Std. X Biology Heredity and Evolution PDF

STD. X (BIOLOGY) 4 GENETICS Syllabus : Some other important terms used in genetics : Heredity : The phenomenon by which living 4.1 Heredity genetics and varia...

STD. X (BIOLOGY) 4 GENETICS Syllabus : Some other important terms used in genetics : Heredity : The phenomenon by which living 4.1 Heredity genetics and variation organisms transmit parental characteristics 4.2 Inheritance of traits - Mendel's contribution or traits to the successive generation is 4.3 Monohybrid cross, dihybrid cross called heredity or inheritance. 4.4 Mendel's law's of inheritance Inherited traits: The characteristics or traits which are transmitted form parents 4.5 Sex determination in human beings to their offsprings are known as inherited INTRODUCTION : traits. For example, eye colour tongue rolling In the previous chapter you have already and right or left handedness. studied that all living organisms produce offspring similar to th em. I t means th at al l living organisms produce young ones that resembles themselves to a large extent. For example human child bears all the basic features of a human being. Similarly, a wheat plant produces wheat grains, which in turn grow into wheat plants. Free and attached earlobes are two variants However, though offsprings resemble their found in human beings parents, they are not identical. They usually Chromosomes : Filamentaous thread-like differ among themselves and also from their bodies parent in the nucleus of a cell, composed of chromation material (DNA- parents. There may be differences in the facial protein complex). features, height, weight, eyes, hair and so on. Variation : Different characteristics or traits All these are individual variations in physical among individuals of a species. appearance. However, despite having individual Gene : A unit of inheritance forming part of variations among the different members of a chromosome. It is a section of DNA found offsprings, minimum essential features are on chromosomes. Genes are passed from same in all the individuals of a species. parents to the offsprings via chromosomes 4.1 Heredity, Genetics and variations : parent in the nuclei of the parents’ gametes. Like begets like is a well-known dogma Gametes : Male or female sex cells. associated with heredity. It means that all living Alleles : Alleles are alternate forms of a gene or a pair of matching genes. For organisms tend to produce young ones that example, free and attached earlobes are the resemble themselves. Heredity is the cause of two different forms of a gene found in human similarities between individuals of a species. beings. Out of these two genes, one is a Continuity or transmission of characters or traits dominant gene and other is a recessive gene. from one generation to another is termed as Dominant gene : The gene which decides heredity. In other words, the phenomenon by the appearance of an organism even in the which living organisms transmit parental presence of an alternative gene is called a characteristics to the successive generation is dominant gene. The free earlobe in human called heredity or inheritance. beings is a dominant character. It is The branch of science which deals with the represented by a capital letter ‘F’. mechanisms responsible for similarities and Recessive gene : The gene which cannot dissimilarities among closely-related species is express it self in the presence of the dominant gene is called a recessive gene. It called genetics. Genetics not only deals with the can decide the appearance organism only inheritance of features from parents to offsprings in the presence of another identical gene. but also explains species. Thus, we can say that The attached earlobe in human beings is a genetics is the science of heredity and variation. recessive character. It is represented by W. Bateson first coined the term genetics corresponding small letter ‘f’. in 1905. It is derived from the Greek word Genotype : The genetic constitution of an genesis meaning to grow into or to become. organism. For example, FF, Ff or ff.  Phenotype : Outward visible expression of offsprings besides a number of similarities in the genes, which is an inherited feature in an offsprings. individual’s appearance. For example, free Sometimes, it so happens that the offsprings or attached earlobes. are not alike, they show some differences Homozygous: Both alleles of a gene are amongst them. If you observe, one or the other identical. For example, FF. kind of variation is present among the siblings. Heterozygous: The two alleles of a gene are Even twins are not exactly same. However, such variation among offsprings is not observed in dissimilar, i.e. one dominant and one those organisms that reproduce asexually. This recessi ve allel e fo r a particul ar is because only one parent is contributing the characteristic. For