Chapter 16 : HEREDITY AND EVOLUTION

1. Define Sex Determination. What are the methods (mechanisms) of sex determination in different organisms?
   ANS
   
   • The mechanism to determine the sex of an individual is known as sex determination.
   • Different species use different mechanisms for sex determination.
     • In some species, the temperature at which fertilized eggs are kept determines whether the organism developing in the egg would be male or female.
     • In human beings the sex is determined genetically.
     • In animals like snail, the individual can change the sex, indicating that their sex is not determined genetically.
2. Answer in short.
   1. Name the ancestor of Broccoli, Kohlarabi, Kale, Cauliflower and Cabbage.
   2. Name at least two organisms which are now extinct and are studied from their fossils.
   3. Name five varieties of vegetables which have been produced from 'Wild cabbage' by the process of artificial selection.
   4. Choose the one term from the following which includes the other three: Broccoli, Wild cabbage, Cauliflower, Cabbage
   
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   1. Wild cabbage
   2. Ammonite, Trilobite, Dinosaur
   3. Broccoli, Kohlrabi, Kale, Cauliflower and Cabbage
   4. Wild cabbage
3. Explain : Variation.
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   • The occurrence of differences among the individuals of same species is known as variation.
   • Organisms which reproduce asexually or vegetatively show very little variation.
   • Organisms which reproduce sexually show distinct variations.
   • In sexual reproduction, crossing over of genes during meiosis gives rise to new combinations of characteristics. This results in variation among the individuals of species.
   • All these variations, however, do not have equal chance of survival in the environment in which they find themselves.
   • Different individuals get different advantages of these variations.
   • Selection of variants by environmental factors forms the basis of evolutionary process.
4. Write a note on Heredity.
   ANS
   
   • Heredity means continuity of features from one generation to another.
   • Heredity can be defined as "the transmission of characters from parents to the offspring" or "the tendency of every individual to resemble his/her parents".
   • Due to heredity, eggs laid by a sparrow hatch into sparrows only and a dog gives birth to pups only.
   • Hereditary information is present in the fertilized egg (zygote) which develops into an organism of a particular type only.
5. Write a note on Sex Determination.
   ANS
   
   • In humans, the sex is determined by the genes located on chromosomes.
   • Genes are inherited from parents.
   • Out of 23 pairs of chromosomes, 22 pairs are autosomes which are similar in male and female.
   • In female, the 23rd pair has two similar chromosomes called X-chromosome.
   • In male, one chromosome in 23rd pair is X chromosome and the other is Y chromosome which is smaller in size.
     
     
   • All eggs (ova) of female are similar. Each egg contains 22 autosomes and one X chromosome.
   • 50% sperms in male have X chromosome and 50% have the Y chromosome (along with 22 autosomes).
   • If a sperm carrying X chromosome fertilizes an egg, the zygote develops into a female.
   • If a sperm carrying Y chromosome fertilizes an egg, the zygote develops into a male.
6. Explain the following as Evidence of Evolution: (i) Homologous organs (ii) Analogous organs (iii) Fossils
   ANS
   
   1. HOMOLOGOUS ORGANS
      • The organs having the same internal structure but different functions are called homologous organs.
      • The basic design of the bones of forelimbs of a frog, a lizard, a bird, a bat and a man is similar in in their internal structure but they perform different functions.
      • This is an evidence that the forelimbs of all these organisms have evolved from a common ancestor with same basic internal structure.
   2. ANALOGOUS ORGANS
      • The organs with different basic design but similar appearance and similar function in different organisms are called analogous organs.
      • The wings of insects and the wings of birds have different structure but they perform similar function.
      • The presence of analogous organs in different animals give the evidence that they may not have evolved from a common ancestor but they perform similar function to survive in prevailing environment.
   3. FOSSILS
      • The impressions of dead organisms that lived in the past are known as fossils.
      • The dead bodies of organisms are decomposed by micro-organisms. Sometimes, due to environmental conditions, these bodies do not decompose completely and are preserved in the earth.
      • For example, a dead leaf trapped in mud is not decomposed completely and the mud forms impression of the leaf by setting around it as a mould. It then hardens to form a rock and retains the impression of the leaf. Thus, fossil of a leaf is formed.
      • The age of the fossils can be estimated by carbon dating method. The age is determined by the radioactive isotope of carbon (C-14).
      • Ammonite, Trilobite and Dinosaur are the examples of fossils. The age of their fossils is determined by carbon dating method.
7. Define 'Speciation'. Explain how speciation occurs.
   ANS
   
