Biology 1003 - Meiosis and Mendel's Theory

Week 8: Meiosis and Mendel

Overview of Topics Covered

  • Chapters: 5.5 (Meiosis), 6.1 (Mendel)

  • Key Themes: Meiosis as a method of gamete formation, Mendelian genetics and inheritance patterns.

5.5 Meiosis Overview

  • Definition of Meiosis: A type of cell division that reduces the chromosome number by half, creating four haploid cells from one diploid cell.

    • Haploid: Cells with one set of chromosomes (23 in humans).

    • Diploid: Cells with two sets of chromosomes (46 in humans).

  • Purpose of Meiosis: To produce gametes (sperm or eggs) from germ cells located in the testes or ovaries in mammals.

  • Comparison with Mitosis: Similar in that cells divide, but meiosis consists of two rounds of division (meiosis I and meiosis II) and involves homologous chromosomes.

Gamete Statistics from Monohybrid Crosses

  • Monohybrid Crosses: Examples of crossing traits in pea plants:

    • Seed Shape: Round x Wrinkled

    • F1: All round

    • F2: 5474 round, 1850 wrinkled

    • Seed Color: Yellow x Green

    • F1: All yellow

    • F2: 6022 yellow, 2001 green

    • Pod Shape: Inflated x Constricted

    • F1: All inflated

    • F2: 882 inflated, 299 constricted

    • Pod Color: Green x Yellow

    • F1: All green

    • F2: 428 green, 152 yellow

Stages of Meiosis

Summary of Meiosis

  • Two Major Divisions:

    1. Meiosis I: Separates homologous chromosomes.

    2. Meiosis II: Separates sister chromatids.

  • DNA Replication: Occurs prior to meiosis I, not meiosis II.

  • End Result: From one diploid cell to four haploid cells (one diploid cell becomes one haploid egg plus two polar bodies or four haploid sperm).

Stages of Meiosis I

  1. Prophase I:

    • Chromosomes condense, nuclear membrane dissolves, centrioles migrate, spindle fibers form.

    • Unique to meiosis: Homologous chromosomes pair and exchange segments (crossing over).

    • Human oocytes are held at this stage until puberty.

  2. Metaphase I:

    • Paired homologous chromosomes align at the equatorial plane.

    • Independent assortment occurs.

  3. Anaphase I:

    • Homologous chromosomes separate and move to opposite poles, sister chromatids remain attached.

  4. Telophase I and Cytokinesis:

    • Nuclear membranes reform, cytokinesis occurs, resulting in two haploid cells with sister chromatids.

Stages of Meiosis II

  1. Prophase II:

    • Brief interphase with no DNA replication and preparation for division.

  2. Metaphase II:

    • Chromosomes align individually across the equatorial plane.

  3. Anaphase II:

    • Sister chromatids separate and move to opposite poles.

  4. Telophase II and Cytokinesis:

    • Resulting in four haploid cells (human oocytes produce only three due to polar bodies).

Human Genetic Variation from Meiosis

  • Independent Assortment of Chromosomes: The first meiotic division allows random distribution of 23 pairs, resulting in 2232^{23} (over 8 million) possible combinations of gametes.

  • Crossing Over: Occurs during prophase I, allows for new combinations of genes.

  • Genetic Combination: Each individual inherits genes from both parents leading to diverse offspring.

6.1 Mendel’s Theory of Inheritance

  • Definition of Heredity: The transfer of traits from parent to offspring.

  • Traits and Alleles:

    • A trait is a heritable feature.

    • Phenotype: The visible expression of a genotype.

    • Alleles: Variants of a gene that comprise an organism's genotype.

    • Genotype Types:

    • Homozygous: Same variant of an allele (e.g., AA or aa).

    • Heterozygous: Different variants of an allele (e.g., Aa).

  • Dominant vs. Recessive:

    • Dominant Allele: An allele whose phenotype is expressed even with one copy.

    • Recessive Allele: An allele expressed only when two copies are present.

Mendel’s Pea Plant Experiments

  • Gregor Mendel: The father of genetics, studied inheritance patterns using pea plants.

  • True-breeding plants: Homozygous for a trait, resulting in consistent phenotype across generations.

  • Cross-breeding observed two generations:

    • F1 Generation: First offspring generation, majority phenotype displayed (dominant).

    • F2 Generation: Offspring of F1 plants, where the recessive phenotype reappeared in a 3:1 ratio for dominant to recessive traits.

  • Example: Crossing true-breeding purple flowers with true-breeding white flowers shows:

    • F1 Generation: All purple flowers.

    • F2 Generation: 3/4 purple flowers, 1/4 white flowers.

Mendel’s Phenotype Ratio in the F2 Generation

  • Mendel’s findings revealed:

    • A consistent proportion of traits in the F2 generation, aligning with a ratio of 3:1 (dominant to recessive).

    • Genotypic Ratio in F2:

    • 1PP:2Pp:1pp1PP: 2Pp: 1pp (Homozygous dominant: Heterozygous: Homozygous recessive)

    • Phenotypic Ratio: Approx. 3 purple: 1 white in plant flowers.

Conclusion and Importance of Mendel’s Work

  • Mendel’s experiments laid the groundwork for classical genetics, demonstrating how traits are passed down from one generation to another.

  • His work is essential for understanding inheritance patterns and genetic variation in all organisms.