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Monday, September 28, 2009

Law of Segregation (Short Note)

Monday, September 28, 2009 - 0 Comments

The factors for contrasting characters remain associated in pairs in the somatic cells of each plant throughout its whole life. Later in its life history when spores (and subsequently gametes) are formed as a result of reduction division, the factors located in homozygous chromosomes become separated out and each of the four spores (and gametes) will have only one factor (tallness or dwarfness) of the pair but not both i.e. a gamete becomes pure for particular character. This law is also called law of purity of gametes. Mendel also experienced on other pairs of alternative characters and he found that in every case the characters followed the same scheme of inheritance. Thus in the garden pea he discovered that coloured flower was dominant over white flower; yellow seed over green seed and smooth seed over wrinkled seed.

Law states that ‘the genes or factors representing a pair of contrasting characters, when brought together by the union of two gametes coexist in the cells of the offspring and later become separated in the gametes, so that each male or female gamete carries only one gene for a member of the pair but never both.

Test Cross (Short Note)

Tall plants produce in F2 generation may either have TT or Tt genotypes. Mendel devised simple way to distinguish the genotype. It is known as test cross. The organism of dominant phenotype (tall) but unknown genotype is crossed to homozygous recessive individual. For example a tall plant is crossed with a dwarf plant. If the tall plant is homozygous (TT), all F1 plants would be tall. But if the tall plant is heterozygous (Tt), one half one F1 plants would be tall (tt) and one half dwarf (tt) exhibiting ratio of 1:1. This ratio is called test cross ratio.

Back Cross (Short Note)

The term test cross is often used interchangeably with back cross. However two are not necessarily the same. The test cross is a cross of an individual of dominant phenotype to homozygous recessive. On the other hand back cross is mating of an individual of known phenotype to anyone of homozygous parents. For example in above mentioned example the cross between F1 hybrid to homozygous recessive individual is test cross but if we cross the F1 hybrid to a homozygous tall (TT), it is a back cross. Back cross may have special value in some genetic analysis such as Casey incomplete dominance. Because the homozygous necessive individual is also one of the parents, therefore test and back crosses are regarded same.

Multiple Alleles (Short Note)

A gene for a trait having three or more allelic forms are called multiple alleles. A well known example of multiple alleles in human beings is that of blood groups. The type of blood group that a person may have, depends upon the presence or absence of certain specific substances in the red blood cells. These substances are antigens and are of two kinds Antigen A and Antigen B. A person with Antigen A has blood group A and that with antigen B, has blood group B. A person with A and B antigens has blood group AB. Similarly a person lacking both antigens falls in blood group O. It is now known that there are three alleles responsible for this trait Gene 1A and 1B are responsible for the synthesis of antigens A and B respectively. Their alternative genes (i) synthesize no antigens at all. As only two alleles can occur in the individual, therefore, three alleles can have only four possible combinations. Alleles 1A and 1B are co dominant and both are dominant over allele ‘i’ Blood groups are helpful in determining the paternity.

Co-Dominance (Short Note)

It may be described as phenomenon of inheritance in which both alleles of a contrasting character are dominant and express themselves in heterozygous individual neither masking the effect of one another. In a cross between a tree breeding short horn red cattle and a true breeding white short horn cattle, the offspring have roan colour. A close examination of the skin or roan coloured animal shows that the animal does not possess an intermediate shade of skin colour, but is appears so because of mixture of red hair and white hair. It is clear that none of the two genes is dominant over the other. Such pairs of alleles of a gene are said to be ‘Co dominant’ and the phenomenon as ‘Co Dominance’.

Evidences of Evolution

According to theory of evolution organisms living today have arisen from earlier types of organisms by a process of genetic change that has occurred over a period of several billion years. The fact that all organism have arisen from a common ancestor explains why they have the same mechanisms for the storage and utilization of genetic information, many of the same types of cellular organelles and similar types of enzymes and metabolic pathways. Evolution explains how a single species can give rise to numerous other species. The biologists may argue over the mechanisms of evolution; they agree that all life descended with modification from single common ancestor.

(1) EVIDENCES FROM PALEONTOLOGY

It is study of fossils. Fossils are remains of life from the past. Fossils are formed when organisms become buried in sediment, the calcium in bone and other hard tissue is mineralized and the sediment is converted into rock. They may include entire organism, hard skeletal structures and casts petrifications, impressions, imprints and fossilized faecal pellets. Earliest known fossil plants are about 410 million years old. Seed bearing plants with feather like leaves similar to living cycads were abundant in Mesogoic era. Oldest fossils of flowering plants or angiosperms are from early cretaceous period.