example, Ff. germ cells for reproduction. However, in case of In this chapter we shall study about the organisms reproducing sexually, greater diversity mechanism by which variations are created and in variations is produced. how they are carried forward from one generation Any variations that have been acquired by to another. We shall also study that how individuals through reproduction and passed on persistent variations across several generations to next succeeding generations are called lead to evolution. heritable variations or germinal variations (since Accumulation of variation during reproduction germ cells contribute in such variations). Re producti on i s the fundamental However, any variation that remains limited to characteristic of all living organisms which only one generation and does not pass onto next involves the transmission of genetic material generation offsprings is called somatic or non- fro m on e ge neration to another. Thu s, heritable variation. reproduction ensures the continuity of race (or Another question which has always arisen species) after the death of the particular in the mind of scientific community is about the organism. Had there been no reproduction, the diversity of living organisms in different periods concepts of heredity, variation and evolution of geological time scale. Has the diversity of living would not have existed. organisms undergone changes over the period of At this stage, there may be many questions time? arising in your mind, such as: The available information and the gathered How do features of a particular organism get evidences from authentic sources suggest that transmitted from parents to offsprings? the present diversity of living organisms has evolved over a period of time. The branch of Is there any principle or rule which directs biology that deals with the changes and its causes the transmission of features of an organism in the diversity of living organisms over the from one generation to the other? period of time is called evolution. Variation is a Do these features remain same throughout necessity for organic evolutions. the lifespan of the individuals? Why is there variation among offsprings of 4.2 Inh eritan ce of traits - men del's same parents? Why is that even twins are contributions not alike? We have been using the principles of It was Gregor Johannes Mendel (1822-1884) inheritance for thousands of years. But as a field whose pioneering work helped us get answers to of science, genetics did not begin until 1866, the year Mendel published the conclusions of his some of these questions in the year 1900. This famous experiments on the common garden pea discovery gave birth to one of the most important plant. branches of biology called genetics. The information to be inherited is present in the sex cells (or gametes) of the parents. These gametes are the link for passage of characters or traits from parents to offsprings. Inheritance from the previous generation provides the basic design of body as well as any subtle changes in it for the next generation. During the process of sexual reproduction, the hereditary information is transmitted from parents to the offsprings through their gametes. The contribution made by two parents also brings about variation among Gergor Johannes Mendel  Gregor Johannes Mendel was a monk in an In pea plants, due to self-fertilization, it is Augustinian monastery at Brunn, Austria. He easy to get pure li nes for seve ral had great interest in plant breeding and generations. hybridization experiments of different plant As it is an annual plant, it is possible to study varieties. He had very keenly observed that there several generations within a short span of are many distinct varieties of features or time. characters in a number of plants. In order to The flowers of pea plants are adequate in understand the principle of inheritance of size and easy to handle. features from one generation to the another, Each plant can produce large number of Mendel conducted a series of hybridization seeds in a single generation. experiments. These were conducted over a period Mendel selected seven pairs of contrasting of eight years on the common garden pea plant, characters which are listed in Table. Pisum sativum. He was the first to systematically study and explain the mechanism of transmission of characteristics from parents to the offsprings, generation after generation. Mendel published his work in the annual proceedings of the Natural History Society of Brunn. Mendel is considered as the Father of Genetics or Modern Genetics. He was the first to introduce the concept of genes as the basic unit of heredity’. Mendel called genes as ‘factors’. Selection of garden pea as the experimental plant was based on Mendel’s meticulous and careful observations about its features. Can you think what essential features