Ch2

Thomas Morgan in the Fly Room 1916

  • Focus on Chromosomal Basis of Mendelism

  • Non-disjunction as proof of the Chromosome Theory of Heredity.

Single Gene Inheritance

  • Chapter References:

    • Griffiths et al., Introduction to Genetic Analysis, 11th and 12th Editions.

  • Key Topics Covered:

    • Chromosome Behavior in Meiosis

    • Chromosomal Theory of Inheritance

    • Sex Linkage and Determination

    • Ploidy and Cytogenetics

    • Non-Disjunction and Disease

Learning Objectives

  • Understand meiosis in relation to Mendelian predictions.

  • Identify sex linkage signatures in genetics.

  • Comprehend the role of sex chromosomes in sex determination (humans & flies).

  • Recognize implications of chromosome non-disjunction.

  • Define karyotype and ploidy.

  • Work through inheritance patterns in sex-linked traits.

Mendelian Laws

  1. First Law: Alleles of a single gene segregate independently.

  2. Second Law: Alleles of different genes segregate independently.

Biological Processes Behind Mendelian Laws

  • Mitosis and Meiosis explain inheritance.

  • Mitosis: Involves diploid (2n) mother cells producing identical diploid daughter cells.

  • Meiosis: Divides diploid meiocytes into non-identical haploid (n) cells through two divisions: Meiosis I & II.

Chromosome Structures: Chromatids vs. Homologs

  • Sister chromatids: identical copies post-division.

  • Homologs: chromosome pairs, one from each parent.

Meiosis Overview

  1. Interphase: Chromosomes unpaired and DNA replicated.

  2. Prophase I: Homologs undergo synapsis and crossing over (genetic exchange).

  3. Metaphase I: Homologs align on the metaphase plate.

  4. Anaphase I: Homologs separate and move to opposite poles.

  5. Telophase I and II: Cells divide resulting in four haploid cells.

Genetic Variation from Segregation

  • 23 pairs of homologs lead to various combinations (2^23 or 8,388,608 possibilities) during Meiosis I.

  • Progeny exhibit distinct phenotypic ratios due to independent assortment.

Chromosomal Theory of Inheritance

  • Chromosome behavior during meiosis reflects gene inheritance.

  • Non-disjunction events serve as proof of chromosomal linkages to inheritance patterns.

Drosophila as a Genetic Model

  • Thomas H. Morgan's contribution to genetics via Drosophila studies, highlighting:

    • Quick generation time.

    • Various observable phenotypes (e.g., eye color).

  • Eye color example:

    • White-eyed mutation investigation through control crosses.

Sex-Linked Traits in Drosophila

  • Mutations showing different inheritance patterns based on sex chromosomes.

  • Females passing X-linked traits to sons (not affecting daughters in manner similar to males).

  • Non-disjunction leads to rare but viable sex-linked anomalies (e.g., white-eyed females).

Sex Determination Systems in Humans

  • XX for females, XY for males; SRY gene critical for male development.

  • Non-disjunction can lead to conditions such as Turner Syndrome (X0 individuals).

Cytogenetics and Karyotyping

  • Use of stains to analyze chromosomes for abnormalities.

  • Karyotype representation for diagnosing genetic disorders.

Ploidy Alterations and Their Consequences

  • Importance of ploidy in development and abnormality occurrences.

  • Definitions of euploid and aneuploid conditions.

  • Monosomy vs. trisomy terms and examples in human conditions (Down Syndrome).

Chromosomal Rearrangements

  • Breakdown of gene functions due to inversions, duplications, and deletions (e.g., Cri du chat syndrome).

  • Impact of changes in gene dosage on phenotype.

Conclusion

  • Chromosomal behavior underpins Mendelian genetics and clarifies the interrelationships between heredity, evolution, and organism development.

  • Study of chromosomal theories has cemented the foundation for understanding genetic inheritance and variability.