NOTES PREPARED BY
ASHAQ HUSSAIN BHAT
TEACHER SCHOOL
EDUCATION DEPARTMENT
JAMMU AND KASHMIR
Genetics And Heredity
Genetics It is the science which deals with the principle and mechanism of biological inheritance which includes both heredity and variation. The term genetics was coined by William Bateson in 1906. The basic principles of genetics were discovered by Gregor Johann Mendel & are therefore called as father of genetics.
Heredity: A recognizable feature of a human being like height, complexion, shape of hair, colour of eyes and shape of nose and chin are called characters or traits. The transmission of characters from the parents to their off springs is called heredity.
Variation: The difference in the characters among the individuals of a species is called variation. e.g.: Human height is a trait which shows variation.
Chromosome: It is a thread like structure in the nucleus of a cell formed of DNA which carries the genes.
Gene: It is a unit of DNA on a chromosome which governs the synthesis of a protein that controls a specific character or trait of an organism. Genes are actually units of heredity which transfer characters from parents to their off springs during reproduction.
Dominant Gene: The gene which decides the appearance of an organism even in the appearance of an alternative gene is known as dominant gene.
Recessive gene: The gene which can decide the appearance of an organism only in the presence of another identical gene is called recessive gene.
Genotype: It is the description of genes present in an organism. It is always in a pair of letters such as TT, 17, Tt. Thus, the genotype of tall plants could be TT or Tt where as that of dwarf plant is tt.
Phenotype: The characteristic which is visible in an organism is called phenotype, e.g. Being tall or dwarf are phenotypes of a plant because these traits can be seen by us or these are visible to us.
First Filial Generation (F1 generation): - (Latin word: Filia-daughter, filus-son). when two parents cross or breed to produce progeny or off springs, then their progeny is called F1 generation
Second Filial Generation (F2 generation): - when the first generation progeny cross or breed themselves to produce second progeny, then this progeny is called F2 generation.
Hybrid: A new form of plant resulting from a cross or breeding of different varieties of a plant is known as a hybrid,
Mendel's Contributions:-
Gregor Johann Mendel (1822-1884) is called the “Father of genetics”. His contributions to study of inheritance paved the way for our basic understanding of how traits are inherited from one generation to the next. Mendel did much of his work with easily obtained local organisms, especially garden peas. He also did genetic work with other plants and honeybees.
Why did Mendel selected pea plants for his experiments.
Mendel selected pea plants for his experiment because of the following reasons:
i) The flowers of pea are bisexual.
ii) Self-pollination takes place in the
iii) Germination of pea-seeds is quick and easy.
iv) The plants possess a no of contrasting characters i.e; tall v/s dwarf stem; violet v/s white flowers; axial v/s terminal pod ; inflated v/s constricted pod : green v/s yellow pod; round /wrinkled seed; yellow v/s green seeds.
Define monohybrid cross or law of segregation? (Law of Purity of Gametes).
Monohybrid cross means that cross which involves a single pair of contrasting character. Mandel did his experiment on pea-plants because that was easily available. He selected two sets of pea plants out of which 1 set consisted of tall plants and the other of dwarf. Both the sets were pure plants ie; by a process of self-pollination, the tall plants produced tall plants and the dwarf plants produced only dwarf plants. He first cross pollinated only tall plants with dwarf plants and collected all the seeds on ripening. He raised the plants from these seeds and called this generation as first filial generation or F1 generation He saw that all the plants of F1 generation were tall, whereas, they had the characters of both types of plants. He self- pollinated these plants and collected the seeds and sowed them in the soil. In this way, he raised the second filial generation or F2 generation which consisted of both tall and dwarf plants in the ratio of 3:1 (1:2:1) respectively. On self- pollinating these plants, he found that the dwarf plants of F2 generation produced all dwarf plants, while in the tall plants, one was pure tall and the others were hybrid tall. Mandel explained the above results in the following ways: The character of dwarfness remained hidden to the dominant character of the tall plants and so it could not express itself in the F1 generation. In the F2 generation, the characters segregated and produced both tall and dwarf plants. On the basis of these observations, Mandel gave his Law of segregation which states as follows: "The pair of contrasting characters of a hybrid separate from each other in the next generation". This can be shown by the figure as follows:Fig
Define dihybrid cross or law of independent assortment?