should be there in such an organism ? Such an organism should essentially have these features given as under: shorter lifespan so that a large number of generations can be studied and examined; presence of contrasting variants of features; and ease of rearing or cultivation. Since almost all these features were present in the garden pea plant, Mendel selected it for his experiment. Experimental plant : Mendel conducted his experiments on Pisum sativum, the garden pea plant, for the following reasons: Pea plants have several distinct types or varieties. All the varieties have sharp contrasting characteristics, such as colour and shape of seeds. Pea plant bears bisexual flowers with each flower having both the male and the female parts. The structure of the flower is such that it completely encloses the reproductive organs until fertilization, which ensures self- pollination. Mendel's experiments : 8. He used the seeds of F2 generation to raise 1. Men del desi gned his cro ss-breeding the F3 generation (by self-pollination of F2 experiments taking various characteristics plants). The analysis of traits or features in meticulously. a particular generation was very carefully 2. In some of the crosses, only one trait like done by observing them and keeping their size of the plant (tall or dwarf), shape of seeds records. Some features such as seed shape (round or wrinkled) or colour of flowers or see d colour cou ld be immediately [purple (violet) or white] was considered. The recorded, whereas other features like plant crosses where only one trait was focused size or flower colour were analyzed after were termed as monohybrid crosses and the planting the seeds and observing the offspring was called a hybrid. features in the next season. 3. In other crosses, he took two or more traits into consideration for his experiments. Thus, the crosses in which two traits were taken into consideration were designated as dihybrid crosses. The crosses in which three traits or four trai ts w ere take n in to consideration were termed as tri-hybrid and tetra-hybrid crosses, respectively. 4. The most important precaution that is to be taken, in a cross breeding experiment between two varieties or traits is to avoid self-fertilization. For this purpose, Mendel removed the anthers of the flowers, selected to be the seed parents, well before the female part of the flower, gynoecium became mature. Thi s process is cal led as emasculation. Further, such flowers were also covered to avoid entry of any foreign pollen grain from outside. 5. Men del crossed two plan ts showi ng contrasting expressions of a single trait and named the cross as reciprocal cross. For example, he crossed true breeding tall stem variety plant (which he called tall plant) with a true-breeding short stem variety plant Steps involved in cross-pollination of plants (dwarf plant). The offsprings were termed as 4.3 Monohybrid cross : hybrid. He performed the experiment by In earlier section, you have already studied transferring pollen grains from the anther about monohybrid cross. Mendel cross-pollinated of the dwarf plant to the stigma of the tall two plants showing contrasting or alternating plant. Self-pollination was prevented by expression of a single trait or feature. A cross removing all the stamens from the tall plant. between two parents representing contrasting The plants of parental generation were forms of a single trait or feature is called designated as P. The seeds from the tall plant monohybrid cross. were then collected and sown. Mendel performed cross-pollination between 6. He found that all plants which grew from the flowers of plants raised on a particular these seeds were tall plants. The plants in characteristic. In a cross between round and this generation were called F1 generation wrinkled seed, Mendel cross-pollinated flowers of (F1 progeny) or first filial generation plants. plants raised on the respective type of seeds 7. In order to completely analyze his results, (round and wrinkled). Before making such crosses Mendel allowed F: plants to self-fertilize and Mendel ensured that all the plants involved in used the seeds of F: generation to raise the the crosses are of pure line. For this purpose, F2 generation. These plants were called F2 seeds of only those plants were used in the generation (F 2 progeny) or second filial parental generation which produced the desired generation plants. trait for at least six generations.  