Mendelian Genetics and Punnett Squares Study Notes

Mendel's Pea Plant Experiment:
- P Generation: True-breeding plants (genetically uniform to ensure consistent traits).
- F₁ Generation: Offspring resulting from crossing the P generation.
- F₂ Generation: Offspring resulting from the self-pollination of the F₁ generation.
- Observed Ratio: 705 purple to 224 white flowers (approx. 3:1 ratio).
- Result: Established the concept of dominant vs. recessive inheritance.

Genotype: The genetic makeup regarding a specific trait (represented by two letters).
- Homozygous: Genotype with two identical alleles (e.g., PP or pp).
- Heterozygous: Genotype with two different alleles (e.g., Pp or hybrid).
Phenotype: The observable physical appearance (e.g., Purple vs. White flowers).
- Note: Can be influenced by environmental factors.
Alleles: Alternative versions of a gene (Dominant vs. Recessive).
Diploid Nature: Organisms have two copies of each gene, one from each parent.

The Law of Segregation:
- During meiosis, the two alleles for a trait segregate so each gamete receives only one allele.
- Monohybrid Cross Example (Yy \times Yy):
  - Phenotype Ratio: 3 Yellow : 1 Green
  - Genotype Ratio: 1 \text{ YY} : 2 \text{ Yy} : 1 \text{ yy}
The Law of Independent Assortment:
- Alleles for different genes assort independently during gamete formation.
- The inheritance of one trait does not affect the inheritance of another.

Dihybrid Cross: A cross involving two traits.
- P Generation: YYRR \times yyrr
- F₁ Generation: All YyRr (Gametes: YR, Yr, yR, yr)
- F₂ Generation Ratio: 9 \text{ Yellow/Round} : 3 \text{ Green/Round} : 3 \text{ Yellow/Wrinkled} : 1 \text{ Green/Wrinkled}
Probability: The likelihood of a specific outcome.
- In genetics, the separation of alleles during gamete formation is a probabilistic event.

Define the Cross: Write out the genotypes of both parents (e.g., Hh \times hh).
Setup the Grid: Draw a 2 \times 2 grid (for monohybrid) or 4 \times 4 (for dihybrid).
Distribute Alleles: Place one parent's alleles on top and the other's on the side (Law of Segregation).
Fill the Boxes: Combine the top and side alleles (always write dominant letters first).
Analyze Ratios:
- Genotype Ratio: Count Homo-dominant, Heterozygous, and Homo-recessive.
- Phenotype Ratio: Determine the physical traits based on dominance rules.