   • The process by which new species develop from the existing species is known as speciation.
   • When the population of a species splits into two groups separated geographically, new species are formed.
   • The geographical isolation leads to reproductive isolation and there is no gene flow between two separated groups of the population.
   • Some variation arises in individuals due to natural selection.
   • Due to the process of random change in gene frequency (genetic drift), the individuals (of two groups) become so different that they cannot reproduce with each other. This may take thousands of years.
   • There can be more ways like DNA changes, the change in the number of chromosomes, the inability of the germ cells of two isolated groups to fuse with each other, etc. which can lead to speciation.
   • Thus, new species come into existence.
8. Write a note on 'Expression of inherited characters' OR Explain how genes control characteristics.
   ANS
   
   • Gene is a unit of heredity.
   • Genes are located on DNA in a linear order.
   • Each gene synthesizes a specific protein responsible for the expression of a characteristic.
   • Consider the height of a plant as a characteristic. The height of a plant depends upon the amount of a particular growth hormone.
   • The synthesis of hormones is catalyzed by specific enzymes and the synthesis of enzymes is regulated by genes located on DNA.
   • If the enzyme is synthesized in large amount, a lot of hormone will be produced and the plant will be tall.
   • If the gene has an alteration that makes the enzyme less efficient, less hormone will be produced and the plant will be dwarf.
   • Thus, characteristics (traits) are controlled by genes.
9. Write a brief note on Evolution.
   ANS
   
   • Evolution is gradual formation of new species from pre-existing primitive organisms by constant and relatively long time changes.
   • Evolution is the sequence of gradual changes in primitive organisms from which new species have been formed.
   • It is a constant process that started since the origin of life and took millions of years. It is continuous and will occur in future, too.
   • The great variety of living organisms on earth is the result of evolution.
10. Explain : Acquired characteristics (traits).
    ANS
    
    • Acquired trait means a trait developed by an organism in response to the environment.
    • It is not inherited from parents.
    • For example, if the weight of an organism is reduced due to starvation, it would not change the DNA of germ cells. Thus, reduced weight is not a trait inherited by the future generation of starving organism.
    • Other examples of acquired traits are: (i) Cut tail of a mouse (ii) a man knowing how to swim (iii) a man speaking German language (iv) a scar on the body of a man due to accident.
    • These all are traits which cannot be passed on to the next generation as these are changes in non-reproductive cells of the body.
11. Explain : Inherited Characteristics (Traits).
    ANS
    
    • A trait of an organism caused by some change in its DNA is known as inherited trait.
    • Inherited traits can be passed on to the progeny of the organism.
    • Example: Suppose there is a population of red beetles in green bushes. If a colour variation arises during reproduction and a beetle green in colour is produced. Since this change is the result of a change in the DNA of reproductive cells, it is an inherited trait which can be passed on to the next generations.
    • This is the essence of the concept of evolution.
12. Write a brief note on Evolution of eyes.
    ANS
    
    • The eye is an important organ for animals.
    • It is a so complicated organ that it cannot have been generated by a single DNA change.
    • The eyes of animals have been created in stages over many generations.
    • First of all, eyes were formed in planaria (flat worm). These eyes were very simple just like 'eye spots' which could detect light.
    • These eyes provide a survival advantage to planaria.
    • The development of eyes seems to be a very popular adaptation.
    • The eyes in animals like insects, octopus, invertebrates and vertebrates have quite different structures suggesting that the evolution of eyes has occurred in stages.
13. Write a brief note on Evolution of Feathers.
    ANS
    
