week 2

Mendelian Genetics Overview

• By mid-19th Century, inheritance mechanisms were poorly understood; believed to be blending.

• Gregor Mendel proposed particulate inheritance in 1866, suggesting specific genetic units control traits passed through generations (first concept of a gene).

Mendel's Three Postulates of Transmission Genetics

1. Segregation

   • Paired factors segregate randomly during gamete formation.

   • Each gamete receives one form of the trait.

2. Independent Assortment

   • When considering multiple traits, factors for each trait assort independently into gametes.

3. Dominance

   • Expression of one trait can exclude another (dominant vs. recessive).

Model Organism

• Common Garden Pea (Pisum sativum)

  • Mendel developed pure-lines of pea types before conducting crosses.

  • Example cross: Round, smooth seeds with wrinkled seeds resulted in all round seeds (dominant round seed trait).

  • Reciprocal crosses produced same results, demonstrating dominance.

Definitions

• Dominant Trait: Trait expressed in the presence of another.

• Recessive Trait: Trait not expressed when dominant trait is present.

• Formula for appearance (phenotype): P = G + E + (G * E)

  • P = phenotype; G = genotype; E = environment.

Influence of Environment on Genotype

• Environmental conditions can significantly impact crop growth and well-being (e.g., drought, soil nitrogen levels).

• A plant's inherent genome also affects health and productivity, influencing traits such as grain yield and nutrient content.

• Example: Two corn hybrids with a disease-resistance gene yield equally without disease but differ under epidemic conditions.

Types of Traits

• Qualitative Traits:

  • Easily inherited, expressed in discrete forms, observable irrespective of environmental conditions.

  • Examples: Flower color, leaf pubescence.

• Quantitative Traits:

  • Influenced by multiple genes, expressed continuously, affected by environmental variations.

  • Important for yield and stress tolerance.

Genetic Variation Sources

1. Gene recombination during meiosis.

2. Variations in chromosome number (ploidy levels).

3. Mutations.

4. Biotechnology.

Terminology

• Allele: Alternate forms of a gene controlling the same phenotype.

• Homozygous: Two identical alleles (e.g., HH or hh).

• Heterozygous: Different alleles (e.g., Hh).

• Filial: F1 generation progeny; F2 generated from self-fertilized F1.

Mendel's Observations in Pea Plants

• When crossing round and wrinkled peas, dominant round seeds appeared in F1; wrinkled trait re-emerged in F2 at a 3:1 ratio.

• Mendel's chart recorded ratios for six traits, with a general trend of 3:1 (dominant to recessive).

• Continued self-pollination in the F2 generation confirmed the segregation ratios.

Verification and Conclusion of Mendel’s Findings

• Mendel posited that genes exist in different forms (alleles) that determine phenotypes; organisms have two copies of each gene (one from each parent).

• Rediscovery of Mendel's work in 1900 led to foundational principles of modern genetics; studies confirmed predictions across various species, not limited to plants.

Principles of Independent Assortment

• Mendel's second law states that traits segregate independently during gamete formation.

• Dihybrid cross example: Round yellow peas crossed with wrinkled green peas produced an F2 generation demonstrating a 9:3:3:1 ratio.

Utilizing Punnett Squares and Testcrosses

• Punnett Squares serve as a visual representation of allele combinations in offspring, enabling predictions of ratios in progeny.

• Testcross: Cross unknown genotype dominant phenotype with known recessive phenotype to determine genotype.

  • Ratio outcomes help identify homozygous vs heterozygous status.

Chi-Square Analysis in Genetics

• Chi-square (χ2) is a statistical tool to compare observed vs expected ratios.

• Steps: Hypothesize a ratio, calculate χ2, compare with analytical tables for significance.

• χ² = ∑(O_i – E_i)²/E_i, where O_i = observed value (actual value) and E_i = expected value and sigma= sum

• Degrees of freedom help determine significance level in observed data.