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

Disclaimer: may from time to time provide links to third party Internet sites under their respective fair use policy and it may from time to time provide materials from such third parties on this website. These third party sites and any third party materials are provided for viewers convenience and for non-commercial educational purpose only. Compete does not operate or control in any respect any information, products or services available on these third party sites. makes no representations whatsoever concerning the content of these sites and the fact that has provided a link to such sites is NOT an endorsement, authorization, sponsorship, or affiliation by with respect to such sites, its services, the products displayed, its owners, or its providers.