History of Genetics

• Genetics is the biology of heredity, studying processes like the inheritance of traits, characteristics, and diseases
• Genetics considers the biochemical instructions that convey information from generation to generation
• Tremendous strides in science and technology have enabled geneticists to demonstrate that genetic variation is related to disease and survival capacity
• Important advances in genetics research include deciphering the genetic code, isolating disease-causing genes, and successfully cloning plants and animals
• The history of genetics study spans about 150 years, progressing from population studies to exploring inheritance at the molecular level
• Ancient civilizations observed patterns in reproduction, leading to early beliefs about heredity and transmission of information from parent to child
• Particulate theories in ancient Greece posited that information from each part of the parent had to be communicated to create corresponding body parts in the offspring
• Preformationist theories during the Renaissance proposed that specialized reproductive cells contained preformed offspring, which would grow into new organisms with traits similar to the parent
• Aristotle's theory of epigenesis described a gradual generation of offspring from an undifferentiated mass by the addition of parts
• Charles Darwin and Alfred Russel Wallace introduced the theory of natural selection in 1858, stating that better-adapted individuals are more likely to survive and pass their traits on to the next generation
• Cell theory, developed in the 19th century, asserts that all living things are composed of cells, the basic units of life
• Cell components include the nucleus, which controls cellular activities, and mitochondria, which produce energy for the cell
• The germplasm theory of heredity, proposed by August Weismann, states that genetic information is contained in germ cells and passed unchanged from one generation to the next
• Weismann's theory of reduction division, now known as meiosis, explains how genetic information is halved in gametes to avoid giving offspring a double dose of heredity information
• Gregor Mendel, known as the father of genetics, conducted experiments with pea plants, leading to the discovery of fundamental principles of heredity
• Gregor Mendel's 1865 paper 'Experiments in Plant Hybridization' established the basic tenets of heredity:

  • Two heredity factors exist for each characteristic or trait
  • Heredity factors are contained in equal numbers in the gametes
  • Gametes contain only one factor for each characteristic or trait
  • Gametes combine randomly, no matter which hereditary factors they carry
  • Different hereditary factors sort independently when gametes are formed
• Mendel's first principle of heredity, the law of segregation, states that hereditary units, now known as genes, are always paired and separate during cell division, with the sperm and egg each receiving one gene of the pair
• Mendel's law of independent assortment establishes that each pair of genes is inherited independently of all other pairs
• Mendel's law of dominance asserts that heredity factors (genes) act together as pairs, with only the dominant trait appearing in the hybrid offspring
• Mendel's experiments with pea plants demonstrated that recessive traits that disappear in the F1 generation may reappear in future generations in predictable percentages
• William Bateson, a British geneticist, coined the term genetics in 1905 and made significant contributions to the progress of genetics by translating Mendel's work and promoting his principles
• In 1908, Godfrey Harold Hardy and Wilhelm Weinberg independently developed the Hardy-Weinberg equilibrium, a mathematical formula describing gene actions in populations
• The Hardy-Weinberg equilibrium advanced the application of Mendel’s laws of heredity from individuals to populations, improving geneticists’ understanding of mutations, natural selection, and hereditary adaptations
• The Hardy-Weinberg equilibrium enables geneticists to determine whether evolution is occurring in populations
• Walter Stanborough Sutton's research using grasshoppers demonstrated that chromosomes exist in pairs, are structurally similar, and proved the relationship between Mendel’s laws of heredity and the role of the chromosome in meiosis
• Thomas Hunt Morgan, along with his students, confirmed Mendel's findings by breeding fruit flies and observing patterns of inheritance, leading to the chromosomal theory of inheritance
• Morgan's research with fruit flies confirmed that genes are the fundamental units of heredity found in chromosomes, specific genes are located on specific chromosomes, and genes are actual physical objects
• Morgan's work led to the ability to predict the distribution of specific traits and characteristics through quantifying genes, qualifying genetics as a science
• Barbara McClintock described the exchange of genetic information and discovered jumping genes, known as crossing over or recombination
• Frederick Griffith's experiments with Streptococcus pneumoniae provided the first tangible evidence linking DNA to heredity in cells, laying the groundwork for researching the biochemical basis of heredity in bacteria
• Oswald Avery, Colin Munro Macleod, and Maclyn McCarty confirmed that DNA was the transforming factor in Griffith's experiments, establishing DNA as the molecular basis for genetic information
• Linus Pauling determined that sickle-cell anemia was caused by a change in a single amino acid of hemoglobin, leading to the understanding that genetic information directs protein synthesis and mutations can cause genetic disorders
• Martha Cowles Chase and Alfred Day Hershey's experiments definitively proved that DNA was genetic material, dislodging virus particles that infect bacteria, establishing DNA as the genetic material
• Oswald Avery's 'Waring blender experiment' definitively proved that DNA, not viral protein, directed the growth and multiplication of new viruses inside bacteria
• Watson and Crick's 1953 model of DNA described the double helix structure, with each rung containing an A-T pair or a G-C pair, consistent with Chargaff's rules
• Watson and Crick's model of DNA enabled scientists to understand functions like carrying hereditary information, directing protein synthesis, replication, and mutation at the molecular level
• Changes in the sequence of nucleotide pairs in the DNA double helix, as proposed by Watson and Crick, would produce mutations
• In 1956, Vernon M. Ingram identified the single base difference between normal and sickle-cell hemoglobin, showing that a mutation of a single letter in the DNA genetic code could cause a hereditary medical disorder
• In 1958, Meselson and Stahl's experiment demonstrated that DNA replication in bacteria is semi-conservative, with one strand remaining intact and combining with a newly synthesized strand
• In the early 1960s, Crick, Nirenberg, Gamow, and other researchers detected a direct relationship between DNA nucleotide sequences and the sequence of amino acid building blocks of proteins