Heredity and Evolution: Meaning of Heredity, Aristotle՚s Model of Inheritance (For CBSE, ICSE, IAS, NET, NRA 2022)

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  • The passing on of traits from parents to their offspring is called Heredity.
  • Heredity is also known as inheritance or biological inheritance.
  • This takes place either through sexual reproduction or asexual reproduction.
  • The genetic information of the parents is acquired by the offsprings.
  • Genetics is the study of heredity in Biology.
  • Cryptophasia is the language which can be understand only by the twins.
  • Twin formation can be of two types:


  • The twins are the ones which are formed from one zygote.
    • Splits into two embryos.
    • Twins are identical to each other.


  • The twins are fraternal in dizygotic twins.
  • The twins are not identical.
  • Reproduced from separate eggs being released simultaneously from the ovary.
  • Subtypes of twins:
    • Boy-boy
    • Boy-girl
    • Girl-girl

Asexual Reproduction

  • This reproduction involves the fusion of gametes or change in the number of chromosomes.
  • It is the primary form of reproduction for single-celled organisms such as archaea and bacteria.
  • Several eukaryotic organisms including plants, animals, and fungi can also reproduce asexually.


Asexual Reproduction Occurs in Following Processes

Aristotle՚s Model of Inheritance

Aristotle՚s Model of Inheritance

Modern Development of Genetics and Heredity

  • The gap between experimental geneticists and naturalists was bridged by the modern synthesis.
  • With known genetic mechanisms including the observational evidence of naturalists, all evolutionary phenomena can be explained.
  • The evolution process is gradual:
    • Small genetic changes
    • Recombination ordered by natural selection
  • Selection is overwhelmingly the main mechanism of change.
  • The primacy of population thinking:
    • The genetic diversity carried in natural populations.
    • The strength of natural selection in the wild.

Common Genetic Disorders

Down Syndrome

  • It is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21.
  • It is also known as trisomy 21.
  • It is usually associated with:
    • Physical growth delays.
    • Mild to moderate intellectual disability.
    • Characteristic facial features.

Sickle Cell Disease

  • It is a group of blood disorders typically inherited from a person՚s parents.
  • Sickle cell anaemia (SCA) is the most common type.
  • Sickle cell disease results in an abnormality in the oxygen-carrying protein haemoglobin.
Sickle Cell Disease


  • PKU is an inborn error of metabolism.
  • It is a genetic disorder inherited from a person՚s parents.
  • It results in decreased metabolism of the amino acid phenylalanine.
  • Untreated, PKU can lead to:
    • Intellectual disability
    • Seizures
    • Behavioral problems
    • Mental disorders

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