(2) EVIDENCES FROM COMPARATIVE ANATOMY

Comparing the structures of the parts of the bodies of different organisms is probably the most commonly used evidence of evolution. In order to gather comparative evidence for evolution, biologists study external characteristics, examine bones and teeth dissect organs systems, study sections of tissue under the light microscope and examine finer details of cells and tissues under the election microscope.

(i) Homologous Structures:

Comparative anatomy reveals that certain structure features are basically similar. For example the basic structure of all flowers consist of sepals, petals, stamens, stigma, style and ovary yet the size colour, number of parts and specific structures and different in different species. Organ having similar basic pattern but modified into different forms to perform characteristics functions are called homologous structures.

(ii) Adaptive radiation:

When a group of organisms share a homologous structure which is differentiated to perform a variety of different functions, it illustrates a principle known as adaptive radiation variations between different species within the class enable them to adapt to different habitat.

Analogous Structure: Similar structures, physiological process or modes of life in organisms that differ in their structural but show adaptations to perform the same functions are called analogous structures. E.g. Presena of thorns o plant stens and spines on animals. The existence of analogous structures suggest convergent evolution. In this type of evolution. The environments act through the agency of natural selection favouring advantageous variations.

(3) EVIDENCE FROM COMPARATIVE EMBRYOLOGY

The evidence of progressive development on the basis of embryology can be seen in plants. the gametophytes of primitive mosses and forms are represented by Protonema produced by germination of the spores has similar structure, physiology and pattern of growth to filamentous green algae from which they thought to be evolved. Alternation of generation in plant life cycles and homologous variation in it are considered to be due to adaptations to various environmental conditions e.g. Gymnosperms are regarded as a group in-between land plants and plants which still requires water for fertilization. In cycas male gametophyte resemble pollen grains of angiosperms in that it is distributed by wind. As it develops pollen tube is formed but instead of carrying gametes it act as hanstorium for absorption of food. The gametes are flagellated and swim to ovule to fertilize the egg present there. Cycas therefore represents intermediate group of plants. it suggests that gymnosperms are adapted to different environment and are phylogenic intermediate forms between non vascular plants and angiosperms. Many of these intermediate forms are extinct and are represented by fossils.

(4) EVIDENCE FROM COMPARATIVE BIOCHEMISTRY

As techniques of biochemical analysis have become more precise, this field of research provided evidences for evolution. The occurence of similar molecules in complete range of organisms suggest the existence of biochemical homology in similar way to the anatomical homology shown by organs and tissues. Most of the research which has been carried out on comparative biochemistry has involved analysis of primary structure of protein molecules such as cytochrome, haemoglobin and most recently nucleic acid molecules particularly r RNA.

Variation and Kinds of Variation

Term variation means the differences in characteristics shown by organisms belonging to the same species. Organisms within the same species vary from one another variations may be

(1) Genetic Variations

(2) Environmental Variations

(1) GENETIC VARIATIONS

Variation between individuals are caused by genes. Most genes have different forms called alleles. In sexually reproducing organisms these alleles are reshuffled each time a new organism is produced. Reshuffling occurs during meiosis as a result of crossing over and independent assortment. The random fusion of gametes from two parent also produces new combinations of alleles in the offspring. Another source of genetic variation is mutation, crossing over, independent assortment and random fusion produce new combinations of alleles while mutations produce completely new alleles. Genetic variations are inheritable therefore when organisms produce they pass on some of their genes to their offspring genetic variations affect DNA of the organisms.

(2) ENVIRONMENTAL VARIATIONS

Some variations among individuals are not caused by their genes. Plants exhibit such variations. They may differ in size and colour of leaves because one may be growing in shade and nutrient deficient soil and other in light and nutrient rich soil. Such differences that arise during organisms life time are called environmental variations. These variations do not affect DNA of organisms, therefore are not inherited.

A study of phenotypic difference in any large population shows that two types of variation occur

(1) Discontinuous variation

(2) Continuous variation

(1) Discontinuous Variations: There are certain characteristics within a population which exhibit a limited form of variation. Variations in this case produce individuals showing differences with no intermediate forms. For example a person belongs to one of the four blood groups in ABO blood system, A, B, AB and O. There are no in-betweens and only a few clearly defined groups of individuals. Such variations are called discontinuous variations.

