Genetics / Mendel's Laws of Inheritance
Deviations from Mendel's finding
Since the rediscovery of Mendel's laws in 1900, cases have been reported, which gave additional information regarding the patterns of inheritance. The following ideas of Mendel had to be modified.
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Mendel found that each of the seven characters considered was controlled by a single gene. Although gene is a functional unit at the biochemical level, character may be the result of coordination of functions of several genes. Therefore, for a particular character there may be two or more than two genes responsible and these genes may interact in different manners (consult
Lethality and Interaction of Genes).
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For each of the seven pairs of characters examined, it was observed that one allelomorph dominated over the other, so that F1 exhibits one or the other alternative phenotypes represented in the parents.
However, soon after rediscovery of Mendel's laws, experiments were available to show that in some cases the F1 individual showed the phenotype which was intermediate between the two parents.
For instance in four-o'clock plant (Mirabilis jalapa) it was found that when plants with red flowers were crossed with those having white flowers, plants with pink flowers were obtained in F1generation. This would then give rise to red, pink and white flowered plants in 1 : 2 : 1 ratio in the F2 generation. Similarly in snapdragon, plants With broad leaves and plants with narrow leaves give rise to plants with intermediate leaves in F1 generation. Likewise, plants with red flowers and plants with white flowers give rise to those with pink flowers. If a dihybrid cross is made using broad leaves and red flowers in one parent (BBRR) and narrow leaves and white flowers in the other parent (bbrr), F1individuals (BbRr) will have intermediate leaves and pink flowers. In the F2 generation nine phenotypes corresponding
to nine genotypes will be observed. A summary showing nine different genotypes and phenotypes is given in Table 2.9.
- In each of the seven pairs of characters studied by Mendel, there were only two alternative forms for each character. This meant that only two alleles were present for each character. This also led to a belief that for each character there were two alternative forms, one dominant over the other. Later work showed that for a character there can be several phenotypes e.g. for rabbits the body colour can be of four or more types. Therefore, concept of alternative allelomorphs had to be modified by the concept of multiple allelism.
- According to the Law of Independent Assortment, any two or more than two pairs of characters assort independently of each other. Exception to this phenomenon were discovered due to linkage and the associated phenomenon of crossing over.
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