Principle of inheritance and variation

• Genetics 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
• 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
• Mendel's experiments had a large sampling size, which gave greater credibility to the data that he collected
• The confirmation of Mendel's 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
• True-breeding line


  One that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations
• Contrasting traits selected by Mendel


  • Smooth or wrinkled seeds
  • Yellow or green seeds
  • Inflated (full) or constricted green or yellow pods
  • Tall or dwarf plants
• Steps in making a cross in pea

  1. Collect the seeds produced

  2. Grow them to generate plants of the F1 generation
• Mendel observed that all the F1 progeny plants were tall, i.e. one of its parents; none were dwarf
• Mendel self-pollinated the tall F1 plants and to his surprise found that in the F2 generation some of the offspring were dwarf; the character that was not seen in the F1 generation was now expressed
• Genotype


  The genetic makeup of an organism
• Phenotype


  The observable physical or biochemical characteristics of an organism
• Dominant


  One member of a pair of factors that dominates the other (recessive)
• Recessive


  One member of a pair of factors that is masked by the dominant factor
• Mendel crossed a tall plant with a dwarf plant, this he called a test cross
• Test cross


  Crossing an organism showing a dominant phenotype (and whose genotype is to be determined) with the recessive parent
• Mendel proposed two general principles or laws of inheritance: the First Law or Law of Dominance and the Second Law or Law of Segregation
• Law of Segregation


  The alleles do not show any blending and that both the characters are recovered as such as in F2 generation
• Mendel also worked with and crossed pea plants that differed in two characters, as is seen in the cross between a pea plant that has seeds with yellow colour and round shape and one that had seeds of green colour and wrinkled shape
• Mendel found that the seeds resulting from the cross of the parents, had yellow coloured and round shaped seeds
• Law of Independent Assortment


  When two pairs of traits are combined in a hybrid, the segregation of one pair of character is independent of the other pair
• To understand the Law of Independent Assortment, compare the chromosomes of four different colour in the left and right columns
• Meiosis


  1. Prophase

  2. Metaphase

  3. Anaphase
• During Anaphase of meiosis, the two chromosome pairs can align at the metaphase plan independently of each other
• The Law of Independent Assortment states that when two pairs of traits are combined in a hybrid, the Segregation of one pair of character is independent of the other pair of character
• Chromosomal Theory of Inheritance

  • Developed by de Vries, Correns and von Schema in the 1800s
  • Advancements in microscopy led to the discovery of chromosomes
  • Chromosome movement during meiosis was observed and linked to Mendel's laws
• Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carried
• Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance
• The chromosomal theory of inheritance, developed by Thomas Hunt Morgan and his colleagues, led to discovering the basis for the variation that sexual reproduction produced
• Morgan worked with the tiny fruit flies, Drosophila melanogaster
• Linkage refers to the frequency of recombination between genes and their physical distance on the same chromosome
• Polygenic inheritance refers to traits that are controlled by multiple genes
• Pleiotropy


  The effect of a single gene on multiple phenotypic expressions
• An example of pleiotropy is the disease phenylketonuria, caused by a mutation in the gene for the enzyme phenylalanine hydroxylase, which is characterised by mental retardation and reduced skin/hair pigmentation
• Sex Determination


  • Cytological observations led to the concept of genetic/chromosomal basis of sex determination
  • Henking observed a specific structure (X-body) that was present in 50% of sperm
  • The X-body was later identified as the X-chromosome
• In the XO type of sex determination, seen in grasshoppers, males have only one X-chromosome besides the autosomes, whereas females have a pair of X-chromosomes
• In the XY type of sex determination, seen in humans, both males and females have the same number of chromosomes, but the males have an X-chromosome and a smaller Y-chromosome, while females have a pair of X-chromosomes