Biology genetics

• 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
• Early beliefs about heredity
• 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 the corresponding body part in the offspring
• Preformationist theories proposed that the parent's body carried highly specialized reproductive cells containing preformed offspring
• Aristotle was a preformationist, suggesting that offspring gradually generated from an undifferentiated mass by the addition of parts
• Cell theory
• In 1665, Robert Hooke observed cells through a microscope
• Matthias Jakob Schleiden and Theodor Schwann theorized that all living things are composed of cells
• Rudolf Virchow launched theories of biogenesis, positing that cells reproduce themselves
• Cells are the basic units of life
• Germplasm Theory of Heredity
• August Weissmann's germplasm theory of heredity stated that genetic code for each organism is contained in its germ cells
• Weissmann observed that genetic material did not double when cells replicated, suggesting biological control of chromosomes during gamete formation
• Weissmann refuted the idea that physical characteristics acquired through environmental exposure were passed from generation to generation
• Gregor Mendel
• Mendel conducted experiments with pea plants to study inheritance
• He observed that traits come in pairs, with one trait assuming dominance over the other
• Mendel's experiments showed a 3:1 ratio of tall to short plants in the F2 generation
• He theorized that characteristics are conveyed from one generation to the next through pairs of traits, with one being dominant and the other recessive
• 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
• Mendel's first principle of heredity, the 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 will be present in half the sperm or egg cells
  • New cells will have a unique pair of genes, half from one parent and half from the other
• Mendel's 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 a cross occurs between organisms pure for contrasting traits, 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 found on specific chromosomes
  • Traits found on the same chromosome are not always inherited together
  • Genes are actual physical objects
• Sturtevant's most notable contribution to genetics was gene mapping, 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
• Barbara McClintock described methods of exchange of genetic information and discovered jumping genes, which could be transposed into new positions
• McClintock's research with Harriet Creighton provided evidence of genetic information crossing over during meiosis
• Frederick Griffith demonstrated that DNA was the transforming factor in bacteria, not just a protein
• Oswald Avery, Colin Munro Macleod, and Maclyn McCarty confirmed that DNA was the transforming factor in bacteria
• Phoebus A. Levene discovered the structure of DNA nucleotides in 1929
• Theophilus Shickel Painter made the first estimate of human chromosomes in 1929, later corrected to 46 chromosomes
• Linus Pauling determined that sickle-cell anemia was caused by a single amino acid change in hemoglobin
• Martha Cowles Chase and Alfred Day Hershey provided definitive proof in 1950-1952 that DNA was genetic material