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.