    • An evolutionary change produced in an organism for one purpose may later on become more useful for an entirely different function. Evolution of feathers is an example of this fact.
    • In some dinosaurs feathers provided insulation in cold weather but they could not be used for flying. Later, they might have become useful for flying.
    • Birds, however, adapted to flight. Thus, presence of feathers in birds indicates that birds are closely related to reptiles (like dinosaurs).
14. Explain : Evolution by Artificial Selection.
    ANS
    
    • The evidences for evolution suggest that very dissimilar looking structures can evolve from a common ancestor.
    • Different looking organisms can in fact be created from the same ancestor by artificial selection in place of natural selection.
    • The farmers have produced entirely different looking vegetables like cabbage, broccoli, cauliflower, kohlrabi and kale from the plant of wild cabbage by artificial selection.
    • Some farmers, in an attempt to have very short distance between the leaves of wild cabbage, produced the commonly used 'cabbage'.
    • Some farmers opted for the arrested flower development of wild cabbage and produced 'broccoli'.
    • Some farmers opted for sterile flowers of wild cabbage and developed 'cauliflower'.
    • When farmers opted for swollen parts of wild cabbage, it resulted in the evolution of 'kohlrabi'.
    • Those farmers who wanted to grow large leaves of wild cabbage produced a leafy vegetable called/kale'.
    • Thus, wild cabbage is the common ancestor and cabbage, broccoli, cauliflower, kohlrabi and kale are the varieties of wild cabbage which have been obtained by artificial selection.
15. Write a note on Human Evolution.
    ANS
    
    • To study human evolution tools like digging of earth, time-dating, study of fossils and DNA sequencing have been used.
    • There is a great variety of human forms and features across the earth.
    • This diversity of features was the basis of different human races. The human races were identified commonly on the basis of the colour of their skin. They were named as white, black, yellow and brown.
    • It is now a well-known fact that the sop called human races have not evolved differently. All human beings are a single species called 'Homo sapiens'.
    • It has been established scientifically that we lived for past few thousand years and our genetic footprint can be traced back to our African roots,i.e. we all came from Africa.
    • A couple of thousand years ago, some of our ancestors left Africa while others stayed back. Those who left Africa spread over the planet from Africa to different parts of Asia, Australia and America.
    • They went in groups, sometimes separating from one another and mixing with one another. Some of them even moved in and out of Africa.
    • This led to evolutionary changes like in all other species.
16. Explain Mendel's contribution.
    ANS
    
    • Mendel performed experiments for explaining the transmission of characteristics (traits) from parents to their offspring.
    • He performed experiments on Pisum Sativum (Garden Pea) and worked out the main rules of inheritance.
    • He studied the inheritance of some of the pairs of contrasting characters like tall and short plants, white and violet flowers, round and wrinkled seeds, axial and terminal flower positions, etc.
    • When Mendel crossed a tall plant (with TT traits on DNA) with a dwarf plant (with tt trait on DNA), the F₁ generation was found to be tall. There were no dwarf or medium height plants in F₁ generation. Thus, only one of the two parental characters was seen in F₁ generation.
    • When the plants of F₁ generation were used to obtain F₂ generation, 25% of the plants were dwarf and 75% were tall.
    • Thus, in F₁ generation both the characters of tallness and dwarfness were inherited from the parents but only the character of tallness was expressed because tallness was dominant trait and dwarfness was recessive trait. [Dominant trait is denoted by capital letter (T) and recessive trait bt small letter (t)].
    • The plants having TT or Tt traits for height were tall and those with tt trait were dwarf. A single trait T is enough for making a plant tall but for dwarfness tt traits are required.
      
      
    • In other experiment, Mendel selected two different traits in a plant. He crossed a plant having yellow and round seed with a plant having green and wrinkled seeds.
    • In F₁ generation, all the plants were with yellow and round seeds indicating that yellow and round are dominating traits.
    • When the plants of F₁ generation are self-pollinated, the plants along with parental combinations (yellow and round seeds, green and wrinkled seeds) new combinations (yellow and wrinkled seeds, grren and round seeds) are also formed in F₂ generation.
    • This indicates that yellow seeds/green seeds traits and round seeds/wrinkled seeds traits are independently inherited.
      
      
      
      
      
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