Monohybrid cross step 1 : Cross-pollination between tall and short plants Monohybrid cross between tail and short plant : depending on their percentage. A single copy 1. For a monohybrid cross between tall and of the trait (in this case T) is enough to short (dwarf) plant, Mendel performed the make tall plant while both copies of the trait experiment by transferring pollen grains have to be ‘t’ for the plant to be short. Thus, from the flower (anther) of a dwarf plant to the traits like ‘T’ (expressed trait) are the stigma of the previously emasculated dominant traits while traits like ‘t’ (repressed flower of a tall plant. It was ensured that the trait) are rece ssive traits. In the F 2 seed bearing plant does not self-pollinate. generation, it was found that there is one After the pollen grains were transferred, the plant with genotype ‘TT’, two plants with pollinated flower (seed parents) was properly genotype ‘Tt’ and one plant with genotype ‘tt’. covered and seeds were allowed to mature. So, 1 : 2 : 1 is the genotypic ratio of the The seeds obtained were of F1 generation. monohybrid cross. The traits of tallness or All the seeds of F1 generation were carefully dwarfness are inherited separately and are observe d an d th e observatio ns w ere not mixed together. recorded. In this experiment, Mendel Dihybrid cross : observed that all the plants of F1 generation A dihybrid cross is one in which two varieties were tall. of pea plants having two contrasting characters 2. Mendel further planted the F1 seeds and are crossed to study inheritance of two pairs of flowers were again allowed to self-pollinate traits simultaneously. Contrasting or alternating to produce the F2 generation plants. In the forms of two traits or features (i.e. heterozygous F2 generation, it was found that tall plants for two pairs of alleles) are simultaneously and short plants were obtained in the ratio considered in the hybridization experiment in of 3:1 (3 tall : 1 short). So, 3:1 is the dihybrid cross. For example, two plants differing phenotypic ratio of the monohybrid cross. In in two characters like seed shape (round/ other words, three fourth plants were tall wrinkled) and cotyledon colour (yellow/ green) and one-fourth were short in F2 generation. were crossed together. Thus, the short plant trait that disappeared 1. Mendel selected a pure line variety of peas in first generation reappeared in second for yellow round seeds and another for green generation. Mendel called the expressed trait wrinkled seeds. In the parental generation, of ‘tallness’ as the dominant trait and cross-pollination was made between flowers repressed trait of ‘shortness’ (or dwarfness) of plants raised using pure line seeds of as recessive trait. This indicates that both round shape and yellow colour with wrinkled tal lness an d sh ortn ess (dwarfne ss) shape and green colour. characters were inherited in F1 generation He crossed these plants and observed plants, but only the tallness trait was that all seeds had the features of only one expressed. The offsprings inherit two copies parental type in the F1 generation which had of traits which maybe identical or different  Monohybrid cross step 2 : self-pllination of F1 generation plants yellow coloured and round shape seeds. This In a dihybrid cross, when combination of showed that: contrasting pairs of two traits were taken Yellow colour was dominant over green together, only one variety of each trait which is a recessive characteristic. appears in the F1 generation. Round shape was dominant over wrinkled The other variety of each trait reappears in shape. So, the cross would be represented the F2 generation on the same lines of the as in Figure. dihybrid cross. 2. In the next step, Mendel self-pollinated Ho weve r, the prese nce of two n ew these hybrids obtained in the F1 generation. combinations of the two contrasting pairs of When these F1 generation seeds were cross- traits in the F2 generation also occurs. bred to raise the F 2 generation, the F 2 4.4 Mendel's laws of inheritance : progeny showed four different kinds of Mendel postulated three laws of inheritance phenotypes of seeds. It was observed that not on the basis of his monohybrid and dihybrid only both the parental types (round seeds of experiments. yellow colour and wrinkled seeds of green col our) were presen t, but two n ew Law I: Law of dominance : combination of traits (round seeds of green When two alleles of contrasting expression colour and wrinkled seeds of yellow colour) of a hereditary trait are brought together by also appeared. fertilization, only one is expressed, while the Thus, there were yellow round, yellow other is suppressed. The characteristic which is wrinkled, green round and green wrinkled seeds expressed, is called dominant or expressive in the ratio of 9 : 3 : 3 : 1, respectively. Of these, characteristic, and the characteristic which is two are of the parental P1 types and two are new repressed (not expressed), is called recessive or combinations or recombinants. The dihybrid ratio suppressive characteristic. This is Mendels first is, therefore, 9 : 3 : 3 : 1. law of heredity - law of dominance. It states that Interpretation of mendel's observations : when two homozygous individuals with one or On the basis of the