Gregor Mendel and Modern Genetics

Mendelian Genetics

• Background: Gregor Mendel, known as the "Father of Modern Genetics", conducted experiments with garden peas from 1857 to 1865, leading to foundational concepts in heredity.

• Key Definitions:

  • Diploid Organisms: Inherit two copies of a gene (alleles), one from each parent. Each allele may be the same or different.
  • Gene: Heritable factor transmitted from parents to offspring, now identified as a gene.
  • Alleles: Variants of a gene. Dominant alleles mask the effects of recessive ones.

Advantages of Garden Pea for Experiments

• Controlled Mating: Allowed careful observation of trait inheritance.
• Distinct Traits: Traits like flower color (purple or white), seed shape (round or wrinkled), and stem length (tall or dwarf) were easily identifiable.
• Short Generation Time: Facilitated quick observation of outcomes.
• Large Number of Offspring: Provided ample data for analysis.

Mendel's Experimental Design

• P Generation: True-breeding parents.
• F1 Generation: Offspring of P generation; all exhibited dominant traits (e.g., purple flowers).
• F2 Generation: Resulted from self-pollination of F1 generation; exhibited a 3:1 phenotypic ratio of dominant to recessive traits.

Alleles and their Effects

• Dominant Alleles: Cover up the expression of other alleles, represented by uppercase letters (e.g., S).
• Recessive Alleles: Masked by dominant alleles, represented by lowercase letters (e.g., s).
• Genotype vs. Phenotype:
  • Genotype: Genetic makeup (e.g., BB, Bb, bb).
  • Phenotype: Observable characteristics (e.g., purple or white flower color).

Genetic Terminology

• Homozygous: Having two identical alleles for a trait (e.g., PP or aa).
• Heterozygous: Having two different alleles for a trait (e.g., Bb).

Mendelian Ratios

• Studied conditions like cystic fibrosis, a recessive trait requiring two recessive alleles for expression.
• Predicted offspring ratios using Punnett squares, e.g., for cystic fibrosis:
  • Genotypic ratio: 1 CC : 2 Cc : 1 cc (25% cc for cystic fibrosis).
  • Phenotypic ratio: 3 without cystic fibrosis : 1 with cystic fibrosis (75% : 25%).

Mendel's Second Law: Independent Assortment

• Two pairs of traits assort independently when gametes are formed, resulting in various combinations observed in F2 generations.
• Example: Dihybrid cross led to a phenotypic ratio of 9:3:3:1.

Non-Mendelian Patterns

• Incomplete Dominance: Traits blend; for example, crossing red and white flowers results in pink flowers (1:2:1 ratio).
• Codominance: Both alleles expressed equally; e.g., AB blood type from A and B alleles.

Influence of Multiple Genes (Polygenic Inheritance)

• Traits influenced by multiple genes (e.g., skin color, height) show a continuum of phenotypes.
• Environmental Factors: Phenotypes can also change due to environmental influences, such as temperature affecting fur color in Himalayan rabbits.

Pleiotropy

• Single genes can influence multiple traits. For instance, the sickle-cell allele affects hemoglobin and leads to various health issues (e.g., anemia, pain).

Multifactorial Inheritance

• Conditions such as heart disease and diabetes arise from interactions between multiple genes and environmental factors.