(2) Continuous Variations: The characteristics show complete gradation from one extreme to the other without any break. For example human skin or eye colour cannot be categorised into clearly defined colours. Continuous variations may be caused by genes or by environment of by both. Variation in leaf length is oxford ragwort plant is caused by environment entirely because all cells in the plant were produced by mitosis from a single zygote and so contain exactly the same genes.

Sources of variations

Genetic variation do not occur in asexually reproducing organisms as replication of DNA is perfect. Any apartment variation in these organisms may be due to influence of environment. There is opportunity for genetic variation to arise in sexually reproducing organisms.

The main causes of variation are:

(1) Gene Recombination

(2) Mutations

(1) Gene Recombination: Gene recombination originate as a result of crossing over during meiosis, random migration of chromosomes at the time of call division and random fusion of male and female gametes during fertilization.

(2) Mutation: The gene reshuffling do not generate major changes in the genotype, which are necessary to give rise to new species. These changes are produced by mutations.

A mutation is a change in the amount or the structure of DNA of an organisms. This produces change in the genotype which may be inherited by cells derived by mitosis or meiosis from the mutant cell. Mutation may result in the change in appearance of characteristic in a population. Mutations occurring in gamete cells are inherited, where as those occurring in somatic cells can only be inherited by daughter cells produced by mitosis. They are called somatic mutations.

A change in the amount of arrangement of DNA is known as chromosomal mutation or chromosomal aberration where as change in structure of DNA at a single locus is called point mutation or gene mutation. The term mutations is usually used to describe gene mutation.

Chemical Mutagens: Certain chemicals such as nitrous acid, hydroxylamine, dimethyl solfonate, methyl ethyl sulphate, acridive etc act as mutagens. In addition a variety of other chemical substances including mustard gas, Caffeine, formaldehyde, Colchinine, certain components of tocacco and increasing number of drugs, food preservatives and pesticides also cause mutations.

Induced Mutation: A mutation that is produced artificially during experimentation by using mutagens such as X-rays, ultra violet rays etc are called induced mutations.

Spontaneous mutation: The mutation which arises for no apparent reason and cause a genetic alteration is called spontaneous mutation. It is random and natural. It may occur at any site on a chromosome. It is found that majority of them produce just a slight effect and are harmful.

Evolution and Theories of Evolution

Evolution means development of life with time. Evolution means that living things change. A species may slowly change into new species or even into two or more new species. It means that the plants and animals that are present on earth now are not the first plants and animals, it also means that many plants and animals that once flourished are no longer alive, as one species evolves into other species, the original species no longer exists. The knowledge about these extinct species is provided by their remains preserved in the rocks called fossils Theory of evolution has special place in the study of history of life. It helps us to understand the unit as well as the diversity of plants and animals. But evolution has not made the organisms completely different DNA always carries the instructions for inheritance. Evolution provides an unifying picture of life. Life has common pattern because all life is interrelated through evolutionary descent. This is evident from classification which has its scientific basis in evolutionary interrelationship. By evolution scientists try to discover how this process of change occurs.

(1) THEORY OF EVOLUTION BY NATURAL SELECTION

Charles Darwin during voyage accumulated geological and fossil evidence that supported the idea that life changes with time. He studied flora and fauna of main land South America. He found evidence development of a variety of forms among finches (a kind of birds) from single ancestral group due to adoption to feeding on different kinds of food. He also studied the effect of light on plant growth that later led to discovery of plant hormones, plants of various habitats. He said there is contest among the members of a population for food to which called ‘Survival of the fittest’. He found that same struggle exists among all living things. He suggested that all species show variations with time. Some variations are of advantage in struggle for existence. Organisms with favourable variations are most likely to breed and to pass on their favourable characteristics. In this way new species arise from existing ones because the natural selects the fittest, evolution by ‘natural selection’.

LAMARCK’S THEORY OF EVOLUTION - LAMARCKISM

French naturalist Lamarck in 1809 gave hypothesis based on two conditions.

(1) The use and disuse of parts

(2) The inheritance of acquired characters.

According to Lamarck changes in the environment may lead to changed patterns of behaviour which can necessitate new or increased or disuse of certain organs or structures. Extensive use would lead to increased size, disuse would lead to degeneracy and atrophy. These traits acquired during life time of the individual were believed to be heritable and thus transmitted to offspring.