analysis of the results of more sets of contrasting characteristics are the monohybrid and dihybrid crosses, following crossed, the characteristics which appear in the conclusions can be drawn: F1 hybrids are dominant and those which do not In a monohybrid cross, when a cross is made appear in F1 generation are recessive. between the contrasting pair of a trait, only Law II: Law of segregation : on e of the traits appe ars, in the F 1 According to Mendel, each organism that generation. reproduces sexually by producing gametes has The trait, which was not present in the two factors (genes) for a characteristic. Of these, offspri ng o f a particul ar cross in F 1 one is inherited from the male parent and other generation, again reappears in the F 2 from the female parent. These are alleles for a generation.  characteristic. If the alleles are similar, they are in homozygous condition. And, if the alleles are different, they are in heterozygous condition. All gametes produced by a homozygous individual will have similar alleles. Gametes of a heterozygous individual will be dissimilar (different/not alike). On the basis of results obtained from monohybrid cross, Mendel formulated the second law of inheritance - law of segregation. It states that when a pair of allele is brought together in a hybrid, the members of the allelic pair remain together without mix ing and separate or Differences between dominant and recessive gene Chacteristic Dominanat Recessive gene gene expression the gene which the gene which decides the can ex pre ss appearance of itself only in an organism the absence of a even in the dominant gene. presence of an alternative gene. homozygous it produces its or phenotype in it produces its heterozygous both hetero- phenotype condition zygous as well on ly in as in homozy- homozygous gous condition. condition. representation it is it is represented represented by by a lower case a upper case le tter. For (capital) letter. example, ‘t’ for For example, dwarf plant. T for tall plant. segregate from each other when the hybrid forms gametes. Since each gamete is pure for a characteristic, the law is also known as law of purity of gametes. Law III: Law of independent assortment : It states that, when a dihybrid organism forms gametes, Dihybrid cross between parents with yellow each gamete receives one allele from each round and green wrinkled seeds allelic pair (or each characteristic), and the assortment of alleles of different Do You Know ? characteristics during gamete formation is independent of their parental combinations. Transmission of traits : All living organisms produce their own kind. So, there must be some common thing that makes an offspring similar to its parents. In scientific terms, we call it trait or characteristic during sexual reproduction. That means, both that are passed from parents to the offspring parents contribute a copy of the same gene. during sexual reproduction. These traits are During meiosis, a gamete receives only one transferred through genes located on their chromosome of a pair. Hence, each gamete (germ chromosomes. cell) has only one allele of the pair of alleles for a characteristic. When two germ cells combine, the y wi ll resto re the n ormal nu mber of chromosome in the progeny and ensure the stabil ity of the D NA o f th e specie s. The characteristics thus travel from parents to the offsprings in the form of genes on chromosomes. They are being transmitted physically. Thus, genes are the physical basis of heredity. Sex determination in lower animals : In most of the sex ual ly repro duci ng organisms male and female reproductive organs are present separately. Different animals have different methods of sex determination. In some animals like turtles, Chrysemys picta, the temperature during the time of fertilization of eggs determines the sex. If eggs are incubated at lower than 28 °C, males are produced and at over 33 °C, females are developed. In honeybees, males are haploid while females are diploid. In Bonellia, if larvae grows alone, it develops into a female, while if it enters the body of female it develops into a male. 4.5 Sex determination in human beings -son or daughter? Determining the sex of an individual at prenatal (before birth or during pregnancy) stage is called sex determination. In a number of organisms, one specific pair of chromosome plays What are genes ? a significant role in the determination of sex of Cellular DNA is the information source for the organisms. These chromosomes are named making protein in the cell. A section of DNA on a as sex chromosome. In human beings, there are chromosome which provides information for one 23 pairs of chromosomes, out of which one pair protein is called gene for that protein. Genes are is sex chromosome. There are two types of sex the units of heredity. They are located in a linear