined dihybrid ratio = (3:1) × (3:1) = 9:3:3:1
Common MCAT Trap
- 1:2:1 is the GENOTYPIC ratio of a monohybrid cross; 3:1 is the PHENOTYPIC ratio under complete dominance — don't mix these up when a question specifies genotype vs. phenotype.
- Independent assortment only applies to genes on different chromosomes (or far apart on the same one) — genes close together on the same chromosome are linked and violate the 9:3:3:1 expectation.
- The law of segregation is about ONE gene's two alleles separating; the law of independent assortment is about MULTIPLE genes assorting independently of each other — distinct laws, don't conflate them.
Quick Recall
Why can a recessive trait disappear in F1 but reappear in F2?
What Punnett square result signals two genes might be linked rather than independently assorting?
What's the biological basis of independent assortment?