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  • Mendel
    • Mendel studied the process of heredity in selected features of the garden peas
      • Pisum Sativum
      • Why peas?
        • They were easy to grow, their pollination could be controlled
          • Easy observable characteristics
        • First scientist to build up data on which sound scientific conclusions could be based
    • He isolated pea plants which were pure breeding (homozygous)
      • He referred to each characteristic as a trait and he chose those which had two contrasting features
        • e.g. stem length, which could have either be long or short and flower colour, red or white
          • He then selected two pure breeding plants with alternative expressions of a particular characteristic e.g. tall/dwarf plant
    • He collected seeds produced by the female parent and grew them to give the first generation offspring (F1)
      • Recorded their characteristics and then crossed  two pants from this generation.
        • As it crossed plants from the same generation = self-cross
          • Again seeds were produced, collected and grown the following year to give the second generation offspring (F2)
    • His results showed that the F2 plants were a mixture of tall and dwarf plants - in approx. ration of 3:1
      • Similar crossings involving other single characteristics produced similar results all producing approx. ratio of 3:1 in the F2
        • 1. There were no plants of intermediate height (no blending)
        • 2. There were no dwarf plants in the F1 generation, though they reappeared in the F2
    • From the first observation, Mendel concluded that characteristics are not blended together but that they are determined by definite discrete particles which he called 'factors' - genes.
      • From the second observation -  factor of dwarfness must be carried in the F1 generation, but is 'hidden' by the factor of tallness
        • The dwarf factor is expressed only in the absence of the tall factor, therefore the plants must  carry two factor for each characteristic, one factor coming from each parent
          • All F1 plants are tall, the factor for tallness must be dominant to the factors for dwarfness, which is the recessive factor
            • Mendel combined these conclusions in his first law of inheritance, the law of segregation, which states that.
              • The characteristics of an organism are determined y factors (alleles) which occur in pairs. Only one of each pair of factors (alleles) can be responded in a single gamete
    • Black cross test
      • One common genetic problem is that an organism which shows a dominant character can have two possible genotypes.
        • For example, a plant producing seeds with round coats could either be homozygous dominant (RR) or heterozygous (Rr)
          • The appearance of the seeds (phenotype) is identical in both cases.
            • It is often necessary however, to determine the genotype accurately.
              • This may be achieved by crossing the organism of unknown genotype with one whose genotype is accurately known
                • The genotype that can be positively identified from its phenotype alone is the homozygous recessive genotype (rr)
                  • In the case of the seed coat, any pea seed with a wrinkled coat must have the genotype (rr) By crossing the dominant character, the unknown phenotype can be identified.
                    • The offspring comprise equal numbers of plants producing round seeds and ones producing wrinkled seeds. If some of the plants produced seeds with wrinkled coats, then the unknown genotype must be Rr
                      • Such a plant with its rr genotype can only be produced if both parents donated an r gamete.
                        • The only way a plant which produces round seeds can donate such a gamete is if it is heterzygous (Rr)
    • Dihybrid Inheritnce
      • Dihybrid inheritance is the inheritance of two characteristics, each controlled by a different gene at a different locus
        • In one experiment Mendel studied dihybrid inheritance by crossing plants from two pure breeding strains:one tall with purple flowers, the other dwarf with white flowers
          • All of the offspring in the F1. gen. were tall with purple flowers, these being dominant characteristics
            • The F1 generation were self-crossed, producing the following phenotypes and ratios in the F2 generation
              • He concluded from his results that the two pairs of characteristics behave quite independently of each other.
                • This led him to formulate his second law of inheritance, the law of independent assortment which states...
                  • Either one of a pair of contrasted characters may combine with either of another pair
                    • With our current knowledge of genetics, the law could now be re-written as...
                      • Either one of an alleic pair may combine randomly with either of another pair


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