chromosomes - X and Y. A female contains two fashion on chromosomes. Thus, chromosomes X-chromosomes (i.e. homomorphic), while a male are the carriers of genes. These are located contains one each, i.e. X and Y-chromosome (i.e. within the cell nucleus. he tero morphic). Re st of the 22 pairs of In normal condition, each gene has two chromosomes are exactly similar and are called alternative forms of a character producing the autosomes. different effects. These alternative forms are called alleles. In a diploid organism like humans, all chromosomes are present in pairs within the cell nucleus. Therefore, all genes are also in pairs, i.e. each characteristic has two alleles, each present on one chromosome of the pair. Two alleles of a pair are always located at the same position on the chromosome pair. Both the parents contribute equally to the DNA of progeny Sex chromosomes in humans  At the time of reproduction, during gamete gradual change or variation in the organism formation these paired chromosomes separate. generation after generation. It means that the Thus, two types of gametes are formed in males organisms of present day have arisen from (one containing X-chromosome while the other ancestors that were more simple in organisation. containing Y-chromosome). Half of the male The following are responsible for evolution: gametes or sperms carry the X-chromosomes and 1. Variations in the gene pool of members of a rest half have Y-chromosomes. All the female population gametes or ovum carry one X-chromosome. 22 2. Natural selection favouring accumulation of pairs of autosomes are equally distributed in both advantageous variations sperm and ovum, and hence they are similar in 3. Genetic drift or chance selection terms of autosomes. Thus, the female has 44 + The word evolution has been derived from a XX chromosomes and male h as 44 + XY Latin word evolvere (Latin: ‘e means out and chromosomes. ‘volvo’ means to roll) means to unroll or unfold to How is it determined if the child would be reveal modifications or gradual changes. male or female? Therefore, evolution is defined as a naturally The sex of the offspring will be determined occurring slow, continuous and irreversible by the type of chromosome (X or Y) inherited process of change. from father. At the time of fertilization, when the sperm An illustration to understand evolution : and the egg unite to form a zygote, each In the previous sections you have studied individual inherits one of the two possible that variation during reproduction occurs due to combinations of sex chromosomes. two reasons A zygote (XX) with two X-chromosomes (one because of errors in DNA copying, and from father and one from mother) develops as a result of sexual reproduction. into a girl while a zygote, (XY) with one X- Let us take an example to understand some chromosome (from mother) and one Y- consequences of variation in evolution. chromosome (from father) develops into a boy. 1. Let us consider a group of nine red-coloured beetles living in some bushes of green leaves. They repro duce sexu ally and therefore generate variations. 2. Imagine that these beetles can be eaten by crows. 3. As a result, the population of these beetles becomes less and fewer beetles are available to reproduce. 4. Now , wh at w ill happen i f di fferent Sex determination in human beings evolutionary phenomenon takes place in these beetle populations. Evolution : Scientists have long tried to discover the Situation 1 - Heritable advantageous variation origin of living organisms. Various observations is favoured by natural selection about the varieties of plants and animals, the A colour variation occurs during reproduction di versitie s in the ir stru ctures and the in the population of red beetles. As a result, reproductive patterns, lead to the concept of one beetle becomes green-coloured instead evolution. of red. In the beginning of this chapter, you have Thi s ne wly deve lope d green beetle studied that during sexual reproduction there is reproduces and passes its green colour on a tendency of variation in the living organisms to its progeny. of a species. The process of evolution involves a  This green body colour is heritable and has Situation 3 - Non-heritable acquired variation survival advantage. These green-coloured disappears under favourable conditions beetles can easily mask themselves in the In this case, no colour variation occurs in green-coloured bushes. Thus, the crows the body of red beetles. However, the bushes cannot see green-coloured beetles on green suffer from a plant disease and the food leaves of the bushes and cannot eat them. material for beetles is reduced. There is scarcity of food and as a result, the On the other hand, progeny of red-coloured ave rage size an d we ight of beetles beetles continues to be eaten since they are decreases. However, no genetic change