HEREDITY AND EVOLUTION - III : LONG ANSWERS

GIVE ANSWER IN DETAIL

*INDEX-TOPIC SEARCH

1. Explain : Heredity and variation.
   

   ANS :
   HEREDITY :
   

   
   
   • 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.
     

   
   VARIATION :
   

   
   
   • 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.
     

   
   

   
   
2. Explain : Mendel's concept of heredity.
   

   ANS :
   

   
   
   • Mendel proposed that for regulation of every character in an organism there is a pair of factors.
   • During Mendel's time, nothing was known about chromosomes and DNA.
   • T.Boveri and W.S.Sutton proved that a gene is a part of chromosome.
   • Thus, Mendel indicated the path of transmission of factors from parental generation to offsprings through gamates.
   • Both Mendelian factors(genes) as well as chromosomes are present in pairs. They get separated from each other during meiosis when a haploid(n) gamate is formed.
   • During fertilisation, gamates fuse to form a zygote(2n) carrying characters which later on get transmitted to the offspring.
     

   
   

   
   
3. Write a note on sex determination and chromosomes OR Explain the role of chromosomes in sex determination.
   

   ANS :
   

   
   
   • The phenomenon determining whether the developing embryo will be a male or a female is known as sex determination.
   • During the investigation on grasshoppers, one additional unpaired chromosome was observed in the male. This additional chromosome is called X-body and is known as sex chromosome since it is associated with sex determination.
   • The second sex chromosome is called X-chromosome if it is similar to the previous one and is called Y-chromosome if it differs from the X-chromosome.
   • In humans, the female has 2A + XX chromosomes and all the ova are A + X type.
   • The human male has 2A + XY chromosomes. This leads to 50% sperms of A + X type and 50% of A + Y type.
   • The fertilisation of an ovum by a sperm of A + X type produces a female whereas the fertilisation of an ovum by a sperm of A + Y type produces a male.
   • In some species, like turtle Chrysema Picta and lizard Agama agama, the sex determination is regulated by environmental factors.
     
     

   
   

   
   
4. What is biological evolution ? Mention the evidences supporting evolution and different theories of evolution.
   

   ANS :
   

   
   
   • The sequential and well-organised process of gradual structural changes in species is called biological(organic) evolution.
   • Evidences of comparative anatomy, comparative embryology, vestigial organs, fossils, connecting links, physiology and taxonomy support biological evolution.
   • The various theories of evolution are :
     
     
     
     1. Lamarck's theory of inheritance of acquired characters.
     2. August Weismann's Germplasm theory.
     3. Darwin's theory of Natural Selection.
     4. Hugo De Vries' Mutation theory.
        
     
     

   
   

   
   
5. Explain : Chromosome is the physical basis of heredity.
   

   ANS :
   

   
   
   • The discovery of the process of sexual reproduction involving gamates established the concept of transmission of genetic material.
   • Mendel proposed that there is a pair of factors for regulation of every character though during his time little was known about chromosomes and DNA.
   • T.Boveri and W.S.Sutton experimentally proved that gene is a part of chromosomes.
   • Mendelian factors(genes) as well as chromosomes are found in pairs but they get separated during meiosis forming haploid(n) gamates.
   • During fertilisation, gamates fuse to form zygote(2n) in which the characters are stored and transmitted to the offspring.
   • It was proved that the genes are located on chromosomes and the number of chromosomes is fewer than that of genes because the number of characters in an organism is very large. Thus, there are 23 pairs of chromosomes in human being but the total number of genes present on them is about 30,000 to 40,000.
   • It has been established that gene is a segment of a large molecule called DNA(DeoxyriboNucleic Acid).
   • DNA is the important component for the expression of genetic information from one generation to the other. Every chromosome possesses one molecule of DNA.
   • In some viruses, RNA(RiboNucleic Acid) is the genetic material. Such viruses are called retrovirus. The virus causing AIDS in human beings is a retrovirus.
   • Thus, chromosome is the physical basis of heredity.
     

   
   

   
   
6. Describe the structure of chromosome.
   

   ANS :
   

   
   
   • Chromosomes are carriers of genes.
   • Chromosomes can be identified by staining.
   • In prokaryotes only one chromosome occurs whereas in eukaryotes many chromosomes occur in pair.
   • The paired structure of chromosomes is known as diploid(2n) and is seen in somatic cells. They are called homologous chromosomes as both the chromosomes of a pair are exactly similar.
   • The number of diploid chromosomes is fixed for a species.
   • In reproductive cells the chromosomes are haploid(n), i.e. they are half in number.
   • During cell division(metaphase), different chromosomes get separated from each other and their morphological study becomes possible.
   • A group of minimum two chromatids forms one chromosome.
   • The diameter of each chromatid is 60 A°.
   • Chromatids are joined with each other by a dark region called centromere or primary constriction. Some chromosomes have additional secondary constriction.
   • Variation can be seen in position and number of centromere.
   • The length of chromosomes is 0.2 - 5.0μ (micron) andtheir diameter ranges from 0.2 - 2.0μ.
     