Weisman theory of continuity of Germplasm

Weisman proposed theory of continuity of germplasm. He postulated that somatic or body acquired characters resulting in variation did not affect the germ or gamete cells (in other words DNA, the genetic material) which are responsible for transmission of genetic characters. The germ cells are protected and nourished by somatic cells and are passed on intact and unmodied from generation to generation. Weisman on the basis of his theory argued that bodily modifications brought out by the environmental changes and by the use and disuse of organs cannot affect the germ cells and therefore cannot be transmitted to the next generation.

According to Weisman only germ cells of an individual contain all the determinants for all hereditary characters but the somatic cells contain only those determinants which determine the development of particular characters of the tissues or organs which they form. For example the nucleus of nerve cell contains those determinants which control the development of characters of nerve cell. Same nucleus behaves differently in different cells because of differences in their cytoplasm.

Dominance, Incomplete Dominance, Co-Dominance, Test Cross and Back Cross

DOMINANCE

If two different alleles of the same gene (T and t) come together by fusion of gametes, they co exist as such although one of them expresses itself called dominant and the other is unable to express itself and is called recessive. If F1 plants are tall therefore the allele of tallness is dominant and allele of dwarfness is recessive. Therefore Tall character is dominant on recessive.

INCOMPLETE DOMINANCE

In case of flower colour Mendel found that red colour of flower was dominant over white colour of flowers. White coloured flowers reappeared in F2 following Mendel’s principle of segregation. Later a deviation to this rule was observed in a plant species commonly called four o clock (Mirabilis Jalapa). The plant produces flowers with red and white colours. When pure breeding red (R) flowered plants were crossed with pure breeding white (r) flowered plants, F1 plants were Pink (Rr) flowered. Appearance of pink flowers, an intermediate shade between red and white provided relief to those who believe in blending inheritance. But when F1 hybrids were crossed (Rr × Rr), F2 generation showed a phenotypic ratio of 1 red: 2 pink: 1 white instead of typical 3: 1 ratio. Although the ratio contradicted Mendel’s principles but it provides strong support to the concept of particular inheritance.

Appearance of red and white factors (genes) in F2 indicates that these have not been altered or blended white present together in the pink flowered individuals. F2 reds and whites are same as the parental red and white. This was declared a case of ‘incomplete dominance’. In this case neither of the genes is dominant to the other. Each expresses itself in the presence of its allele to produce intermediate effect. Another important aspect of incomplete dominance is that phenotypic ratio (1: 2: 1) is the same as genotypic ratio (1 homozygous dominant: 2 heterozygous dominant: 1 homozygous recessive) resulting from cross when two monohybrids are crossed, when dominance is incomplete, a cross of two monohybrids (Rr × Rr) gives a phenotypic ratio of 1: 2: 1 which is identical to genotypic ratio. The heterozygous individual shows incomplete dominance. Where as when the dominance is complete the phenotypic (3: 1) and genotypic ratio are complete.

CO DOMINANCE

In case of blood types in human, both genes (A & B) produce an effect in a heterozygous individual. This is called co dominance. The genes which govern A and B blood types are alleles. Each control the formation of different red blood cell protein or antigen. Antigen in case of person having blood group A and antigen b in individuals with blood group B. Neither gene is dominant to the other. Heterozygous individuals with blood group AB contains both antigens a and b. Both proteins are detected in equal amounts in the red cells.

TEST CROSS

Tall plant produced in F1 generation may either have TT or Tt genotypes. Mendel devised a simple way to distinguish the genotype. It is known as test cross. The organism of dominant phenotype (tall) but unknown genotype is crossed to homozygous recessive individual. For example a tall plant is crossed with a dwarf plant. If the tall plant is homozygous (TT), all F1 plants would be tall. If the tall plant s heterozygous (Tt) one half of F1 plants would be tall (Tt) and one half dwarf (tt), exhibiting ratio of 1: 1. That ratio is called ‘test cross’.

BACK CROSS

The term test cross is often used interchangeably with back cross. However, two are not necessarily the same. The test cross is a cross of an individual of dominant phenotype to homozygous recessive. On the other hand back cross is mating of an individual of known phenotype to any one of homozygous parents. For example in the above mentioned example the cross between F1 hybrid to homozygous recessive individual is a test cross, but if we cross the F1 hybrid to homozygous tall (TT), it is a back cross. The back cross may have special value in genetic analysis such as in case of incomplete dominance. Because the homozygous recessive individual is one of the parents therefore test cross and back crosses are regarded same.