Dihybrid cross is a cross between two sets of plants having two pairs of true breeding characters. For this cross, two sets of plants were selected by Mandel. One set of plants was true breeding for round and yellow seeds and another set was true for wrinkled and green seeds. These plants on self-pollination produced seeds of their own kind. So, they were pure, when these two sets of plants were cross pollinated, he get the plants in the F1 generation which produced round and yellow seeds. This proves that yellowness and roundness are dominant over green and wrinkled characters.
In F2 generation, four kinds of seeds were produced:
(I) Round & yellow (9)
(ii) Round & green (3)
(iii) Wrinkled & yellow (3)
(iv) Wrinkled& green (1)
On the basis of this Mandel gave the Law of independent assortment which states as "when two pairs of characters are contributed by the same parents, and in a cross, they assort independently of each other in future generation as independent factors".
What is Sex determination?
Sex determination is a process by which the sex of a person is determined. Genetics is involved in the determination of sex of a person. This can be explained as: The chromosomes which determines the sex of a person are called sex-chromosomes. There are two types of sex-chromosomes, one is called X-chromosome and another is called Y-chromosome.
(a) A male has one X-chromosome and one Y-chromosome. This means that half of the male gametes or sperms will have X-chromosome and the other half will have y-chromosome.
(b) A female has 2 X-chromosomes but no Y-chromosomes. This means that all the female gametes called Ova or egg will have only X-chromosome. The sex of a person depends on what happens at fertilization:
(i) If a sperm carrying X-chromosome fertilizes with an Ovum or egg which carries Xchromosome, then the child born will be a girl or female. This is because the child will have xx combination of sex chromosome.
ii) If a sperm carrying Y-chromosome fertilizes with an Ovum or egg which carries X- chromosome, then the child born will be a boy or male. This is because the child will have XY combination of sex chromosome. Therefore, it is the sperm which determines the sex of a child.
It has been found that, in a turtle (chrysemapicta), high incubation temperature leads to the development of female off springs. While on the other hand, in case of lizards (Agama agama), high incubation temperature leads to the development of male offspring's. In some animals, such as snails, an individual can change sex, indicating that sex of is not determined genetically in such animals,
How blood groups are inherited?
A person has one of the 4 blood groups: A, B, AB or 0. This blood group system is controlled by a gene which has three different forms denoted by the symbols IÀ, IB& IO The genes IA and IB show no dominance over each other i.e. they are co-dominant. However, genes IA and I both are dominant over the gene IO
Although there are 3 genes for blood but a person can have only 2 of them. So, the blood group of a person depends on which two forms of the genes he possesses.
(i) If the genotype is IA IBor IAIO, then the blood group of the person is A.
(ii) If the genotype is IB IBor IB IO, then the blood group of a person is B.
(iii) If the genotype is IA IB, then the blood group of a person is AB.
(iv) If the genotype is IO IO, then the blood group of a person is O.
What is acquired trait and inherited trait?
(I) A trait of an organism which is not inherited but develops in response to the environment is called an acquired trait. E.g. If a beetle does not get sufficient food for a considerable time, its weight will be reduced, due to starvation. The low weight of this beetle is an acquired trait which has been acquired in response to environment which contained insufficient food. Again, suppose the tail of a mouse gets cut. The cut tail of a mouse is also an acquired trait which has brought about by some agents in its environment. The acquired trait of an organism cannot be passed on to their future generation.
(II) A trait of an organism which is caused by a change in its gene or DNA is called an inherited trait. It can be passed on to the progeny (next generation) of the organism because they have produced changes in the genes of an organism. E.g. suppose, there is a population of red beetles but a colour variation arises during reproduction, so that there is a beetle which is green in colour. The green colour of beetle is an inherited trait which has been passed on from its parents. The inherited traits of an organism can be passed on to future generation because they are produced by the change in the genes present in different cells.
Define Evolution. What are various evidences which support evolution?
