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Heredity and Evolution I Class -10 I Science Chapter 9

Heredity and Evolution



Heredity (inheritance) means transmission of characters from parents to offspring.

Inherited traits are characteristics that are passed from parents to their children e.g. Hair color, eye color and muscle structure and bone structure etc.  Inherited traits, also termed as hereditary traits or family traits and are genetic characteristics that you get from your parents. 

Genetics is the study of heredity and other variations.

Organisms that are identical copies of one another are known as clones. They are exact copies of each other.


Evolution means the change in the characteristics of a species over several generations and development of new species. It naturally occurring slow, continuous and irreversible process of change.

The gradual change of living organisms from pre-existing organisms since from beginning of life is called organic evolution.

Accumulation of Variation during Reproduction:

The occurrence of small changes or differences among the individuals of a species is called variation. Hereditary variations is one of the importance reason in the process of evolution of a new species.

There are two kind of variations - somatic variations and genetic variation.


·    Somatic variations: It occur in the somatic cell of the body. They are not inherited or transmitted to the next generation. So, they are also known as acquired traits. Ex: cutting tail of dog.


·    Gametic variations: It occur in the gemate cells/reproductive cell of the body. They are inherited in the next generation. So, they are known as inherited traits.

 Importance of Variations: 

·         It is the process of evolution of a new species.

·         It improves the chances of the survival of the organism according to the changing environment.

 Reproduction is a vital activity for every living organism to maintain its existence on earth. In this process one or more offspring are produced from the parent organisms. During reproduction, offspring are not identical to their parent organism; they're a little different. The extent of the variation is determined by the mode of reproduction. 

Reproduction is classified into two types: asexual and sexual reproduction;

Asexual Reproduction

Asexual reproduction involves a single parent so minor variations do occur due to inaccuracies in DNA replication.

  •      Variations are fewer.
  •      It occurs due to small inaccuracies in DNA copying.


Sexual Reproduction

Sexual reproduction involves two parents and the offspring’s genotype is contributed by two parents and hence chances of variations are very high.

  •    Variation is large
  •    It occurs due to crossing over the separation of chromosomes.

 Rules for the Inheritance of Traits – Mendel’s Contributions

  •  Gregor Johann Mendel (1822-1884) first worked out scientific experimental study on heredity.
  •   Mendel, an Austrian Augustinian monk noticed variations in the characteristics of garden pea plant (Pisum sativum) which was cultivated in his monastery garden.
  •  Mendel was keen to know the results of crossing of pea plants with the variation in traits.
  • The visible contrasting characters observed by Mendel in the garden pea plants were given below:
  • Mendel choose the garden pea plant of Pisum sativum for his experiments.
  • He selected tall and dwarf plants and allowed them to grow naturally.
  • As pea plants produce seeds only by self-pollination (asexual  reproduction), he noticed that tall plants always produced tall plants generation after generation under natural conditions and dwarf plants always dwarf plants generation after generation.
  •  Mandel termed the tall and dwarf plants as wild types or pure breeding varieties.  Further He crossed a tall plant with a dwarf plant, produced progeny (off springs) and calculated the percentage of tallness and dwarfness in subsequent generations. 
  •   When a pure breeding tall plant was crossed with a pure breeding dwarf plant, all plants were tall in the first filial generation (F1) i.e., there were not any medium height plants or dwarf plants.
  •   This shows that only one of the parental traits were seen and not the mixture of the both parental traits.
  •   When such a F1 tall plant was allowed to have self-pollination, both the tall and dwarf plants developed in second filial generation (F2) in the ratio of 3:1.
  •   This represents that both tallness and dwarfness were inherited in the F1 plants but only the tallness trait was expressed.
  •   The first experiment of Mendel carried out for inheritance of a single trait (Height of the plant Tall/Dwarf) is called Monohybrid Cross.






Related topics of CBSE CLASS 10

  • Life Process
  • Control and Coordination
  • How do Organisms Reproduce?
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