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BIOLOGY Though she herself broods her eggs how different her offsprings are! Grandma! Whom do I look like? Didn't you notice the conversation between the grandmother and the child and the thoughts of the zebra and the hen? Are the lines on all zebras alike? Aren't there resemblances between the hen and the chickens? How is it that the grandchild has features of the grandmother besides those of her father and mother? Don't you find such examples in your surroundings? The similarities between parents and children are due to the transmission of characters from parents to offsprings. But are the characters of parents alone transmitted to offsprings? Note down 94 Who drew the lines on their bodies so exactly! Father’s colour-Mother’s hair --Then --------------eyes, your Grandma's your opinion. ................................................................... Why is it that only some of the characters of parents are expressed in offsprings? How is it that differences also occur along with similarities? Such questions and the assumptions related to them have arisen since very ancient times. But an effort to find a scientific explanation to these was initiated by Gregor Johann Mendel. Hence he is known as the Father of Genetics. Collect more information on Mendel's contributions based on the description given below. BIOLOGY Gregor Mendel - The Father of Genetics Gregor Johann Mendel was born in a middle class peasant family at Brunn in Austria (now Czech republic) in 1822. After school education, he studied theology and became a priest. He also worked as a teacher after studying Physical Science and Natural Science at the Vienna University. The experiments conducted in pea plants while being a priest and the inferences he arrived at, elevated him to the status of the Father of Genetics. Mendel analysed the inheritance of 7 pairs of contrasting characters in pea plants in detail. The colour of the flower (red, white), seed colour (yellow, green), shape of the seed (round, wrinkled), height of Gregor Johann the stem (tall, short) etc. were some of the contrasting characters Mendel subjected to hybridisation experiments. From his experiments he arrived at certain inferences. — A pair of factors responsible for each character is present in all organisms. He indicated these factors by using symbols. E.g. Tallness Dwarfness - TT - tt Red flower White flower - RR - rr — One factor in each pair is received from the mother and the other from the father. — There will not be any change in the result of the experiment whether contrasting characters are from the female parent or the male parent. — Factors of both the contrasting characters (qualities) will be present in plants of F 1 generation, but only one character will be expressed. Even though Mendel's findings were published in 1866, it didn't get enough recognition. Mendel died in 1884. Sixteen years after his death, scientists like Hugo de Vries, Carl Correns and Erich Tschermak independently discovered Mendel's laws of inheritance and reformulated them. With that Gregor Johann Mendel was accepted as the Founder of Heriditary Science. Genetics, the branch of science which deals with heredity and variations was formed only after that. From his experiments in pea plants, Mendel formulated certain laws. These are known as Mendel's laws of inheritance. Examine Illustrations 7.1, 7.2, 7.3 and 7.4. Fill them up suitably and analyse on the basis of indicators. Prepare notes on the basis of your understanding of the relationship between the results of the experiments and Mendel's laws. 95 BIOLOGY Experiment - 1 Cross Pollination between Tall and Dwarf Pea Plants Female parent (Tall) male parent (dwarf) TT tt Gametes T t (.........................................) Tt First filial generation (F1 generation) Illustration 7.1 Indicators — Which is the character expressed in F1 generation? — Which is the character not expressed? — What is the difference between the factors contained in the Tall female parent and the offsprings of F 1 generation? 96 Law of Dominance When a pair of contrasting characters combines, only one is expressed (dominant character) and the other remains hidden (recessive character). BIOLOGY Experiment - 2 Self Pollination of F 1 generation F1 generation (.........................................) Tt Female gametes Male gametes t T .......... ......................................... ......................................... TT .......... ......................................... ......................................... .................. .................. Tt Second filial generation (F2 generation) Illustration 7.2 Indicators — What are the differences observed in the progenies of the second generation from the first generation? — What is the ratio in which tall and dwarf plants are formed in the second generation? — Is the genetic constitution of all tall plants of the second generation the same? — How do the factors seen as pairs in the female parent (Tt) appear in the gametes? Law of Segregation During gametogenesis, the pair of hereditary factors that determine the characters segregate from each other, and only one factor enters each gamete. 