The word evolution has been derived from a Latin word 'evolvere' which means to 'unroll' or'unfold'. Evolution is a sequence of gradual changes which take place in the primitive organisms over millions of years in which new species are produced. As the evolution is of living organisms, it is also called organic evolution,
Evidences which support Evolution:-
These evidences have been categorised under the following heads:
1. Morphological and Anatomical evidence.
2. Embryological evidence and
3. Paleontological evidence.
(1). Morphological and Anatomical evidence: - These evidences are based on the similarities in the external and internal features of the different kinds of organisms. These features are:
(a) Homologous organs:- The organs which perform different functions in different species but have similar basic structure and similar embryonic origin are called homologous organs. E.g.: the forelimbs of a man, a lizard, a frog, a bird and a bat seem to be built from the same basic design of bones but they perform different functions i.e., the forelimbs of humans are used for grasping, the forelimbs of a lizard are used for running, the forelimbs of a frog are used to prop up the front end of its body and also act as shock absorbers when the frog lands back on ground after a leap, whereas the forelimbs of a bird and a bat are modified for flying.
(b) Analogous organs:- The organs which are quite different in fundamental structure and embryonic origin but perform same function in different species are analogous organs. E.g.: the wings of an insect and a bird have different structures i.e. the insects have a fold of membranes as wings which are associated with a few muscles, whereas a skeleton, flesh and feathers support wings of a bird, but they perform the same function of flying.
(c) Vestigial organs: - The organs which occur in reduced form and are useless to the possessorbut are homologous to the fully developed, functional organs in the ancestors or related forms are called vestigial organs. E.g. vermiform appendix of the large intestine and nictitating membrane in the eye of human beings are vestigial organs.
2. Embryological evidence: - Embryological refers to the study of development of an embryo of organism from fertilised egg to young one. The early embryos of all the vertebrates resemble in shape and structure. The resemblance of early embryos of fish, salamander, tortoise, chick, rabbit and man is so close that it is very difficult to distinguish them from each other. This similarity among the early embryos shows that all the vertebrates have evolved from a common ancestor. These observations led Ernst Haeckel to propose a law which is popularly known as the biogenetic law. This law can be briefly put in three words- Ontogeny repeats phylogeny. It simply means that an organism repeats its ancestral history during its embryonic development.
3. Paleontological evidence:- Palaeontology is the study of past life based on the fossil record.
- Fossil:- The fossils are the remains or impressions of the dead animals and plants that lived in the remote past. Fossils occur in sedimentary rocks. They provide direct evidence of evolution. Various kinds of fossils have been unearthed from sedimentary rocks. These include many fossil invertebrates (e.g. Ammonites, trilobites), fossil fish (knightia) dinosaurs skull of Raja saurus (from Narmada valley) etc.
- Age of fossils: - There are two ways to know the age of fossils.
1. First way is relative. If we dig the earth to locate fossils, the fossils we find closer to the surface are more recent than the fossils we find in deeper layers.
2. Second way is to date the rocks or fossils. Dating the rocks or fossils is done by working out the ratio of uranium to lead in a rock or fossil. It is so because radioactive uranium decays spontaneously to lead at a constant and known rate. This property is used to determine the age of rocks or fossils. Potassium, argon and carbon dating is also done for calculating the age of fossils.
Examples: - The fossil bird, Archaeopteryx lived about 180 million years ago. It has feathers, fused bones, beak and other bird characters. But it also had teeth in the jaws, claws on free fingers, a long tail etc which are found in reptiles. Thus, Archaeopteryx represents a stage midway between the reptiles and birds, and is often called the lizard bird. This example provides a clue that the birds have evolved from reptiles.
What are fossils? How they are formed and also discuss the types of fossils?
The remains or impressions of dead animals or plants that lived in remote past are known as fossils.
Fossilization. Fossilization of a dead organism usually begin when the dead organism is buried before extensive decay sets in. The organism sinks into bog or marsh or to the bottom of a lake, sea, or river. In some cases it is buried in sand. Even after the burial, decay occurs and the soft parts of the body are generally destroyed. The hard parts, however, survive as fossils. Water, mud or sand hardens to rock and the organic remains are safely preserved. Fossilization is a random process, only those organisms which happen to die in a spot where they can be buried before other organisms destroy them are fossilized.
