Geneticists

• Genetics is the biology of heredity
• Geneticists study hereditary processes such as the inheritance of traits, distinctive characteristics, and diseases
• Genetics considers the biochemical instructions that convey information from generation to generation
• Advances in science and technology have shown that genetic variation is related to disease and that varying genes improves a species' ability to survive environmental changes
• Important advances in genetics research have occurred since the mid-twentieth century
• The history of genetics study spans about 150 years
• Scientific research in genetics became increasingly more specific over time
• Ancient civilizations observed patterns in reproduction
• Ancient Greek particulate theories posited that information from each part of the parent had to be communicated to create corresponding body parts in the offspring
• Preformationist theories in the Renaissance proposed that specialized reproductive cells contained preformed offspring that 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
• Hooke's observations of cells in cork led to the speculation that living and nonliving tissue was composed of cells
• Schleiden and Schwann theorized that all living things are composed of cells and developed cell theory
• Virchow proposed theories of biogenesis and described cells as the basic units of life
• Key cell components include the nucleus, which controls cellular activities, and mitochondria, which produce energy for the cell
• Germplasm theory of heredity
• Weissmann's germplasm theory of heredity stated that genetic information is contained in germ cells and is conveyed unchanged from one generation to the next
• Weissmann observed that genetic material did not double during cell replication, indicating a biological control of chromosomes during gamete formation
• Weissmann's theory was confirmed by the process of reduction division (meiosis) to avoid giving offspring a double dose of heredity information
• Gregor Mendel
• Mendel conducted experiments with pea plants to study inheritance systematically
• He observed that traits come in pairs, one from each parent, and one trait may dominate over the other
• Mendel's experiments showed a 3:1 ratio of tall to short plants in the second generation (F2) when breeding tall and short plants together
• His observations about purebred plants conveying traits accurately represented a novel idea in inheritance
• Mendel's Laws 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, regardless of the hereditary factors they carry
  • Different hereditary factors sort independently when gametes are formed
• Law of Segregation:

  • Hereditary units (genes) are always paired
  • Genes in a pair separate during cell division, with sperm and egg each receiving one gene of the pair
  • Each gene in a pair is present in half the sperm or egg cells
  • New cells have a unique pair of genes, half from each parent
• Law of Independent Assortment:

  • Each pair of genes is inherited independently of all other pairs
  • Recessive traits that disappear in the F1 generation may reappear in future generations in predictable percentages
• Law of Dominance:

  • Heredity factors (genes) act together as pairs
  • When organisms pure for contrasting traits cross, only the dominant trait appears in the hybrid offspring
• Chromosome Theory of Inheritance:

  • Genes are the fundamental units of heredity found in chromosomes
  • Specific genes are located on specific chromosomes
  • Traits on the same chromosome are not always inherited together
  • Genes are physical objects
  • Genes are found in pairs in germinal matter and separate when germ cells mature
• Sturtevant's most notable contribution to genetics was providing a detailed outline and instruction about gene mapping
• Gene mapping is the process of determining the linear sequence of genes in genetic material
• Sturtevant began construction of a chromosome map of the fruit fly in 1913, completed in 1951
• Sturtevant is often referred to as the father of the Human Genome Project
• Barbara McClintock described key methods of exchange of genetic information
• McClintock observed colored kernels on maize that should have been clear, hypothesizing the loss of genetic information leading to jumping genes
• McClintock's work on crossing over or recombination earned her the Nobel Prize in Physiology or Medicine in 1983
• Frederick Griffith demonstrated the transfer of genetic material between strains of bacteria
• Griffith's experiments with Streptococcus pneumoniae showed that DNA was the transforming factor, linking DNA to heredity in cells
• Oswald Avery, Colin Munro Macleod, and Maclyn McCarty confirmed that DNA was the transforming factor in Griffith's experiments
• Their studies showed that DNA from one strain of bacteria could transform a harmless strain into a deadly strain