Plant Breeding Techniques

Plant breeding techniques are as under:

(1) Selection (2) Hybridization (3) Mutation breeding

(4) Polyploid breeding (5) Tissue culture (6) Genetic engineering

(1) SELECTION

It is the process of isolating desirable genotypes from undesirable ones. Selection is oldest method and basis of all plant breeding techniques. The genotypes most suitable for human needs are isolated. It is carried out in a number of ways depending on the mode of reproduction and types of breeding method used. Selection tends to change gene frequencies and is thus responsible for creating new gene pools. A plant breeder selects plants according to his specific needs, for example higher grain yield, sugar or potato content or early maturity. Since artificial selection tends to change gene frequencies in genotypes, changes in the genetic make up of new varieties are expected.

(2) HYBRIDIZATION

If sufficient genetic variability is not available for effective single plant or mass selection, the variability can be created artificially through hybridization. The varieties or species with desirable characteristics are selected and crossed to produce hybrids. The populations are heterozygous and plants with combined desirable features of the parents are selected for testing and further growth. The hybridization result in a single true breeding strain possessing best characteristics of the parent varieties. Large number of plants are used in these crosses because the greater the number of plants used, the greater the chances of obtaining the desired combination of characters and the easier the development of a variety with good agronomic qualities.

(3) MUTATION BREEDING

Mutation is sudden change in hereditary material of a cell. Mutation involves:

(a) A change in gene from one allele to other.

(b) Rearrangement of chromosome material.

(c) Loss or duplication of chromosome segment.

Changes in genes are often called point mutations. The use of mutation in plant breeding is popularly known as ‘mutation breeding’. Mutations are induced through physical mutagens, as x-rays, fast neutrons, thermal neutrons, ultra violet radiation and beta radiation. In addition to physical mutagens large numbers of chemical mutagens are also used to induce mutations in crop plants. Whole plant or any part of plant can be treated by radiation seeds, are most commonly used for irradiation. They offer a number of advantages. They are easily to handle and store and can be maintained for extended periods of time. When dry seeds are almost inert biologically and sever environment cause no significant biological damage.

(4) POLYPLOID BREEDING

Polyploid is a condition in which individuals have more than two chromosome sets or genomes in their somatic cells. In contrast to the normal diploid (2n) they may be triploid (3n), tetraploid (4n), pentaploid (5n), and hexaploid (6n) and so on.

Polyploid plants may arise by duplication of the chromosome sets for single species, auto polyploidy, or by combining chromosome sets from two or more species, alloploidy. Alloploidy in more common method of ploidy in nature. An alloploid in which total chromosome compliment of two other species is combined to form a fertile species hybrid is called amphiploid. Many commonly cultivated crop species have evolved in nature as polyploids for example oats, cotton, tobacco, sorghum, Brassica, common wheat, forage grasses and legumes.

(5) TISSUE CULTURE TECHNIQUE

It is commonly used to describe in vitro and aseptic cultivation of any part on a nutrient medium. The technique was first introduced by Haberlandt in 1902. It has provided an efficient selection tool for modern plant breeding.

In part, if a plant breeder wished to propagate a hybrid plant, a method of vegetative reproduction was used, such as taking stem, leaf or root cuttings. This is a relatively rapid way of reproduction in plant, but not all plants can reproduced like this Tissue culture is a method of vegetative reproduction on mass scale. Within a few years several thousand plants, mostly identical to the original plant can be produced. However some mutations may occur giving occasional variant plants. The propagation of plants by tissue culture is also used after genetic engineering.

(6) GENETIC ENGINEERING

The aim of genetic engineering is to remove a gene from one organism and transfer it into another in such a way that the gene is expressed in its new host. The host is now known as transgenic. Genetic engineering provides a way of overcoming barriers to gene transfer between species. Indeed the genes in question have often been taken by organisms in different kingdoms, such as a bacterial gene but into a plant or a human gene into a bacterium. Unlike selective breeding where whole sets of genes are transferred; genetic engineering results only in the transfer of single gene. Genetic engineering must obtain the wanted gene, clone the gene to produce many copies and insert a copy of the gene into the host DNA.

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