97 BIOLOGY Experiment - 3 If more than one character combines? male parent (dwarf with white flowers) Female parent (tall with red flowers) TTRR ttrr TR tr Gametes F1 generation (........................................................) TtRr Illustration 7.3 Indicators — Which are the dominant characters here? — Which are the recessive characters? What is the difference between the factors in the offsprings of F1 generation and factors in the parental plants? — 98 Illustration 7.4. shows the experiment conducted by Mendel subjecting the F1 generation of Experiment 3 to selfpollination. Observe the Illustration and fill up Table 7.1. Prepare notes based on the indicators. ................................................................... BIOLOGY Experiment - 4 Self Pollination of F 1 Generation F1 generation — Which are the new types among the progeny? — Are the characters which remained together in the parental plants of the F1 generation transmitted together to all offsprings? If not how? Note down your inference. ................................................................... Compare your inference with the Law of Independent Assortment. (......................................) Tt Rr Law of Independent Assortment Female gametes Male gametes TR ........ ........ ........ ........ ........ ........ ........ Illustration 7.4 Let us see what happens when these gametes fuse? Male Gametes Female gametes TR TTRR TR Tr tR TTRr TtRR tr TtRr Tr ............... ............... ............... ............... tR ............... ............... ............... ............... tr ............... ............... ............... ttrr Tall with red flowers : ……………. Dwarf with red flowers : ……………. Tall with white flowers : ……………. Dwarf with white flowers : ……………. Ratio of F2 progeny : ………........................ Table 7.1 Indicators — How many types of gametes are formed? Which are they? — How many types of progenies are formed? Which are they? When two or more pairs of characters combine, factors for each character pair segregate and assort independently to pass on to the next generation. Though characters in parents are not expressed in children, sometimes they get expressed in their grand children. What is the reason for this? Discuss and prepare notes based on Mendel's Law of Inheritance. ................................................................... Mendel tried to prove that characters are inherited from parents to offsprings through certain factors. What are these factors? Where are they situated? A lot of research has been done in these directions after Mendel. Many new findings resulted from this. These findings later grew into the extensive branch of science called Genetics. Some important milestones in the history of Genetics are given below. Analyzing them and collecting more information, prepare notes on the development of Genetics. ................................................................... 99 BIOLOGY Milestones in the History of Genetics Johannsen : Coined Gregor Johann Mendel 1866 Formulated the Laws 1909 the term gene to the heriditary factors explained by Mendel. of Inheritance. Friedrich Meischer 1869 Discovered Nucleic acid Johannsen Friedrich Meischer Hugo de Vries, Erich Tschermak, O. Avery : Research under his leadership identified DNA as the genetic material 1900 Carl Correns: Reformulated Mendel's 1943 team Laws. Avery 1953 Hugo de Vries Erich Tschermak Carl Correns James Watson, Francis Crick: Presented the spiral helix model of DNA Walter S.Sutton, Theodor Boveri 1902 Discovered that hereditary factors are located in chromosomes. James Watson & Francis Crick 1970 Walter S.Sutton William Marshall Nirenberg, Har Gobind Khorana Discovered the Genetic Code Theodor Boveri Bateson 1905 Named the new branch of science which deals with the study of heredity and variation as Genetics. William Bateson Marshall Nirenberg Har Gobind Khorana Mendel's hypothesis that inheritance of characters is through certain factors was later proved correct. It was made clear that every character in an organism is controlled by genes which are parts of DNA (deoxyribonucleic acid) molecules. The DNA molecule is formed by two strands of nucleotides. The DNA molecule has the shape of a spiral double helix with two longitudinal strands made up of sugar and phosphate and steps made up of nitrogen bases. This structure was presented by Watson and Crick. Analyse Illustration 7.5. based on the indicators and prepare a note on the DNA molecule. .............................................................................................................................. 