Introduction to Mendelian Genetics

Introduction to Mendelian Genetics

• Before the 20th Century Understanding of Heredity

  • Heredity occurs within species.

  • Traits are transmitted directly from parent to offspring.

  • The belief was that traits blended together in offspring.

Gregor Mendel and His Experiments

• Background

  • Mendel is known for discovering the basic principles of heredity through experiments with garden peas.

• Advantages of Pea Plants

  • Distinct varieties with heritable features (e.g., flower color).

  • Controlled mating through cross-pollination.

  • Both sperm (stamens) and egg (carpels) organs present in each plant.

• Experimental Method

  • Stages of Mendel's Method:

  1. Produce true-breeding strains for each trait.

  2. Cross-fertilize strains with alternate forms of a trait, performing reciprocal crosses.

  3. Allow hybrid offspring to self-fertilize and count offspring's traits.

Mendel's Findings on Monohybrid Crosses

• Monohybrid Crosses

  • Studies only 2 variations of a single trait.

  • Key traits studied:

  1. Flower Color: Purple (dominant) vs. White (recessive).

  2. Seed Color: Yellow (dominant) vs. Green (recessive).

  3. Seed Texture: Round (dominant) vs. Wrinkled (recessive).

  4. Pod Color: Green.

  5. Pod Shape: Inflated (dominant) vs. Constricted (recessive).

  6. Flower Position: Axial (dominant) vs. Terminal (recessive).

  7. Plant Height: Tall (dominant) vs. Short (recessive).

• F1 Generation

  • Offspring from true-breeding strains.

  • Displayed the dominant trait exclusively; no intermediate traits.

• F2 Generation

  • Result from self-fertilization of F1 plants.

  • Recessive traits reappear; consistent ratio found is approximately 3:1 for dominant to recessive traits.

Mendel's Principles

• Five-Element Model

  • Parents transmit discrete factors (genes).

  • Each individual receives one copy of a gene from each parent.

  • Alleles can be homozygous (same) or heterozygous (different).

• Dominance

  • Dominant alleles are expressed, while recessive alleles are masked.

  • Genotype: Total set of alleles an individual carries.

  • Phenotype: Physical appearance influenced by genotype.

• Principle of Segregation

  • Alleles separate during gamete formation.

  • Random combination of alleles occurs during fertilization.

Punnett Squares and Probabilities

• Punnett Square

  • Provides a visual representation of genetic crosses and predicted offspring outcomes.

• Rule of Addition

  • The probability of either of two mutually exclusive events is the sum of their separate probabilities.

• Rule of Multiplication

  • The probability of two independent events occurring simultaneously is the product of their probabilities.

Extensions to Mendel's Model

• Polygenic Inheritance

  • Multiple genes affect a single trait, leading to continuous variation (quantitative traits).

• Pleiotropy

  • A single allele may affect multiple traits.

• Multiple Alleles

  • More than two alleles may exist for a gene (e.g., ABO blood types).

• Codominance and Incomplete Dominance

  • Codominance: Both phenotypes expressed (e.g., AB blood type).

  • Incomplete dominance: Intermediate phenotype (e.g., pink flowers from red and white parents).

• Environmental Influence

  • External conditions can affect phenotypic expression (e.g., temperature impact on coat color).

• Epistasis

  • Interaction between genes can alter expected ratios from independent assortment.

The law of independent assortment states that alleles of different genes assort independently of one another during gamete formation. In meiosis, this law is explained through the separation and distribution of homologous chromosomes. During metaphase I of meiosis, homologous pairs align at the equatorial plane of the cell randomly. Each pair can orient themselves towards either pole, leading to a mix of maternal and paternal chromosomes in gametes. Consequently, the inheritance of one trait does not affect the inheritance of another, allowing for genetic diversity in the resulting gametes. This process ensures that alleles are segregated independently during the formation of gametes, leading to combinations of traits that are different from those of the parents, which is essential for evolution and adaptation.