Structural Biochemistry/Foundation of Genetics

History of Genetics

Gregor Johann Mendel is a German-Czech Augustinian monk is referred to as the father of genetics. Mendelian genetics was founded based on his work with pea plants. His research with the nature of inheritance in plants was first published in 1865 in his paper "Experiments on Plant Hybridization" in the Society of Plant Hybridization journal.

The main idea of Mendelian and classical genetics is that the pattern of inheritance can be described through simple rules and ratios. These rules were derived from Mendel's analysis of seven characteristics of pea plants that include the following traits: 1. Color and smoothness of the seeds 2. Color of the cotyledons 3. Color of the flowers 4. Shape of the pods 5. Color of unripe pods 6. Position of flowers and pods on the stems 7. Height of the plants

Mendelian genetics is founded on two laws. The first law is the law of segregation. The law of segregation states that each individual that is a diploid has a pair of allele (copy) for a particular trait. Each parent passes an allele at random to their offspring resulting in a diploid organism. The allele that contains the dominant trait determines the phenotype of the offspring. In essence, the law states that copies of genes separate or segregate so that each gamete receives only one allele.This occurs during the natural process of meiosis that occurs during sexual reproduction in eukaryotes. The second law states is known as the Law of Independent Assortment. It states that the genes that are for separate traits are passed to the offspring independently of other genes. In other words, the inheritance pattern of another gene has no influence on the inheritance pattern of the gene of interest. Alleles of different genes assort independently during gamete formation. In Mendelian experiments, dihybrid crosses showed that the 9:3:3:1 ratio table is just two genes is independently inherited with a 3:1 phenotypic ratio.

During Mendel's time, it was believed that genetic information was passed down through the concept of blending inheritance. This is the idea that the offspring is the result of the combination or blend of genetic traits inherited from the parents. For example, the flower color of an offspring pink when a white flower and red flower are crossed. This theory was disproved by Mendel who showed that traits are combinations of distinct genes rather than a gradient or continuous blend of traits from the parents.

Significance of Mendel's Work

Gregor Mendel performed a series of breeding experiments on pea plants. Mendel studied how pea plants inherited two observable traits: flower color (white or purple) and the texture of the peas (smooth or wrinkled). Mendel bred many generations and learned that these characteristics were passed on to the next generation in orderly and predictable ratios. He cross bred purple flowered pea plants with white flowered ones and found that the next generation had only purple flowers but directions for making white flowers were hidden somewhere in the peas because when the second generation of only purple flowered peas were bred with each other, some of their offspring had white flowers. The second generation plants displayed the colors in a predictable pattern: 75% purple, 25% white. The same ratio persisted even when the experiment was repeated over and over. Mendel reasoned that “factors,” each of which determined a specific trait, must exist in order to reproduce physical material because they passed from parent to offspring.

Mendel’s experiment was groundbreaking because later on these “factors” were found to be genes and his mathematical rules for inheritance were applied not just to peas but to all plants, animals, and people. It revealed that a common, general principle governed the growth and development of all life on earth.

Following Mendel's Work

Once Mendel's work was recognized the next goal was to determine the molecules in the cell that was responsible for the control of genetic information. In 1910, Thomas Hunt Morgan discovered that genes are present on chromosomes through his experiments with fruit flies and monitoring sex-linked white eye mutations in Drosophila.

Once genes were known to exist on chromosomes which are made up of proteins and DNA, the question scientists were inclined to answer is whether protein or DNA is responsible for the inheritance of genetic material. In 1928, scientist Frederick Griffith showed that dead bacteria could transfer genetic material to "transform" other living bacteria. This interesting theory was reassured through the experiments of Oswald Theodore Avery, Colin McLeod and Maclyn McCarty. These three scientists discovered the molecule that was responsible for the bacterial transformation is DNA. In 1952, the Hershey and Chase experiment showed that DNA is the genetic material of bacteriophages, or viruses that infect bacteria.

Now that DNA had been identified as the genetic material, scientist sought to solve the structure of DNA and to identify mechanisms by which genetic material is inherited. James D. Watson and Francis Crick, back in 1953, determined the structure of DNA using the x-ray crystallography image of a DNA helix imaged by Rosalind Franklin and Maurice Wilkins. They discovered that the structure of DNA is highly adapted to allow for its function especially for DNA replication and for following a set genetic code. After the years of these scientists, modern genetics started to evolve.

Now it is known the genetic information can be passed down through multiple patterns of inheritance including single gene inheritance such as recessive or dominant autosomal inheritance, and recessive or dominant x-linked inheritance. Other patterns of inheritance include multi-factorial inheritance in which genetic factors and environmental factors contribute to the passing of genetic information. A third pattern of inheritance is mitochondrial inheritance. Mitochondria have circular chromosomes that are inherited from the mother. Diseases acquired though mitochondrial inheritance are often concerned with the functionality of heart, skeletal muscle, kidneys, and liver as these organs use large amounts of energy processed by the mitochondria.