Principles of Inheritance and Variation

Sex linked inheritance in humans


  • If body characters are located by certain genes in X chromosomes, it is referred to as X linkage and if the genes for body characters are located in Y chromosomes then it is Y linkage.
  • Certain X linked characters are Haemophilia (bleeders disease); colour blindness, night blindness etc.
  • Y character linkage are associated only with males such as Hypertrichosis.
  • X linkage was first observed in eye colour of drosophila.
  • If the characters in a family donot shift in sexes then the inheritance is non-criss cross inheritance.
    Eg : Hologenic inheritance (mother → daughter → grand daughter)
           Holandric inheritance (Father → son → grandson)
  • In criss cross inheritance, the characters shift in sexes
    Eg : Diagenic (Father → daughter → grand son)
           Diandric (mother → son → grand daughter)
  • Baldness is a sex influenced character as it is differently influenced in both sexes. [ Eg. ‘Bb’ couses baldness in males, but not in Femals]

    Mutation [New sudden Inheritable Discontinuous Variation]

  • First discovered in Oenothera lamarckiana by Hugo de vries, was later was studied by Morgan in Drosophila
  • Somatic mutation (will not pass to offspring) and Germinal mutation (Passes to offsprings) are its two types.
  • Exception in somatic mutation is vegetative propagation and asexual reproduction where mutation is passed onto the next generation.
  • Depending on the Cause, mutation is of 2 types (i)- chromosomal mutation (ii) Gene mutation
  • Chromosomal mutation is of 2 types – (i) Polyploidy (ii) Aneuploidy mutation, under genomic mutation.
  • Polyploidy is the increase in the number of set of chromosomes Eg. Autopolyploidy, Allopolyploidy [Triticale]
  • Hyperploidy [Trisomy (2n + 1), Tetrasomy (2n + 2)] and Hypoploidy → monosomy (2n – 1), nullisomy (2n – 2) are types of Aneuploidy
  • Change in chromosome structure is called chromosomal aberration, which is of 2 types
  • Gene mutation is of following types.
    → Point mutation and gross mutation [change in IBP / change in more than IBP resp.]
    → Substitution is of 2 types Transition and Transversion.
    → Transition is when purine and pyrimidines are replaced by another purine and pyrmidine respectively.
    → If a purine is replaced by pyrimidine and vice versa, it is trans version.
  • Frame shift mutation (most dangerous) is the insertion (or) deletion of 1 or 2 BP. Eg. Muscular dystrophy, Thalassemia etc.
  • The mutation by which stop codons are formed is called Non – Sense – mutation.
  • Substances causing mutation are mutagens – (i) Physical (ii) Chemical.
  • Radiation, Temperature (Q10 = 5) are physical mutagens.
  • Nitrous acid, DDT, Mustard gas, LSD are chemical mutagens
  • UV rays do not react with Thymine, whereas they act on Adenine to convert hypoxanthine, Guanine to X anthine, Cytosine to Uracil.
  • Antibiotics like kanamycin and Streptomycin cause mutation in prokaryote. Paromycin acts on Eukaryotes and causes mutation.

Part-1: Watch this video for the topic from 0:13 to 12:42

Part-2: Watch this video for the topic from 0:10 to 5:25

Part-3: Watch this video for the topic from 0:10 to 14:21

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