Genetics Lecture Notes

Introduction to Genetics

• Importance of Mendel's laws in the study of genetics.
• Key experiments: Monohybrid and Dihybrid crosses.
• Mendel's conclusion: Factors passed from one generation to another.

Mendel's First Law: Law of Segregation

• Definition: During gamete formation, alleles for each gene segregate so each gamete carries only one allele for each gene.
• Understanding Alleles:
  • Genes: Units of heredity carrying genetic information.
  • Alleles: Different forms of genes (e.g., TT, tt, Tt).
  • Example: Chromosomes with height genes (T or t).

Breakdown of the Law of Segregation

1. Gamete Formation: Related to meiosis, producing four cells with half the chromosomes.
2. Alleles of Each Gene: Referring to genes such as C and C for skin color.
3. Segregation: Alleles separate during meiosis.
4. One Allele per Gamete: Each resulting gamete has one allele (e.g., 2 cells with C and 2 cells with c).

• Historical Context: The law was hypothesized by Mendel, later accepted as a fundamental genetic principle.

Mendel's Second Law: Law of Independent Assortment

• Definition: Genes for different traits assort independently during gamete formation.
• Randomness in Nature: Observed in processes like chromosome alignment on the metaphase plate.
• Chromosome Alignment:
  • Random patterns lead to independent segregation of genes (e.g., A/a, B/b, C/c).

Understanding Independent Assortment

• Gene Sets: Example with heterozygous gene sets (AA, BB, CC).
• Assortment Independence: Alleles assort independently; no fixed pattern.
• Best Illustration: Dihybrid crosses rather than monohybrid crosses, allowing for multiple characters to be studied, showing independent assortment.

Mendel's Third Law: Law of Dominance

• Definition: Some alleles are dominant while others are recessive. An organism with at least one dominant allele expresses that trait.
• Example: Tall (TT) and dwarf (tt) plants.
  • Tall phenotype arises from dominant alleles.

Breakdown of the Law of Dominance

1. Dominant and Recessive Alleles: Dominant alleles mask the presence of recessive ones.
2. Phenotypic Expression: At least one dominant allele results in the dominant phenotype.
3. F1 Generation Cross: Tall phenotype in F1 generation indicates dominance.
4. F2 Generation Results: 3 tall plants (TT or Tt) and 1 dwarf plant (tt).

• Conclusion: Law of Dominance is validated through crossing of alleles and observing offspring traits.