Principles of inheritence

• The work of Mendel and others who followed him gave us an idea of inheritance patterns. However 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.
• He met Crick and discovered their common interest in solving the DNA structure.
• Their first serious effort was unsatisfactory. 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.
• 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".
• A critical influence in Crick's career was his friendship with J. D. Watson, then a young man of 23, leading in 1953 to the proposal of the double-helical structure for DNA and the replication scheme.
• Crick was made an F.R.S. in 1959.
• 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
  • The Nobel Prize in 1962
• Have you ever wondered why an elephant always gives birth only to a baby elephant and not some other animal? Or why a mango seed forms only a mango plant and not any other plant?
• Given that they do, are the offspring identical to their parents? Or do they show differences in some of their characteristics? Have you ever wondered why siblings sometimes look so similar to each other? Or sometimes even so different?
• These and several related questions are dealt with, scientifically, in a branch of biology known as Genetics. This subject deals with the inheritance, as well as the variation of characters from parents to offspring.
• Inheritance

  The process by which characters are passed on from parent to progeny; it is the basis of heredity.
• Variation


  The degree by which progeny differ from their parents.
• 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.
• We must, however, recognise that 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.
• During Mendel's investigations into inheritance patterns it was for the first time that statistical analysis and mathematical logic were applied to problems in biology.
• His experiments had a large sampling size, which gave greater credibility to the data that he collected. Also, the confirmation of his inferences from experiments on successive generations of his test plants, proved that his results pointed to general rules of inheritance rather than being unsubstantiated ideas.
• 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 conducted such artificial pollination/cross pollination experiments using several true-breeding pea lines. A true-breeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations.
• Some of the contrasting traits selected by Mendel were

  • Smooth or wrinkled seeds
  • Yellow or green seeds
  • Inflated (full) or constricted green or yellow pods
  • Tall or dwarf plants
• Mendel crossed tall and dwarf pea plants to study the inheritance of one gene.
• He collected the seeds produced as a result of this cross and grew them to generate plants of the first hybrid generation. This generation is also called the Filial1 progeny or the F1.
• Mendel observed that all the F1 progeny plants were tall, like one of its parents; none were dwarf.
• Mendel then self-pollinated the tall F1 plants and 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.
• Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things as 'factors'. Now we call them as genes.
• Genes

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


  Slightly different forms of the same gene which code for a pair of contrasting traits.
• Genotype


  The pair of alleles for a particular trait.
• Phenotype


  The observable expression of a trait.
• Dominant


  In a pair of dissimilar factors, one dominates the other.
• Recessive


  In a pair of dissimilar factors, one is recessed or not expressed in the presence of the dominant factor.
• Mendel observed that the recessive parental trait is expressed without any blending in the F2 generation, and inferred that the alleles of the parental pair separate or segregate from each other during gamete formation.
• 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.