02-Mendel

Gregor Mendel and Mendelian Genetics

• Gregor Mendel (1822-1884): Known as the father of genetics.

• Key Concepts:

  • Characters and Traits

  • Monohybrid Cross Experiments

  • Punnett Square

  • Test Cross

  • Dihybrid Cross and Independent Assortment

  • Trihybrid Cross

  • Meiosis and Independent Assortment

Mendel’s Experimental System

• Organism: Garden pea (Pisum sativum)

  • Easy to grow, reproduces in a single season

  • Self-fertilizing, allowing for pure traits

  • Cross-fertilizable for experiments

• Character Selection: Mendel chose seven distinguishable characters from pure strains:

  1. Seed color: yellow and green

  2. Seed shape: round and wrinkled

  3. Seed coat: gray and white

  4. Pod color: green and yellow

  5. Pod shape: inflated and constricted

  6. Flower position: axial and terminal

  7. Stem length: tall and short

Methods of Cross-Fertilization

1. Removed anthers to prevent self-fertilization.

2. Dusted stigma with pollen from a different plant.

3. Pollen fertilized ova, developing into seeds.

4. Seeds grew into plants.

Monohybrid Cross

• Definition: Experimental mating of two individuals with contrasting forms of a single character.

• Generations:

  • P1: Parental generation

  • F1: First filial generation (offspring from P1)

  • F2: Second filial generation (offspring from F1)

• Ratios:

  • Genotypic ratio: 1:2:1

  • Phenotypic ratio: 3:1

Results of Mendel's Monohybrid Cross

• Crossed homozygous varieties of peas:

  • Round seeds (RR) x Wrinkled seeds (rr)

  • F1 generation: All round seeds (Rr)

  • F2 generation: 3/4 round and 1/4 wrinkled seeds (3:1 ratio)

Mendel’s Explanation of Results

• Unit Factors: Traits are determined by unit factors (alleles).

  • Example: Seed shape R (round) and r (wrinkled)

  • Dominant (R) and recessive (r) factors.

Mendel’s First Three Postulates

1. Unit Factors in Pairs: Genetic characters controlled by unit factors in pairs.

2. Dominance/Recessiveness: One unit factor is dominant over the other.

3. Segregation: During gamete formation, paired unit factors segregate randomly.

Modern Genetic Terminology

• Genes: Units of inheritance (Mendel’s unit factors).

• Alleles: Alternative variants of a gene.

• Genotype: Genetic makeup of an individual.

• Homozygote: Individual with two identical alleles.

• Heterozygote: Individual with two different alleles.

• Phenotype: Physical expression of a trait.

Test Cross

• Definition: Cross of an individual showing a dominant trait with a homozygous recessive individual.

• Used to determine the genotype of the dominant phenotype individual.

• Expected phenotypic ratio: 1:1 for testcross.

Relationships Between Mendel’s Laws and Meiosis

• Genes occur in pairs in homologous chromosomes.

• Locus: Specific location on chromosome.

• Mendel’s Law of Segregation: Alleles segregate during anaphase I of meiosis.

Dihybrid Cross

• Definition: Experimental cross studying two independent characters simultaneously.

• Results follow the product rule for probabilities of each character.

• Phenotypic Ratio: 9:3:3:1.

Mendel’s Fourth Postulate

• Independent Assortment: During gamete formation, unit factors assort independently.

Branch Diagrams and Phenotypic Ratios

• Used for calculating proportions in crosses.

• Individual character ratios combined using the product rule.

Testcross with Two Characters

• Example: Round, yellow vs. wrinkled, green in test cross. Expected phenotypic ratio for testcross: 1:1:1:1.

Trihybrid Cross

• Involves three traits, maintaining Mendel’s postulates across multiple traits.

Meiosis I and Independent Assortment

• Applies to genes on separate chromosomes affecting gamete combinations.

• Orientation of homologous chromosomes leads to variation in gametes.

Practical Example with Guinea Pigs

• Cross between black and white guinea pigs showed:

  • F1: all black

  • F2: 3/4 black and 1/4 white.

• Various crosses within the F2 generation confirmed Mendelian ratios.