Genetics: Foundations and Molecular Basis (BIOL 3416 Study Notes)

Introduction to Genetics

Overview

Genetics is the scientific study of heredity, variation, and the molecular mechanisms underlying the transmission of traits. This field integrates classical principles, molecular biology, and evolutionary theory to explain how genetic information is inherited and expressed.

I. Modern History of Genetics

A. Before 1860 – Cell Theory and Early Observations

• Selective Breeding: Humans have practiced selective breeding for over 10,000 years, notably in crops such as rice, maize, and wheat, to enhance desirable traits.

• Microscopy: Advances in microscopy from the 1590s led to the discovery of the cell nucleus in 1831 and subsequent identification of chromosomes.

• Early Heredity Concepts: While practical applications of genetics were ancient, scientific exploration of heredity principles is a more recent development.

• Example: Ancient farmers selected plants with favorable traits for cultivation, laying the groundwork for genetic studies.

B. 1860-1900: Mendel's Discovery

• Gregor Mendel: In 1866, Mendel published his work on hereditary transmission in pea plants, establishing foundational laws of inheritance.

• Rediscovery: Mendel’s principles were independently rediscovered in 1900 by Correns, de Vries, and von Tschermak, marking the beginning of modern genetics.

• Key Terms: Law of Segregation, Law of Independent Assortment

C. 1900-1944: Major Discoveries and Extension of Mendelian Analysis

• Garrod (1901): Described the inheritance of alkaptonuria, a metabolic disorder, linking genes to biochemical pathways.

• Bateson: Advocated for Mendelism and recognized that alkaptonuria was a rare, recessive trait.

• Chromosome Theory: Flemming, Sutton, and Boveri observed chromosome movement during cell division, correlating it with Mendelian inheritance.

• Additional info: The Chromosome Theory of Inheritance posits that genes are located on chromosomes, which segregate during meiosis.

D. 1944 – Present: Molecular Genetics

• Genes: Defined as physical units of heredity, now known to be specific DNA sequences.

• Chromosomes: Composed of long molecules of double-stranded DNA and associated proteins; contain genes.

• Homologous Pairs: Sexually reproducing organisms possess homologous chromosomes carrying genes for the same traits.

• Genomics and Genetic Engineering: Modern genetics includes the study of entire genomes and manipulation of genetic material for research and biotechnology.

II. Areas of Genetics

Major Subfields

• Classical Genetics: Focuses on gene location and chromosome behavior during inheritance.

• Molecular Genetics: Examines the structure, function, and regulation of genetic material at the molecular level.

• Evolutionary Genetics: Studies mechanisms of evolution and changes in gene frequencies within populations.

Organelle Genetics

• Mitochondria and Chloroplasts: Plant and animal cells contain mitochondria; plant cells also contain chloroplasts.

• Organelle Chromosomes: These organelles have their own circular chromosomes, inherited through the cytoplasm during cell division.

• Additional info: Organelle inheritance is typically non-Mendelian and often maternal.

III. Choices of Model Organisms

Commonly Used Organisms in Genetics Research

• Drosophila melanogaster: Fruit fly, widely used for genetic mapping and mutation studies.

• Caenorhabditis elegans: Nematode worm, model for developmental genetics.

• Arabidopsis thaliana: Model plant species for genetic and molecular studies.

• Escherichia coli: Bacterium, essential for molecular genetics and recombinant DNA technology.

• Viruses: Used to study gene expression, replication, and genetic engineering.

IV. DNA Is the Hereditary Material

Discovery and Structure

• Avery, MacLeod, and McCarty: Identified DNA as the hereditary material, initiating the molecular era of genetics.

• DNA Structure: DNA is a double-stranded helix (DNA duplex), elucidated in the 1950s.

• Universal Hereditary Material: DNA is the genetic material in all organisms; RNA serves this role in some viruses.

• Key Terms: Deoxyribonucleic acid (DNA), Ribonucleic acid (RNA)

• Example: The Hershey-Chase experiment confirmed DNA as the genetic material in bacteriophages.

V. Genetic Information Flow

Central Dogma of Molecular Biology

• Genetic Information Pathways: Information flows from DNA to RNA to proteins.

• Reverse Transcription: In some cases (e.g., retroviruses), RNA can be reverse-transcribed to DNA.

Equations:

• $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

• $\text{RNA} \rightarrow \text{DNA}$

VI. Genes Can Mutate

Mutation and Genetic Variation

• Mutation: Genes are subject to changes (mutations) that can alter their function and lead to genetic diversity.

• Types of Mutations: Point mutations, insertions, deletions, and chromosomal rearrangements.

• Example: Sickle cell anemia is caused by a point mutation in the beta-globin gene.

VII. Traits Are Affected by Genes and Environment

Gene-Environment Interaction

• Phenotype: The observable traits of an organism result from the interaction between its genotype and environmental factors.

• Examples: Height, skin color, and susceptibility to certain diseases are influenced by both genetic and environmental components.

• Additional info: The concept of heritability quantifies the proportion of trait variation due to genetic factors.

Table: Areas of Genetics

Area	Main Focus	Example Organism/Application
Classical Genetics	Gene location, chromosome behavior	Drosophila melanogaster mapping
Molecular Genetics	Structure and control of genetic material	Escherichia coli DNA replication studies
Evolutionary Genetics	Gene frequency changes, mechanisms of evolution	Population genetics in humans
Organelle Genetics	Inheritance of mitochondrial and chloroplast DNA	Maternal inheritance in plants