Unit 5: Heredity - In Depth Notes

Unit 5: Heredity - Overview

  • Focus on essential concepts and mechanisms of heredity, particularly meiosis and genetics.

Meiosis

  • Definition: Meiosis is a two-step cell division process that produces gametes (sex cells).
  • Ploidy: Meiosis reduces ploidy:
    • Most somatic cells are diploid (2n) - possessing two sets of chromosomes.
    • Gametes produced are haploid (n) - possessing one set of chromosomes.

Key Stages of Meiosis

  • Meiosis I:

    • Prophase I: Chromosomes condense; homologous chromosomes pair (synapsis) and crossing over occurs.
    • Metaphase I: Homologous pairs align on the metaphase plate.
    • Anaphase I: Homologous chromosomes are pulled to opposite poles.
    • Telophase I: Chromosomes decondense; two haploid cells form after cytokinesis.

  • Meiosis II:

    • Prophase II: Chromosomes condense again; spindle fibers form.
    • Metaphase II: Chromosomes align along the equatorial plane.
    • Anaphase II: Sister chromatids separate to opposite poles.
    • Telophase II: Chromatids decondense and four unique haploid cells result after cytokinesis.

Genetic Diversity

  • Crossing Over: Exchange of DNA between homologous chromosomes which enhances genetic variation.
  • Law of Segregation: Two alleles for a gene separate randomly into gametes, increasing combinations.
  • Law of Independent Assortment: Different genes independently separate from one another when reproductive cells develop.
  • Nondisjunction: Definition: Errors in meiosis leading to abnormal chromosome numbers.
    • Trisomy: Presence of an extra chromosome (e.g., Down syndrome).
    • Monosomy: Absence of one chromosome.

Genetics Terms and Concepts

  • Key Definitions:
    • Gene: Basic unit of heredity.
    • Allele: Variant form of a gene.
    • Genotype: Genetic constitution of an individual.
    • Phenotype: Observable traits of an individual.
    • Dominant: Allele that masks the effect of another allele.
    • Recessive: Allele whose expression is masked.
    • Homozygous: Two identical alleles for a trait.
    • Heterozygous: Two different alleles for a trait.
    • True-breeding: Organisms that produce offspring identical to themselves.
    • Test cross: Breeding to determine genotype.

Punnett Squares

  • Monohybrid Cross: Cross that examines the inheritance of one trait. Expected ratio: 3:1 (dominant:recessive).
  • Dihybrid Cross: Cross that examines two traits. Expected ratio: 9:3:3:1.
  • Linked Genes: Genes located close together on the same chromosome do not assort independently.

Pedigrees and Inheritance Patterns

  • Pedigrees are diagrams used to track inheritance patterns across generations.
  • Types of Traits:
    • Autosomal Recessive: Possible skipping of generations; unaffected parents can have affected children.
    • Autosomal Dominant: No skipping of generations; affected offspring can sometimes be born to unaffected parents.
    • Sex-linked Traits: More common in males; pattern of inheritance differs for males and females.

Non-Mendelian Genetics

  • Definition: Traits that do not follow Mendelian inheritance patterns. Examples include:
    • Sex-linked traits
    • Mitochondrial inheritance
    • Incomplete dominance: Blending of traits.
    • Codominance: Both alleles express equally in the phenotype.
    • Linked genes: Genes that do not assort independently when located on the same chromosome.