   
   
   
   

   
   
7. Write a note on types of chromosomes.
   

   ANS :
   

   
   
   • Chromosomes are classified depending on the position of the centromere.
   • There are four types : (i)Metacntric (ii)Sub-metacentric (iii)Acrocentric (iv)Telocentric.
   • Metacentric :
     
     
     • The centromere is present exactly in the centre.
     • All four arms of chromatids are equal in length.
       
     
     
   • Sub-metacentric :
     
     
     • The centromere is situated slightly away from the centre.
     • The two arms of chromatids on one side are long and the other two arms are short.
       
     
     
   • Acrocentric :
     
     
     • The centromere is located far from the centre near one end.
     • The two arms of chromatids on one side are very long and the other two arms are very short.
       
     • In some chromosomes, a small filamentous structure possessing a round tip is present on the distal end of short arms. This part is known as satellite.
     • A short filament called nucleolar organiser region connects satellite with the short arm.
       
     
     
   • Telocentric :
     
     
     • The centromere is situated at the tip of the chromosome.
     • The two arms are located only on one side.
       
     
     

   
   

   
   
   
   
8. Describe Watson-Crick model for structure of DNA.
   

   ANS ::

   
   
   • In 1953, Watson and Crick proposed a model showing molecular structure of DNA.
   • In the molecule of DNA two polynucleotide chains run in antiparallel directions and twist around each other to form a spiral helical structure.
   • One nucleotide is linked with another nucleotide by a phospho-diaster bond.
   • In the chain, sugar and phosphate are linked with their nitrogenous bases.
   • Purine type nitrogenous base of one nucleotide links with pyrimidine type of nitrogenous base by weak hydrogen bonds.
   • Adenine(A) links with Thymine(T) by two weak hydrogen bonds whereas Guanine(G) links with Cytosine(C) by three weak hydrogen bonds. Such pairing is known as complimentary pairing.
   • The purine to pyrimidine ratio in each DNA molecule is always 1 : 1.
   • The length of one helix is 34 A° and each helix possesses 10 nitrogenous base pairs.
   • The distance between two adjacent nucleotides is 3.4 A° and the distance between two opposite chains is 20 A°.
     

   
   
   
   

   
   
9. Explain different evidences supporting biological evolution.
   

   ANS ::

   
   
   • Comparative anatomy, comparative embryology, vestigial organs and fossils are major evidences supporting biological evolution.
   • COMPARATIVE ANATOMY :
     
     
     • A number of common features of different kinds of organisms favour evolution.
     • Homologous organs like forelimb of humans, fins of fishes, wings of birds, limb of frog and forelimb of lizard perform different functions but show structural similarities.
     • All these organs might have evolved from a common ancestor from which these organisms have evolved.
     • Analogous organs like wings of birds and wings of insects perform same function of flying but their origins differ. Wings of birds are modifications of forelimb but wings of insects are ectodermal derivatives(i.e they have originated from skin).
       
     
     
   • COMPARATIVE EMBRYOLOGY :
     
     
     • Embryology provides strong evidence of evolution for the origin of different organisms.
     • During the early stage of development, the embryos of different vertebrates show similarities.
     • Later, their development results in clearly visible differences.
     • This is an evidence of the fact that the vertebrates have a common ancestor and origin.
     • Based on this fact Ernst Hackel proposed the 'Recapitulation Theory' which states that "Ontogeny recapitulates phylogeny".
       
     
     
   • VESTIGIAL ORGANS :
     
     
     • Morphological studies have identified some organs which have lost their functions. These organs might have been functional in their ancestors. Such organs are called vestigial organs.
     • Vermiform appendix associated with large intestine is a vestigial organ in humans but it is functional in herbivores like rabbit.
     • Nictitating membrane of eye is a vestigial organ in humans but it is functional in birds and gives protection to the eyes.
     • Thus, vestigial organs support evolution.
       
     
     
   • FOSSILS :
     
     
     • Fossils provide some evidence to support evolution.
     • Fossils of Archaopteryx(now extinct) show that it looked like a bird but it had maximum reptilian characters. This indicates that birds have evolved from reptiles.
       
     
     

   
   
   

   
   
10. 
    

    ANS :