Biology: How Life Works - Meiosis and Mendelian Inheritance

Meiosis

Key Components of Meiosis

Parts of Meiosis

  • Meiosis I:

    • Creates haploid cells.

    • Generates genetically different cells.

  • Meiosis II:

    • Separates duplicated chromosomes formed in Meiosis I.

  • Bivalent: A pair of homologous chromosomes.

  • Chiasmata: Points where crossing over occurs between homologous chromosomes, leading to genetic exchange.

Results of Meiosis

  • Completing Meiosis I and II yields 4 genetically different haploid cells.

Sources of Genetic Diversity

  1. Chiasmata Formation - Leads to genetic exchange.

  2. Independent Assortment - Random arrangement of bivalents during metaphase I, with 223 possible arrangements in humans, resulting in approximately 8 million combinations.

Comparison of Mitosis and Meiosis

  • Mitosis results in two diploid daughter cells, while meiosis results in four genetically diverse haploid cells.

Meiosis and Gender Differences

  • Females:

    • Cytoplasmic division during meiosis is unequal.

  • Males:

    • Cytoplasmic division is equal.

Errors in Meiosis

Nondisjunction

  • Failure of chromosomes to separate properly, resulting in:

    • 2 gametes lacking a chromosome.

    • 2 gametes with an extra chromosome (as in Trisomy 21, or Down Syndrome).

Mendelian Inheritance

True-Breeding Plants

  • Results in consistent trait production across generations, e.g.:

    • Yellow seeds crossed with yellow seeds consistently result in yellow seeds.

    • Green seeds crossed with green seeds consistently result in green seeds.

Generational Terminology

  • P1 Generation: Parent generation.

  • F1 Generation: First generation offspring.

Ratios in F2 Generation

  • Crossing F1 plants results in a 3:1 ratio of dominant to recessive traits, indicating the reappearance of the recessive trait.

Principle of Segregation

  • Each diploid organism carries two alleles for each gene, which separate during gamete formation leading to heterozygosity in offspring.

  • Expected genotype ratios:

    • 1:2:1 (AA:Aa:aa).

    • Expected phenotype ratio: 3:1.

Principle of Independent Assortment

  • Alleles for different traits segregate independently (e.g., round vs. wrinkled and yellow vs. green traits).

Pedigree Analysis in Humans

  • Visualization of trait inheritance patterns:

    • Dominant traits appear in every generation.

    • Recessive traits may skip generations and are often expressed when offspring share affected relatives.

Examples of Pedigree Analysis

  • Brachydactyly (Dominant Trait):

    • Present in every generation, with equal distribution across genders. Offspring of affected and unaffected individuals have a 50% chance of being affected.