Fossils are exposed by natural erosion or excavation. Fossil bearing rocks become exposed by the actions of wind, rain or rivers, or through excavations by scientists. Once exposed, they are studied in detail in terms of their antiquity and characteristics. Such study of fossils is called paleontology.
Types of fossils
There are various kinds of fossils some of the important fossils are:-
1. Ammonite:- These were the invertebrate animals (Molluscus) with a flat, coiled, spiral shell, which lived in the sea. The ammonite fossils are about 180 million years old.
2. Trilobite:- These were marine arthropods which were common between 400 to 600 million years ago.
3. Dinosaurs:- These are extinct carnivorous or herbivorous reptiles. They were first appeared on earth about 250 million years ago and become extinct about 65 Million years ago.
What are the two methods to determine the age of fossils?
Determination of age of fossil?
There are two methods to known the age of fossils.
1. By the relative method.
2. By carbon dating method.
Relative method:-
When we dig the earth, we find fossils at different depths. The fossils which we find closer to the surface of the earth are more recent and the fossil found in deepest layer of earth are oldest one.
carbon-dating method:- Dating of rocks or fossils is done by working on concentration of carbon-14 atoms in fossils. This can be explained as:
All the living objects contain some carbon-14 atoms which are radioactive. When a living object dies and forms a fossil. Its carbon-14 content goes on decreasing gradually. In the carbon-14 dating method, the age of fossils is found by comparing the carbon-14 radio-activity left in fossil with carbon-14 radioactivity found in living object.
Explain various theories which explain origin of life?
Various theories which explain origin of life are as:
(1). Theory of special creation: - It states that life was created by the creator or God out of nothing in the remote past. According to Christian belief, God created the universe, plants, animals and human beings in six natural days. Adam and Eve were the first man and woman created by God. According to Hindu mythology, Lord Brahma created the world with his wishes. Manu and shradha were the first man and woman respectively,
2. Theory of Eternity:-Different living beings, planets, stars etc existed as such from the beginning and would continue to exist till eternity.
3. Theory of Catastrophism - World has experienced several ages. At the end of each age all or most organisms die while at the beginning of next new age they are recreated by the Almighty
4. Cosmozoic theory - Both living and non-living matters were formed simultaneously. Early living objects were resistant spores called cosmozoa, Cosmozoa gave rise to different types of living beings on earth.
5. Theory of panspermia (Extra terrestrial origin): - Primitive life or panspermia came to earth from some other planet in the form of microbes or spores.
6. Theory of spontaneous generation (theory of a biogenesis or Auto biogenesis):- The theory believes that under certain conditions non-living substances gave rise to living beings spontaneously
7. Oparin-Haldane theory of chemical evolution of life: - It believes that life originated on early earth through physicochemical processes of atoms combining to form molecules, molecules reacting to produce inorganic and organic compounds. Organic compounds reacting or interacting to produce all types of macromolecules which organised to form the first living system or cells.
Major support to this theory was provided by Stanley L.miller and Harold C.Urey in 1953. They designed a glass apparatus and simulated the conditions that thought to have existed on primitive earth. They circulated a mixture of methane (CH4), ammonia (NH3) and hydrogen (H2) in the ratio of 2:2:1 and water vapour in the apparatus. These gases were believed to prevail in the primitive earth. The mixture was maintained at a temperature just below 100° C. They provided energy for interaction of gases in the form of electric sparks in the flask. At the end of a week, 15% of the carbon (from methane) had been converted to simple organic compounds such as amino-acids, simple sugars, purines and pyrimidines. These simple organic compounds interacted in oceans to form complex organic polymers such as polysaccharides, polypeptides and proteins, lipids, nucleotides and nucleic acids. These polymers due to intermolecular interactions, formed large colloidal coacervates. These coacervates later organised into the living matter.
What is speciation? Explain with example the generation of new species?
The origin of new species from the existing one due to reproductive isolation of a part of its population is called speciation. Generation of new species: When a population of a species splits into two populations, it cannot reproduce with each other. Then, they can be called two independent species. Then a new species is generated. E.g.
(i) A huge population of beetles feed on bushes spread over a wide mountain range,
(ii) The beetle population becomes very large,
(iii) Individual beetles feed mostly on nearby bushes.
