HEREDITY AND EVOLUTION - II : BRIEF ANSWERS

ANSWER IN BRIEF

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1. Explain : Heredity.
   

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   • Transmission of characters from parents to offsprings is known as heredity.
   • Fertilised egg or zygote carries hereditary information.
   • The zygote develops into a particular type of offspring only.
   • Thus, eggs laid by a sparrow hatch into sparrows and a dog reproduces to give birth to pups only.
     

   
   

   
   
2. Explain : Variation.
   

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   • Variation means some individual diversity in the organism of one generation from that of the parental generation.
   • Variation passes from generation to generation.
   • In sexual reproduction, both similarities and variation are clearly observed among the offsprings of same parents.
   • Due to variation, any two persons rarely show complete similarity.
     

   
   

   
   
3. Explain : The number of chromosomes is fewer than the number of genes.
   

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   • Genes are located on chromosomes.
   • Genes are responsible for specific characters of an organism.
   • The number of chromosomes in an organism is small but the number of characters of an individual organism is very large.
   • For example, there are 23 pairs of chromosomes in a human cell but the number of characters is very large requiring a large number of genes(30,000 to 40,000).
   • Thus, the number of chromosomes is fewer than that of genes.
     

   
   

   
   
4. Explain : Mutation.
   

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   • Gene is a segment of DNA and is a unit of inheritance.
   • The nature of a gene is maintained generation after generation but sometimes it undergoes sudden inheritable changes known as mutation.
   • Mutation is one of the reasons of variation among individuals and species.
     

   
   

   
   
5. Describe the role of environmental factors in sex determination.
   

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   • In some species, the sex determination is regulated by environmental factors.
   • For example, in turtle Chrysema Picta, high incubation temperature results into development of a female.
   • In a lizard, Agama agama, high incubation temperature results into development of a male.
     

   
   

   
   
6. Mention various theories of evolution.
   

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   • Lamarck's theory of Inheritance of acquired characters.
   • August Weismann's Germplasm theory.
   • Darwin's theory of Natural Selection.
   • Hugo De Vries' Mutation theory.
     

   
   

   
   
7. Write a few points on Darwin's theory of Natural Selection.
   

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   • Darwin's theory of Natural Selection describes biological evolution.
   • It explains how evolution takes place with the help of variation.
   • It fails to explain how variation occurs.
     

   
   

   
   
8. Explain : Homologous organs with example.
   

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   • The organs showing similarities in origin but performing different functions are called homologous organs.
   • Forelimbs of human beings, fins of fishes, wings of birds, limbs of frog and forelimbs of lizard have structural similarities due to same origin but they perform different functions.
     

   
   

   
   
9. Explain : Analogous organs with example.
   

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   • The organs showing different origin but performing the same functions are called analogous organs.
   • For example, both birds and insects use wings to fly(same function) but their origins are different. The wings of birds are modifications of forelimbs whereas the wings of insects are ectodermal derivatives(i.e. they have originated from skin).
     

   
   

   
   
10. Write a note on components of nucleotides.
    

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    • DNA is a macromolecule made up of a large number of nucleotides.
    • A nucleotide is made up of three components :
      
      
      
      1. Pentose sugar(deoxyribose sugar)
      2. Phosphate group(H₃PO₄)
      3. Nitrogenous base
    • The nitrogenous bases are of two types :
      1. Purines
      2. Pyrimidines
    • Adenine(A) and Guanine(G) are purines whereas Cytosine(C) and Thymine(T) are pyrimidines.
      

    
    

    
    
11. 
    

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