Mendel

Mendel & Genetics

Introduction to Gregor Mendel

• Modern genetics began in the mid-1800s in an abbey garden.

  • Key figure: Gregor Mendel, a monk.

  • Conducted experiments to document inheritance using pea plants.

• Applied an experimental method:

  • Employed quantitative analysis to collect and count data.

  • His approach is an excellent example of the scientific method.

Mendel's Experimental Work

• Mendel bred pea plants through the following process:

  • Cross-pollinated true breeding parents (P generation).

  • Raised the seeds and observed traits in the first filial generation (F₁).

  • Allowed the offspring (F₁) to cross-pollinate to observe the traits in the next generation (F₂).

Mendel’s Observations and Data Collection

• Mendel collected data for 7 distinct pea traits:

  • The following table summarizes the traits and Mendel’s experimental results:
    | Character | Dominant Form | Recessive Form | F₂ Generation Dominant:Recessive Ratio |
    |------------------------|--------------------------|--------------------------|---------------------------------------|
    | Purple flowers | Purple flowers | White flowers | 705:224 (3.15:1) |
    | Seed color | Yellow seeds | Green seeds | 6022:2001 (3.01:1) |
    | Seed shape | Round seeds | Wrinkled seeds | 5474:1850 (2.96:1) |
    | Pod color | Yellow pods | Green pods | 428:152 (2.82:1) |
    | Pod shape | Inflated pods | Constricted pods | 882:299 (2.95:1) |
    | Flower position | Axial flowers | Terminal flowers | 651:207 (3.14:1) |
    | Plant height | Tall plants | Dwarf plants | 787:277 (2.84:1) |

In-Depth Analysis of Mendel's Findings

• True-breeding Crosses:

  • True-breeding purple-flower peas crossed with true-breeding white-flower peas:

  • Outcome: 100% purple-flower peas in the F₁ generation (hybrids).

  • In F₂ generation: 75% purple-flower peas & 25% white-flower peas yielding a 3:1 ratio.

Implications of Mendel’s Findings

• Traits exist in alternative versions (alleles):

  • Example: Purple and white flower colors.

  • Alleles correspond to different genes varying in nucleotide sequences on homologous chromosomes.

  • Purple-flower allele and white-flower allele are two variations at the flower-color locus.

• Traits are inherited as discrete units:

  • Each characteristic is governed by 2 alleles (1 inherited from each parent).

  • Organisms are diploid, containing 2 sets of chromosomes (homologous pairs).

• Masking of Traits:

  • Some traits are dominant over others (e.g., purple color masks white color).

  • Gene expression characteristics:

    • Dominant allele: Fully expressed.

    • Recessive allele: No noticeable effect; it may produce a non-functional protein.

Understanding Genotype and Phenotype

• Definitions:

  • Phenotype: Description of an organism’s observable traits (e.g., flower color).

  • Genotype: Description of an organism’s genetic makeup (e.g., alleles).

• Mendel's Results Breakdown:

  • F₁: All hybrids have a dominant phenotype.

Genetic Crosses and Punnett Squares

• Crossing Alleles:

  • Using representative letters for alleles (e.g., P for purple and p for white).

  • True-breeding purple-flower peas: PP; true-breeding white-flower peas: pp.

• Punnett Squares:

  • Example crossing Pp x Pp:

  • Male (sperm): P or p; Female (eggs): P or p.

  • Results:

  • PP, Pp, pp classifications.

  • Phenotypic and genotypic ratios:

    • Genotypic: 75% dominant, 25% recessive (3:1).

    • Phenotypic: 1:2:1 ratio for genotypes (1 PP, 2 Pp, 1 pp).

Genotypes Explained

• Definitions:

  • Homozygous: Same alleles (e.g., PP or pp).

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

• Comparison of Genotypes:

  • Two organisms may have the same phenotype (e.g., purple) yet differ in genotype (PP vs. Pp).

Identifying Dominant Genotypes

• It is impossible to determine the genotype of an organism with a dominant phenotype merely by observation (e.g., PP vs. Pp).

Test Cross Method

• A method to determine the genotype of an organism exhibiting a dominant phenotype by crossing it with a homozygous recessive (pp).

• Example of test cross outcomes:

  • If crossed with pp: Possible results can include offspring with genotypes that reveal whether the unknown genotype is PP or Pp.

Mendel’s Laws of Heredity

• Law of Segregation:

  • During gamete production in meiosis, homologous chromosomes separate, leading to each allele for a trait being packaged into separate gametes.

• Law of Independent Assortment:

  • Alleles for separate traits assort independently during gamete formation.

  • Only applies to genes on different chromosomes, resulting in 4 classes of gametes produced in equal amounts.

Monohybrid and Dihybrid Crosses

• Monohybrid Cross:

  • Examines inheritance of single traits (e.g., flower color).

• Dihybrid Cross:

  • Studies inheritance of two different traits (e.g., seed color and seed shape).

• Dihybrid details:

  • Example: True-breeding yellow, round peas (YYRR) crossed with true-breeding green, wrinkled peas (yyrr) producing hybrids (YyRr).

  • Self-pollination results in:

  • F₂ generation: 9/16 yellow round, 3/16 green round, 3/16 yellow wrinkled, 1/16 green wrinkled (ratios: 9:3:3:1).

• Genetic Distribution:

  • Analyzing how alleles segregate from different chromosomes during gamete formation.

Review of Mendel's Laws

• Law of Segregation:

  • Applies to monohybrid crosses focused on single traits.

• Law of Independent Assortment:

  • Applies to dihybrid or more complex crosses involving two or more traits, with each pair segregating independently in meiosis.