Genetics unit POH

• The work of Mendel and others who followed him gave us an idea of inheritance patterns
• The nature of those 'factors' which determine the phenotype was not very clear
• As these 'factors' represent the genetic basis of inheritance, understanding the structure of genetic material and the structural basis of genotype and phenotype conversion became the focus of attention in biology for the next century
• The entire body of molecular biology was a consequent development with major contributions from Watson, Crick, Nirenberg, Khorana, Kornbergs (father and son), Benzer, Monod, Brenner, etc.
• A parallel problem being tackled was the mechanism of evolution
• Awareness in the areas of molecular genetics, structural biology and bio informatics have enriched our understanding of the molecular basis of evolution
• In this unit the structure and function of DNA and the story and theory of evolution have been examined and explained
• James Dewey Watson


  Born in Chicago on 6 April 1928, received B.Sc. degree in Zoology in 1947, received Ph.D. degree in 1950 on a study of the effect of hard X-rays on bacteriophage multiplication
• Francis Harry Compton Crick

  Born on 8 June 1916, at Northampton, England, studied physics at University College, London and obtained a B.Sc. in 1937, completed Ph.D. in 1954 on a thesis entitled "X-ray Diffraction: Polypeptides and Proteins"
• Watson met Crick and discovered their common interest in solving the DNA structure
• Their first serious effort was unsatisfactory, but their second effort based upon more experimental evidence and better appreciation of the nucleic acid literature, resulted, early in March 1953, in the proposal of the complementary double-helical configuration
• The honours to Watson with Crick include: the John Collins Warren Prize of the Massachusetts General Hospital, in 1959; the Lasker Award, in 1960; the Research Corporation Prize, in 1962 and above all, the Nobel Prize in 1962
• Humans knew from as early as 8000-1000 B.C. that one of the causes of variation was hidden in sexual reproduction
• They exploited the variations that were naturally present in the wild populations of plants and animals to selectively breed and select for organisms that possessed desirable characters
• Though our ancestors knew about the inheritance of characters and variation, they had very little idea about the scientific basis of these phenomena
• It was during the mid-nineteenth century that headway was made in the understanding of inheritance
• Gregor Mendel


  Conducted hybridisation experiments on garden peas for seven years (1856-1863) and proposed the laws of inheritance in living organisms
• Mendel's experiments had a large sampling size, which gave greater credibility to the data that he collected
• Mendel investigated characters in the garden pea plant that were manifested as two opposing traits, e.g., tall or dwarf plants, yellow or green seeds
• Mendel selected 14 true-breeding pea plant varieties, as pairs which were similar except for one character with contrasting traits
• Mendel conducted artificial pollination/cross pollination experiments using several true-breeding pea lines
• Mendel observed that all the F1 progeny plants were tall, like one of its parents; none were dwarf
• Mendel found that in the Filial2 generation some of the offspring were 'dwarf'; the character that was not seen in the F1 generation was now expressed
• The proportion of plants that were dwarf were 1/4th of the F2 plants while 3/4th of the F2 plants were tall
• The tall and dwarf traits were identical to their parental type and did not show any blending, that is all the offspring were either tall or dwarf, none were of in-between height
• Genes


  The units of inheritance that contain the information required to express a particular trait in an organism
• Alleles


  Slightly different forms of the same gene that code for a pair of contrasting traits
• Dominant


  The allele that is expressed in the phenotype of the heterozygous individual
• Recessive


  The allele that is not expressed in the phenotype of the heterozygous individual but can be expressed in the homozygous condition
• Monohybrid


  An organism that is heterozygous for genes controlling one character
• Monohybrid cross


  A cross between two individuals that differ in one character
• The production of gametes by the parents, the formation of the zygotes, the F1 and F2 plants can be understood from a diagram called Punnett Square
• The Punnett Square shows the parental tall TT (male) and dwarf tt (female) plants, the gametes produced by them and, the F1 Tt progeny
• When the F1 Tt plant is self-pollinated, the resultant zygotes can be of the genotypes TT, Tt or tt in the ratio 1:2:1
• Monohybrid cross


  Cross between F1 plants of genotype Tt
• Gametes of F1 plant Tt

  T and t in equal proportion
• Fertilisation of F1 plant Tt

  1. Pollen grains of genotype T have 50% chance to pollinate eggs of genotype T and t

  2. Pollen grains of genotype t have 50% chance to pollinate eggs of genotype T and t
• Genotypes of resulting zygotes

  • TT
  • Tt
  • tt
• Ratio of genotypes in F2: 1/4 TT, 1/2 Tt, 1/4 tt
• Phenotype of F1 is 'tall' even though genotype is Tt