easily visible on green leaves even from a occurs despite scarcity of food. distance. After some time the plant disease is cured As a result, there are more and more and there is plenty of food for beetles. Thus number of green-coloured beetles than red- the conditions become favourable for beetles. coloured ones and their population size At this time, the size and weight of beetles increases. It shows that natural selection again increases to their previous limits favoured evolution of green-coloured beetles. when food was plentiful. This shows that decrease in the size of beetles is not heritable but an acquired character. Variation in a population Conclusion : Situation 2 - Heritable variation of neutral What do you conclude from the above three advantage is fixed by chance selection situations? In this case, again a colour variation occurs In first two situations, a rare variation in during reproduction and one beetle becomes colour of beetles becomes a common blue-coloured instead of red-coloured. This characteristic in the population. Thus, newly developed blue beetle sexually frequency of an inherited trait of a certain reproduces and passes its colour to its gene in a population is changed over progeny. generations. This is the basic idea behind The crows can see both blue-coloured and evolution. red-coloured beetles and randomly eat them. In first situation, the variation in colour gave As the population grows there are few blue- advantage of survival to green-coloured coloured beetles and more red-coloured beetles and hence, it became common. The beetles. colour variation was naturally selected and However, an elephant steps on the population was exerted by crows. The crows fed on red- of beetles. Due to this, most of the beetles coloured beetles. More the number of crows, are killed. By chance, all red-coloured more the red beetles will be eaten, and as a beetles are killed while few blue-coloured result, more number of green-coloured beetles survive. The beetle population grows beetles were left in the population. Thus, again, but most beetles are now blue- natural selection is directing evolution in coloured. one direction only that is, forming green- coloured beetle population. Hence, this is known as directional evolution. Thus, there is adaptation in the beetle population to suit their environment. In second situation, colour variation did not From the above example it becomes clear give any advantage of survival to the beetles that in a small population, any accident or chance as crows could eat both red-coloured as well selection can change the gene frequency and as blue-coloured beetles. Survival of blue- coloured beetles was simply a matter of even a character that has no advantage for ch ance whi ch chan ged the common survival get fixed. Such type of selection by characteristics of the resultant population. chance is called genetic drift or chance selection. Since the population was small, the accident could change the frequency of some genes in it, passed to their offsprings through genes in even if these genes gave no survival re producti ve cells during the process of advantage. This is due to genetic drift that re producti on. The chan ge i n th e no n- provides diversity without any adaptation. reproductive body cells of an organism cannot be In third situation, the variation in the beetle inherited by its offsprings. For example, the population was caused due to so me reduced weight of beetles due to lesser food, as environmental factors and there was no studied in situation 3, will not change the DNA genetic change in the beetle population. It of germ cells. Hence, this trait (reduced weight) was simply scarcity of food that acted as a cannot be inherited by the progeny of starving limiting factor for the change in size and beetles. Thus, this is not an example of evolution, weight of beetles. On the availability of food, sin ce the chang e is not inh erited o ver the beetles again became normal-sized. generations. Hence, the experience of an individual during its lifetime cannot be passed Did you know ? to its progeny and they cannot direct evolution. How did life originated on earth? You can take another example to understand There are different theories of origin of life on this. Suppose you breed a group of mice. Then earth. In 1920's, Alexander I. Oparin and J.B.S. Haldane independently proposed that the first you will get all the progeny with tails. If you organic molecules were formed spontaneously remove their tails by surgery in each generation, in the reducing atmosphere that was present would you have tailless mice? No, because it is on the primitive earth. According to this theory, not an inherited trait. The cut tail of mice is an the atoms (carbon, hydrogen, oxygen and acquired trait which is not passed on to the nitrogen) present in the reducing atmosphere