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NEET Biology Principles of Inheritance and Variation

Principles of Inheritance and Variation PDF Notes, Important Questions and Synopsis


  • Heredity is the transmission of characters from one generation to another through direct genetic descendants.

  • Mendel’s Laws of Inheritance:

  • Punnett square is a graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross.
  • Test cross is a cross between the F1 hybrid and its recessive parent. It confirms the purity of the F1 hybrid, whether it is homozygous or heterozygous.
  • Back cross is a cross between the F1 hybrid and one of its parents, which may be dominant or recessive.
  • Incomplete dominance occurs when two parents are intercrossed with each other, and the hybrid produced does not resemble either of its parents but is mid-way between the parents.
  • Multiple allelism occurs when more than two alleles exist at a given locus of a chromosome. In a given individual, only two of these alleles occur, one derived from each parent.
  • Co-dominance is the phenomenon when both alleles of a pair are fully expressed in a heterozygote, so the genes and traits are said to be co-dominant.
  • Linkage is the tendency of the genes to remain together during the process of inheritance.
  • Differences between complete linkage and incomplete linkage:

  • Crossing over is the mutual exchange of segments of non-sister chromatids of homologous chromosomes during the process of meiosis.
  • Chromosomal Theory of Inheritance was proposed by Walter Sutton and Theodor Boveri. Sutton combined the knowledge of chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance.
  • Sex Determination in Humans: 

  • Mutation is an abrupt and discontinuous process where a gene or a chromosome undergoes heritable change in its structure or number.
  • Ways of Chromosomal Mutation: 

  • Mendelian disorders are determined by alteration or mutation in a single gene. 

  • Aneuploidy is a condition where an organism possesses fewer or extra chromosomes than the normal genome number of the species; e.g. (2N – 1) or (2N + 1), where N is the haploid chromosome number.
  • Chromosomal disorders are caused by the absence or excess or abnormal arrangement of one or more chromosomes.