(iv) In that huge population of beetles, there will be sub populations. The male and female beetles meet for reproduction to happen so most reproduction will be within these sub populations.
(v) Occasionally a migrant beetle might go from one side to another or a beetle is picked up by a crow from one side and dropped on the other side. This migrant beetle will reproduce with the local population.
(vi) Over generations, genetic drift will accumulate different changes in each sub population. Also natural selection may operate differently in different geographic locations.
(vii) Together, the processes of genetic drift and natural selection will result in these two isolated sub populations of beetles becoming more and more different from each other.
viii) Eventually, members of these two groups will be incapable of reproducing with each other and two generations of beetles are being generated.
Explain evolution of eye is an example of evolution by stages?
Eye is a complex organ of sight in animals. Flat worms, Planeria have very simple eyes which are, in fact, just eye spots to detect light. Even these rudimentary structures in flat worms provide advantage to the animal to ensure its survival. From this basic design, more complex eyes later evolved in different organisms. For instance, eyes in insects, octopus, other invertebrates and all vertebrates including human beings have different structures and also have separate evolutionary origin. The complex structure of eye these animals has been created in stages over many generations. To conclude, the eyesof insects, octopus and all vertebrates are analogous organs which have developed bit by bit over generations in their own way as adaptation for similar function.
How evolutions of feathers tell us that birds are closely related to reptiles?
Sometimes an evolutionary change produced in an organism for one purpose later on becomes more useful for an entirely different function. E.g. birds evolved feathers as a means of providing insulation to their bodies in cold weather but later on these feathers became more useful for the purpose of flying. Even some dinosaurs had feathers though they could not fly by using these feathers. Birds, however, adapted feathers for flying. The presence of feathers on birds tells us that the birds are very closely related to reptiles because dinosaurs (which had feathers) were reptiles.
What is genetic drift?
of the genes of certain traits when a section of a species population migrates or dies of natural calamity. It alters the gene frequency of the remaining population.
Define Artificial selection. Give the importance of artificial selection.
Artificial selection is the process by which man selects traits useful to him for improving the qualities of domesticated plants and animals. Man selects the individuals having the desired traits and separates them from those which do not possess such characters. The selected individuals are interbred. This process of artificial selection, when repeated for many generations, produces new breed with desired traits. In this way, the wild forms are modified through artificial selection.
Example: If cows with high milk yield are desired, the animal breeders monitor the milk yield of a large no of cows and select those which produce a large quantity of milk. The calves of the high milk yielding cows are interbred to get the next generation of calves. After repeating this process for a number of generations, a breed of high milk yielding cows is produced.
Importance:
(i) By artificial selection, animal breeders have been able to produce improved varieties of several domestic animals such as dogs, horses, pigeons, poultry, cows, goats, sheep etc from their wild ancestors.
(ii) Plant breeders have obtained improved varieties of useful plants, namely, wheat, rice, sugarcane, cotton, pulses, vegetables, fruits and ornamental plants. Many crop plant species, namely, cabbage, kohlrabi, kale, broccoli, cauliflower etc have been produced by plant breeders from a common wild mustard species (Brassica oleracea) by selective breeding.
Can evolution be equated with progress?
Evolution should not be equated with progress. Evolution, in fact, results in the generation of diversity and the shaping of the diversity by environmental selection. The only progressive trend that appears in evolution is the development of more and more complex designs over a period of time. However, many of the older and simpler designs still survive. The newly formed species is not, in any way, better than the older one. It is drift which have together led to the formation of a population that cannot reproduce with the original one. Let us take the example of evolution of human beings and chimpanzees. Human beings are closely related to chimpanzees but have not evolved from them. Both these species had a common ancestor a long time ago. This common ancestor diverged into several forms and each form probably evolved in its own separate way to give rise to the present forms of human beings, chimpanzees and other closely related great apes. In other words, evolution does not proceed as ladder of progress but occurs as branches of trees.
Explain Darwin's theory of natural selection.
Charles Robert Darwin gave the theory of evolution in his famous book "the origin of species". This theory is called so because it suggests that the best adapted organisms are selected by nature to pass on their characteristics to the next generation. It applies to plants as well as animals.
