Biology 30 -Classical Genetics notes

Chapter Overview

• Focus: Inheritance & Genetics (Mendelian concepts and modern implications).

• Page Range: 584-621.

Historical Theories of Inheritance

• Ancient Theories:

  • Early scientists believed that offspring traits came from parents through various mechanisms;

    • Aristotle's Pangenesis: Interaction of purified blood from both parents.

    • Leeuwenhoek: Saw living sperm, theorized existence of a complete 'homunculus'.

    • De Graaf: Proposed that the egg contained the individual, while sperm only stimulated development.

• Blending Inheritance:

  • 1800s theory suggesting parent traits blend irreversibly in offspring.

  • Dominance of these theories until Mendelian genetics emerged.

Gregor Mendel and His Experiments

• Mendel's Contribution:

  • Conducted experiments in pea plants over 7 years in Brno, Czech Republic.

  • Considered the father of genetics due to systematic study of inherited characteristics (heredity).

  • Used true breeding plants to ensure consistency in traits.

• Traits Studied:

  • Focused on seven visual traits, observable in two forms.

  • Noticed characteristic masking in initial generations, with potential reappearance.

  • Shift from blending theory to discrete inheritance units.

Mendel's Key Findings

• Terminology:

  • Dominant traits (expressed in F1) vs. recessive traits (masked in F1).

  • Observations led to a 3:1 phenotypic ratio in second generation (F2) when F1 self-pollinated.

• Laws of Inheritance:

  • Law of Segregation:

    • Traits determined by discrete factors (now called genes).

    • Each individual carries two copies of each factor.

    • During gamete formation, factors segregate randomly.

  • Law of Independent Assortment:

    • Genes do not influence each other's segregation.

Genetic Terminology

• Genes and Alleles:

  • Gene: term coined by Johannsen for Mendel's factors.

  • Alleles: Different forms of a gene (dominant = uppercase, recessive = lowercase).

  • Genotype vs. Phenotype:

    • Genotype: Genetic makeup (AA, Aa, aa).

    • Phenotype: Observable traits.

• Punnett Square:

  • Technique to predict offspring genotypes and phenotypes during crosses.

  • Introduced to analyze monohybrid crosses.

Dominance Types in Alleles

• Incomplete Dominance:

  • Traits blend (e.g., red and white snapdragons yield pink).

  • Seen in certain human traits like sickle cell anemia (protection against malaria).

• Co-dominance:

  • Both traits fully expressed (e.g., roan cattle with red/white) or ABO blood types in humans.

Advanced Inheritance Concepts

• Dihybrid Crosses:

  • Involving two traits leading to phenotypic ratios of 9:3:3:1 in F2 generation.

• Linked Genes:

  • Morgan's discoveries using fruit flies, revealing genes on the same chromosome do not assort independently.

  • Role of crossing over in prophase I of meiosis affects gene linking.

Sex-Linkage

• Sex-linked Traits:

  • More commonly expressed in males; mothers pass traits to daughters and sons.

  • Examples include hemophilia and color blindness.

• X-inactivation:

  • Random inactivation of one X chromosome in females leads to observable phenotype variances (e.g., tortoiseshell cats).

Other Genetic Concepts

• Lethal Alleles:

  • Genes that can cause death (e.g., Huntington's disease).

• Multiple Alleles:

  • More than two alleles for a trait; for instance, eye color in Drosophila.

• Polygenic Traits:

  • Traits controlled by multiple genes leading to continuous variation (e.g., height, skin color).

Genetic Implications in Society

• Artificial Selection:

  • Used for thousands of years, exemplified by Mesoamerican maize breeding.

  • Modern agricultural practices continue this trend with crops and livestock.

• Genetic Counseling:

  • Used to assess risks of genetic disorders; involves pedigree analysis to understand inheritance patterns.

Mendelian Laws Summary

• Key laws arrive from Mendel's peas:

  • Law of Segregation: Traits based on genes that segregate during gamete formation.

  • Law of Independent Assortment: Genes assort independently from each other.