Chapter 14: Mendel and the Gene

Mendel's work on genetics initiated in 1856, focusing on plant breeding, particularly using pea plants (Pisum sativum).

The significance of Mendel's findings was not recognized until years after his death, leading to a temporary setback in the acceptance of his principles.

Self-fertilization: Pea plants can fertilize themselves or can be cross-fertilized with pollen from other plants.
- Female organ receives pollen from the male organ, which produces pollen grains recording sperm cells.
Cross-fertilization: Taking pollen from one individual and transferring it to the female organ of another plant with removed male organs.

The analysis of seven distinct characteristics in pea plants in the F2 generation demonstrates Mendel's principles.
Table 14.1 Results of Mendel's F₁ Crosses for Seven Characters in Pea Plants:
- Flower Color:
- Dominant Trait: Purple
- Recessive Trait: White
- Ratio: 705:224 (3.15:1)
- Flower Position:
- Dominant Trait: Axial
- Recessive Trait: Terminal
- Ratio: 651:207 (3.14:1)
- Seed Color:
- Dominant Trait: Yellow
- Recessive Trait: Green
- Ratio: 6,022:2,001 (3.01:1)
- Seed Shape:
- Dominant Trait: Round
- Recessive Trait: Wrinkled
- Ratio: 5,474:1,850 (2.96:1)
- Pod Shape:
- Dominant Trait: Inflated
- Recessive Trait: Constricted
- Ratio: 882:299 (2.95:1)
- Pod Color:
- Dominant Trait: Green
- Recessive Trait: Yellow
- Ratio: 428:152 (2.82:1)
- Stem Length:
- Dominant Trait: Tall
- Recessive Trait: Dwarf
- Ratio: 787:277 (2.84:1)
Mendel's findings consistently demonstrate the dominant to recessive ratio of approximately 3:1 in the F2 generation, challenging the blending hypothesis.

Alternative Versions of Heritable Factors:
- Traits are determined by genes, with alternative forms known as alleles.
- Variations arise from differences in the DNA sequence.
- Example:
  - Alleles for flower colors: purple (dominant) and white (recessive).
- The enzyme that produces purple pigment is influenced by these alleles.
Dominance:
- Dominant alleles express their traits in the presence of recessive alleles.
- Note: Dominance does not imply commonality; for example, the dominant allele causing polydactyly is rare.
The Principle of Segregation:
- During gamete formation, the alleles for each gene segregate from one another. Each gamete carries only one allele for each gene.
The Principle of Independent Assortment:
- Alleles for different traits segregate independently during gamete formation.
- E.g., the assortment of alleles for yellow (Y) and round (R) seeds corresponds to the behavior of chromosomes during meiosis. This allows dihybrid crosses to be calculated as two independent monohybrid crosses, simplifying complex inheritance problems.

In a dihybrid cross (e.g., YyRr x YyRr), calculate probabilities:
- Yellow and Round:
- rac{3}{4} (YY ext{ or } Yy) imes rac{3}{4} (RR ext{ or } Rr) = rac{9}{16}
- Recessive for both traits:
- rac{¼ (yy)} imes rac{¼ (rr)} = rac{1}{16}

When analyzing crosses involving three segregating genes (e.g., YyRrTt x YYRrtt), one can calculate the probability of particular genotypic combinations:
- Example: Probability of YYrrTt:
- rac{1}{2} (YY) imes rac{1}{2} (rr) imes rac{1}{2} (Tt) = rac{1}{16}
- To find the total probability of being recessive for at least two characters, consider all genotypes.

Recessive for both characters:

Mendelian genetics provide a foundational understanding of heredity and inheritance patterns, establishing principles that still influence genetic analysis today. Mendel's work—while initially overlooked—sets the groundwork for modern genetics and further discoveries in the field.