progeny because cutting the tail does not change of the primitive earth combined with each other the gene of their reproductive cells. Similarly, to form inorganic molecules. The inorganic sun-tanned skin, or scar on the forehead or skin molecules in turn formed simple and then are all acquired traits that cannot be inherited. complex organic molecules. The energy was On the other hand, all those traits that pass from provided by lightning, heat from volcanoes and one generation to next generation through DNA ultraviolet radiation. Thus, primitive life was in germ cells are known as inherited traits. produced by the process of chemical evolution. Stanley L. Miller and Harold C. Urey in 1953 ******* conducted an experiment to support this theory. They created an atmosphere somewhat similar to as presumed to exist on early earth by having mo lecu les (like me than e, ammon ia and hydrogen sulphide but no oxygen) over water. The temperature of the experimental set-up was maintained just below 100 °C. Sparks were also passed through the mixture of gases. At the end of the experiment after one week, it was observed that about 15% of the methane was converted to simple compounds of carbon and amino acids which are required to make protein molecules. This experiment thus provides a great knowledge of how life would have originated on earth from lifeless matter like inorganic molecules in the primitive gaseous and hot atmosphere of earth. Acquired and inherited traits : A characteristic or trait of an organism which is developed in response to the change in external environment and is not inherited is called an acquired trait. You have already studied that the traits or characteristics of parents are  : MULTIPLE CHOICE QUESIONS : 11. Which of the following phylum is known for its asymmetrical body shape? 1. Which of the following is responsible for the a) Mollusca b) Arthropoda transmission of hereditary traits from one c) Annelida d) Porifera generation to the next in living organisms? 12. Name the phylum which has a single a) Genes b) Environment opening for taking food and throwing out c) Mutations d) Hormones faeces? 2. Mendel's experiments were based on the a) Coelenterate b) Porifera study of: c) Annelida d) Aschelminthes a) Human genetics b) Pea plants 13. Name the organism which has flame-cell in c) Bacteria d) Insects its excretory system? 3. In Mendel's experiments, the first filial a) Corals b) Ctenophora generation (F1) resulted from the cross of: c) Roundworm d) Flatworm a) Two purebred parents 14. Which of the following is causative agent of b) Two hybrid parents Trichionosis? c) Two mutants a) Trichinella spiralis d) Two unrelated plants b) Oxytricha trifallax 4. The term "allele" refers to: c) Morus nigra a) Different forms of the same gene d) Agrodiaetus shahrami b) Offspring of two different species 15. Intestinal fluke disease is caused by c) Genetic mutations phylum____ d) Chromosomal abnormalities a) Ctenophora b) Platyhelminthes 5. If a trait is controlled by more than one gene, c) Annelida d) Aschelminthes 16. Which organism has segmental nephridia it is known as: for excretion and osmoregulation? a) Monohybrid inheritance a) Platyhelminthes b) Dihybrid inheritance b) Aschelminthes c) Polygenic inheritance c) Annelida d) Incomplete dominance d) Arthropoda 6. In humans, which of the fol lowi ng 17. In arthropods, Ecdysis or moulting is a chromosomes determine the gender of the process of casting off the skin. offspring? a) True b) False a) X and Y chromosomes 18. Which of the following phylum has a water b) X chromosomes only vascular system? c) Y chromosomes only a) Plantae b) Mollusca d) Autosomal chromosomes c) Echinodermata d) Hemichordata 7. Hemoph ilia is a se x-linked recessive 19. Name the phylum which is characterized by disorder. If a carrier female (XhX) marries a the presence of notochord. normal male (XY), what is the probability of a) Chordata b) Hemichordata their son having hemophilia? c) Echinodermata d) Mollusca a) 0% b) 25% 20. Name the first vertebrate which possessed c) 50% d) 100% amniotic egg? 8. Which of the following disorders is caused a) Amphibia b) Aves by an extra chromosome (trisomy) in c) Reptilia d) Mammalia humans? 21. Whi ch o f th e fo llow ing is known as a) Down syndrome b) Hemophilia scavengers in the marine ecosystem? c) Cystic fibrosis d) Sickle cell anemia a) Lamprey b) Hagfish 9. The process of the transfer of pollen grains c) Scoliodon d) Pristis from the anther to the stigma of a flower is : ANSWER KEY : known as: 1. a) 2. b) 3. a) 4. a) a) Fertilization b) Pollination 5. c) 6. a) 7. d) 8. a) c) Reproduction d) Budding 10. When two genetically dissimilar individuals 9. b) 10. b) 11. d) 12. a) are crossed, the offspring are referred to as: 13. d) 14. a) 15. b) 16. c) a) Purebred b) Hybrid 17. a) 18. c) 19. a) 20. c) c) Mutants d) Clones 21. b)