Introduction to Genetics

• Genetics: Branch of biology focused on heredity and variation of organisms.

• Chromosomes carry hereditary information (genes).

• DNA Structure: Arrangement of nucleotides in DNA leads to the flow of information: DNA → RNA → Proteins.

Chromosomes and Genes

• Chromosome Pairs: Occur in pairs;

  • Homologous Chromosomes: Contain the same genes in the same locations but may have different alleles (versions of the gene).

  • Sister Chromatids: Exact replicas of each other; homologous chromosomes are not identical.

Gregor Johann Mendel

• Born in 1822, Czech Republic; trained in Theology, studied botany.

• Conducted experiments with pea plants for eight years.

• Before Mendel, heredity was thought to be a blending process.

Mendel’s Pea Traits

• Studied seven traits:

  • Seed shape (Round/Wrinkled)

  • Seed color (Yellow/Green)

  • Flower color (Purple/White)

  • Pod shape (Inflated/Constricted)

  • Pod color (Green/Yellow)

  • Stem length (Tall/Dwarf)

Mendel's Discoveries

• Published Experiments in Plant Hybridization in 1866, introducing three Principles of Inheritance.

• Rediscovery of his work in 1900.

• Used mathematics to explain inheritance quantitatively.

Key Genetics Terms

• Gene: A unit of heredity that encodes proteins.

• Genome: The complete set of genes in an organism.

• Alleles: Different versions of a gene located on homologous chromosomes.

• Locus: Specific location of a gene on a DNA strand.

Understanding Genetic Combinations

• Homozygous: Identical alleles for a trait.

• Heterozygous: Different alleles for a trait.

• Dominant: Allele that masks another (phenotype expressed in heterozygous).

• Recessive: Allele that is masked by a dominant allele.

Genotype and Phenotype

• Genotype: The genetic makeup of an organism.

• Phenotype: The observable physical characteristics (Genotype + environment).

• Monohybrid Cross: Involves a single pair of traits.

  • Notation: P = Parental, F1 = First generation, F2 = Second generation.

Monohybrid Cross Example

• Crossing tall (TT) and dwarf (tt) pea plants.

• Resulting F1 generation is all heterozygous (Tt).

Punnett Square

• A method to determine offspring genotypes by visualizing combinations.

• Outcomes of a TT x tt cross: 100% Tt (tall phenotype).

F2 Generation Outcomes

• If F1 (Tt) self-fertilizes: Tt x Tt.

• Genotypic ratio of offspring: 1 TT : 2 Tt : 1 tt.

• Phenotypic ratio: 3 Tall : 1 Dwarf.

Mendel's Principles of Inheritance

1. Principle of Dominance: Dominant alleles mask recessive ones.

2. Principle of Segregation: Alleles segregate into gametes.

3. Principle of Independent Assortment: Genes assort independently during gamete formation.

Dihybrid Crosses

• Involves two traits simultaneously (e.g., flower color and stem length).

• F1 generation results in a dihybrid phenotype.

• Ratios and outcomes for phenotypes and genotypes derived from F2 crosses include 9:3:3:1 ratio.

Test Cross

• Used to determine the genotype of an individual with a dominant phenotype by crossing with a homozygous recessive.

Beyond Mendelian Genetics: Incomplete Dominance

• Example with snapdragons: Mixing red (RR) and white (rr) results in pink (Rr).

• In incomplete dominance, phenotypes blend.