Principles of Inheritance and Variation

Deviations from Mendelism - incomplete dominance, codominance, multiple alleles

  • Incomplete dominance [diversion from mendel] was first discovered in Mirabilis jalapa, when the cross between a red (RR) and white (rr) flowers yield pink flowers (Rr).
  • As 2 genes were incompletely / Partially dominant, blending 9 characters occurred to produce a phenohypic and genotypic ratio of 1 : 2 : 1 [White : Pink : Red]
  • When both the genes are dominant, then both the characters of the genes could be visualized in an organism.
  • When a red and a white flower is mixed (crossed), the new generation flower produced had petals of both Red and white, but not pink. Other eg : Roan. (with the phenotypic and genotypic ratio of 1 : 2 : 1 [Black : roan : white]
    * (Roan : Animal with both black and white colours)
  • Certain genes can have more than 2 alleles because of a mutation in a  gene
    Eg : alleles for blood grouping IA, IB (dominant) and I*(recessive)
  • Four alleles are present for the hair coat colour found in rabbit namely Himalayan (CH), Chinchilla (Cch), Coloured (C+) and Albina (CA)
  • The range of dominance of above alleles is as follows
    C+ > Cch > Ch > CA
  • The possible genotype from the given number of alleles could be identified by a formula \frac{n(n+1)}{2}, when n is the number of alleles.

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Part-2: Watch this video for the topic from 0:08 to 11:57

Part-3: Watch this video for the topic from 0:10 to 7:45

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