- Darwin's theory of evolution can be described as follows:
(i) Within any population, there is natural variation. Some individuals have more favourable variations than others.
(ii) Even though all species produce a large number of offspring's, populations remain fairly constant naturally,
(iii) This is due to the struggle between members of the same species and different species for food, space and mate.
(iv) The struggle for survival within populations eliminates the unfit individuals. The fit individuals possessing favourable variations survive and reproduce. This is called natural selection (survival of the fittest).
(V) The individuals having favourable variations pass on these variations to their progeny from generation to generation,
(vi) These variations when accumulated over a long period of time, lead to the origin of a new species.
Natural selection is the process of evolution of a species whereby characteristics which help individual organisms to survive and reproduce are passed on to their offspring, and those characteristics which do not help are not passed on. Though Darwin's theory was widely accepted, but it was criticised on the ground that it could not explain how the variations arise.
How do traits get expressed?
Deoxyribonucleic acid (DNA) present in the cell is the hereditary material. It is the information source in the cell for making proteins. In fact, the genes determine everything about an organism i.e. they control all the characteristics of the organism. A dominant factor of allele controls the synthesis of a fully functional protein or enzyme to produce its morphological or physiological effect. A recessive factor is able to contribute partially andnthus produces defective, incomplete and less efficient protein or enzyme. Effect of recessive factor only becomes apparent when it occurs in a pair. The plant height depends on the amount of a particular hormone synthesized. If the specific protein needed for this process is synthesized and works efficiently, a lot of hormone will be made. The latter will effect more growth and the plant will be tall. If the gene for tallness is altered the protein now synthesized is less efficient and, therefore, the amount of hormone formed will also be less. Consequently, growth of plant will be less and the plant will be dwarf. According to Mendel's interpretations, both parents contribute equally to the DNA of the progeny during sexual reproduction. Also, both parents help in determining the traits in the progeny. In other words, both parents contribute a copy each of same genes and both male and female gametes, therefore, have only one set of all genes,
Explain briefly structure of DNA.
DNA stands for deoxyribonucleic acid. It was first isolated by the scientist Frederick Meisher from the nucleus of the pus cells in 1869. Later, it was experimentally proved by the scientists Griffith (1928) and Avery, McLeod and McCarty (1944) that DNA is the carrier of the genetic information from generation to generation. It transmits the hereditary characters in a coded language from parents to the offspring. Watson and Crick (1953) came to the conclusion that DNA was made of two helical chains held together by hydrogen bonds created between adjacent nitrogen bases. DNA is made up of very large no of nucleotide units. Each nucleotide unit in a DNA molecule is made up of 3 components.
(1) Deoxyribose sugar: - It is a pentose sugar.
(2) Nitrogenous base: - Each nucleotide unit has nitrogen containing base. In a DNA molecule,
nitrogenous bases are of two types:
(a) Purines: - The purines in a DNA molecule are- Adenine (A) and Guanine (G).
(b) Pyrimidine: - The pyrimidines in a DNA molecule are- Cytosine (C) & Thymine (T).
(3) Phosphate Group: - The phosphate group contains one phosphorous atom and four specially linked oxygen atoms.
Why are human beings who look so different from each other in terms of size, colour and looks said to belong to same species?
Human beings look different from each other in terms of size & colour but belong to same species because on the basis of time dating, study of fossils and molecular phylogeny all the human beings are evolved from the same ancestors which have been traced in Africa. The variation in colour, size and other features among the individuals of human beings is because of migration and settlement in different geographical regions. Some of our ancestors left Africa & spread slowly across the planet while others stayed on. Over a period of time, due to variability in environmental conditions of different regions, our ancestors adopted and developed genetic variations and became different in colour and size. in terms of size & colour but belong to same species because on the basis of time dating, study of fossils and molecular phylogeny all the human beings are evolved from the same ancestors which have been traced in Africa. The variation in colour, size and other features among the individuals of human beings is because of migration and settlement in different geographical regions. Some of our ancestors left Africa & spread slowly across the planet while others stayed on. Over a period of time, due to variability in environmental conditions of different regions, our ancestors adopted and developed genetic variations and became different in colour and size.
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