100 BIOLOGY Indicators Cell Chromosome — Where is the DNA chiefly found? — What are nucleotides? What are its components? — Which are the nitrogen bases that pair with each other? Nucleus DNA ultra structure DNA Specific parts of the DNA are responsible for regulating metabolism and effecting specific characters. These parts are known as genes. How do genes perform their functions? Examine the given description and Illustration 7.6. and prepare a note. ................................................................... Gene Action Nucleotide Phosphate Nitrogen base Deoxyribose Sugar Nitrogen bases - 4 types Adenine-A Thymine - T Cytosine-C Guanine-G Nucleotides in DNA Formation of specific characters and regulation of metabolism are done by proteins. Genes in the DNA determine which types of proteins are to be made. DNA does not manufacture protein directly. DNA makes RNA (Ribonucleic acid) from its own strands. RNA reaches ribosomes and synthesizes proteins by linking together of amino acids. Information for the synthesis of specific proteins is encoded in each gene. Nuclear membrane DNA Different amino acids Protein Molecule RNA Adenine nucleotide Cytosine nucleotide DNA unwinds and RNA is synthesized Ribosome Thymine nucleotide Guanine nucleotide Deoxyribose sugar Illustration - 7.5 Illustration - 7.6 Protein synthesis 101 BIOLOGY Indicators — Where is RNA synthesized? — Where does this RNA reach? — What is the role of RNA in protein synthesis? Haven't you understood that DNA performs its function by synthesizing another nucleic acid called RNA? Observing the differences between RNA and DNA given in Illustration 7.7 fill up the Table 7.2 given below. Strands made up of sugar and phosphate molecule of DNA. Proteins responsible for specific characters are synthesized according to the directions of hereditary factors called genes. Offsprings get these genes from parents. If it is so, should'nt the offspring get characters only from parents? Some times characters different from parents are expressed in offsprings. These are called variations. How do variations occur? Haven't you studied that gametes are formed by meiosis? Homologous chromosomes from the mother and father, pair and exchange chromosomal material during meiosis. Analyse Figure 7.1. and form inferences. Crossing over in Meiosis Different stages Nitrogen bases DNA Chromosome pair RNA Nucleotides in RNA Chromosome from mother Chromosome from father Adenine Cytosine Beginning of crossing over Uracil Guanine Ribose sugar Phosphate The chromosome pairs exchanging genes Illustration 7.7 Characteristics DNA RNA Number of strands Type of sugar Daughter chromosomes after the exchange of genes Nitrogen base Table 7.2 You have understood that specific proteins are responsible for all the processes taking place in the cells and proteins are synthesized according to the instructions 102 Fig.7.1. Gene exchange (different stages of exchange of genetic material by chromosome pairs during meiosis). BIOLOGY Haven't you understood that exchange of genes causes variations in offsprings? Variations are seen in offsprings because characters that are hidden in parents also get transmitted to offsprings. Now we have understood the role of chromosomes and the genes in the development of characteristics of an organism. But due to several reasons there may be changes in chromosome number and structure. Such spontaneous changes are called mutations. These are inherited. Most of the mutations are harmful to organisms. Pictures and the press report given below include some such examples. Find more examples. Sickle cell anaemia widespread among tribals Thiruvananthapuram: Study reports reveal that the genetic disease called sickle cell anaemia is widespread among tribals. Decrease in the oxygen carrying capacity of the red blood cells as it bends like a sickle is the notable feature of this disease. In the gene structure.. Child - male or female? What is the reason behind the formation of a male or female child? Analyse Illustration 7.8. based on indicators and form inferences. ................................................................... Mother Turner syndrome Loss of one sex chromosome (only one X chromosome, no Y chromosome). Deficient females Turner syndrome Chromosomes = 44 + X with sterility and dwarfness. Autosomes = 44 Sex chromosomes = xx Total = 46 XX Female gametes Father XY Autosomes = 44 Sex chromosomes = xy Total = 46 Male gametes Down syndrome One extra autosomal chromosome (three chromosomes in 21st pair, total 47 Chromosomes). Mental retardation, sterility, low immunity. Down syndrome Chromosome = 45 + XX or 45 + XY Female Child XX ................ ................ ................ Illustration - 7.8 103 BIOLOGY Indicators — How many types of sex chromosomes are there? — What are the similarities and dissimilarities between the chromosomes of man and woman? — What is the probability of formation of male and female children? Haven't you understood that sex chromosomes in males play a major role in sex determination? This is the reason for the more or less equal proportion of female- male ratio. How difficult it is to assess the contributions of genetics to human progress! Like any other branch of science, is there a possibility to make genetics also a means of exploitation? Read the article given below, collect additional information and conduct a debate on the topic. The Branch of Science that Transforms the Living World The genetic revolution that was started by Gregor Mendel in pea plants has developed to such an extent that today it influences all sections of society. The knowledge we have gained about genes and the technology that alters genetic structure are progressing every moment. Technology that is used to make desired changes in genetic structure is called genetic engineering or recombinant DNA technology. Right at the outset of the 1970s, the scientific world gained the ability to cut the DNA at specific sites. Enzymes which are used to cut DNA at specific sites are called molecular scissors. Restriction endonuclease enzymes belong to this category. Similarly, enzymes like ligase bind genes. Such enzymes are called molecular glues. Genetic engineering has caused tremendous changes in the field of agriculture and medicine. Insulin, for the treatment of diabetes was extracted from animals. This caused allergy - like problems. But today insulin without any side effects is being manufactured through genetic engineering. The insulin gene separated from human beings is combined with bacterial DNA and using this bacteria, human insulin is manufactured. Bt cotton and Bt Brinjal which triggered so many hot discussions in the field of agriculture are the products of genetic engineering (Bt-Bacillus thuringiensis). The gene when introduced in these plants cause the production of a protein. Since this protein can destroy pests, pest control is possible without pesticides. However studies are yet to be completed to ensure whether it has any side effects in human beings. Bt Cotton Bt Brinjal 104 Another product of genetic engineering is super bugs. These are new types of bacteria produced by combining different types of bacterial genes. Some super bugs eat and destroy petroleum products. This bacterium can be used as a remedy for oil spills BIOLOGY in oceans and for the clearing of obstructions in oil pipes. This super bug was first produced by Anand Mohan Chakrabarty of Indian origin. Many improved agricultural crop varieties could be produced through artificially created mutations. The DNA finger printing which is used to prove disputed parentage and criminal offenses, is a contribution of genetics. By 2003 we attained complete knowledge about the nucleic acid sequence in human chromosomes through the Human Genome Project. This is considered as a major achievement which could result in a major leap in therapeutics. Geneticists all over the world are still at work in their laboratories for many more achievements. But one cannot ignore situations where such achievements may be used for exploitation and selfish motives. What will be the future of the human race, if pathogens, which will not yield to any medicine, are used as biological weapons? What will happen if Dr. Anand Mohan super bugs, that can be used to prevent oil pollution, are introduced Chakrabarty in oil fields? Science always has the potential to be the protector of man. But some times, man misuses science for destructive purposes. While being proud of the boons of science we should be able to use it for the benefit of the human race too. Science is a product of collective human thought. Only when it is used for the benefit of mankind, we can do justice to our ability to think. The individual and the society should be able to approach science with such an attitude. 105 BIOLOGY Follow up Activities 1. The offsprings of two types of hybridization of homozygous black and white rabbits are illustrated. White female rabbit Black male rabbit C Black female rabbit White male rabbit D a. Is the coat colour sex-linked? b. When C and D are crossed, what will be the colour of the coat in the offsprings? 2. A husband, along with other family members, constantly accused his wife of delivering only female children. As a science student what is your response to this attitude. State with definite reasons. 106