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BIOLOGY FOR CLASS IX Chapter #16 Class IX Genes And Inheritance Content Inheritance By Genes Crossing Over And Its Significance Significance Of Crossing Over Brief Structure Of Dna Significance Of Base Sequence Replication Of Dna Common Terms Used In Genetics Mendel And His Laws Of Inheritance Mendel’s Law Of Segregation Law Of Independent Assortment Determination Of Sex In Man By X And Y Chromosomes Sex Linked Inheritance Other Hereditary Diseases In Human Beings GENE Gene is a unit of heredity which is transferred from a parent to offspring and is held to determine some characteristic of the offspring. Some of these traits may be physical for example hair and eye color and skin color etc. On the other hand some genes may also carry the risk of certain diseases and disorders that may pass on from parents to their offspring What is genetic variation? Individuals in a population are not exactly the same. Each individual has its unique set of traits, such as size, color, height, body weight, skin colour and even the ability to find food. Sometimes, offspring’s of the same parents still differ a lot among themselves. You can find that among 3 sisters, one may be very tall, the other may have dark hair and the third may have a rounded nose tip. Such differences in individuals from the same parents are called variation. Characteristics or traits that are inherited are determined by genetic information. Some other traits like dialect or accent, scars, skin texture or even body weight may be determined by some external or environmental factors. How genetic conditions are inherited? Each cell in the body contains 23 pairs of chromosomes. One chromosome from each pair is inherited from your mother and one is inherited from your father. The chromosomes contain the genes you inherit from your parents. There may be different forms of the same gene – called alleles. For example, for the gene that determines eye colour, you may inherit a brown allele from your mother and a blue allele from your father. In this instance, you will end up with brown eyes because brown is the dominant allele. The different forms of genes are caused by mutations (changes) in the DNA code. Crossing Over Crossing over, or recombination, is the exchange of chromosome segments between nonsister chromatids in meiosis. Crossing over creates new combinations of genes in the gametes that are not found in either parent, contributing to genetic diversity. Significance of Crossing Over a.It produces new individuals having new combinations of traits. b. Crossing over has helped in establishing the concept of linear arrangement of genes. c. The frequency of crossing over helps in the mapping of chromosomes. i.e., determining the location of the genes in the chromosomes. d. Selection of useful recombination by geneticists has brought about green revolution in our country. The Structure of DNA DNA is made up of six smaller molecules -- a five carbon sugar called deoxyribose, a phosphate molecule and four different nitrogenous bases (adenine, thymine, cytosine and guanine). Using research from many sources, including chemically accurate models, Watson and Crick discovered how these six subunits were arranged to make the the structure of DNA.. The model is called a double helix because two long strands twist around each other like a twisted ladder. The rails of the ladder are made of alternating sugar and phosphate molecules. The steps of the ladder are made of two bases joined together with either two or three weak hydrogen bonds Nucleotides The basic building block of DNA is called a NUCLEOTIDE. A nucleotide is made up of one sugar molecule, one phosphate molecule and one of the four bases. Here is the structural formula for the four nucleotides of DNA. Note that the purine bases (adenine and guanine) have a double ring structure while the pyrimidine bases (thymine and cytosine) have only a single ring. This was important to Watson and Crick because it helped them figure out how the double helix was formed. Watson Crick Model of DNA Structure In 1953 Watson and Crick surprised the scientific world with a concise one page paper in the British Journal Nature. The paper reported their molecular model of DNA, the double helix, which has since become the symbol of molecular biology. The beauty of the model was that its structure suggested the basic mechanism of DNA replication. Watson and Crick suggested ladder type organization of DNA. Each molecule of DNA is made up of two poly nucleotide chains which are twisted around each other and form a double helix. The uprights of the ladder are made up of sugar and phosphate part of nucleotide and the rungs are made up of paired nitrogenous bases. The pairs are always as follows. Adenine always pars with thymine and cytosine with guanine. There is no other alternate possible two polynucleotide chains which are complimentary to each other, are held together by hydrogen bonds. There are two hydrogen bonds between A = T, and three between C = G. Both polynucleotides strands remain separated by 2OA´´ distance. The coiling of double helix is right handed and complete turn occurs after 34A´´. SIGNIFICANCE OF BASE SEQUENCE Watson Crick model explained chargaff’s rules. Wherever one strand of DNA molecule has an A, the partner strand has a T and G is one strand is always paired with a C in the complimentary strand. Therefore in DNA of any organism, the amount of academic equals the amount of thymine and the amount of guanine equals the amount of cytosine. Although the basepairing rules dictate the combinations of nitrogenous bases that form the rungs of the double helix; they do not restrict the sequence of nucleotide along each DNA strand. Thus the linear sequence of four bases can vary in countless ways and each gene has unique order, or base sequence. Watson Crick model suggested that the basis or occupying the genetic information is complimentary one chain of DNA molecule may have any conceivable base sequence but this sequence completely determines that of its partner in the duplex. If the sequence of one chain is ATTGCAT, the sequence of its partner in duplex must be TAAGGTA. Each chain in duplex is a complimentary mirror image of the other. To copy the DNA molecule one need only unzip it construct new complimentary chain along each naked strand. DNA REPLICATION DNA replication is the process by which DNA makes a copy of itself during cell division. The first step in DNA replication is to ‘unzip’ the double helix structure of the DNA? molecule. This is carried out by an enzyme? called helicase which breaks the hydrogen bonds? holding the complementary? bases? of DNA together (A with T, C with G). The separation of the two single strands of DNA creates a ‘Y’ shape called a replication ‘fork’. The two separated strands will act as templates for making the new strands of DNA. One of the strands is oriented in the 3’ to 5’ direction (towards the replication fork), this is the leading strand?. The other strand is oriented in the 5’ to 3’ direction (away from the replication fork), this is the lagging strand?. As a result of their different orientations, the two strands are replicated differently Basic term of genetics ALLELES An alternative form of a gene that occurs at the same locus on homologous chromosomes, e.g., A, B, and O genes are alleles. ChromosomeRod-shaped structures within the cell nucleus that carry genes encoded by DNA. Co-dominantGenes are co-dominant if both alleles are expressed in the heterozygous state, e.g., K and k genes GameteA reproductive sex cell (ovum or sperm) with the haploid number (23) of chromosomes that results from meiosis. GeneA segment of a DNA molecule that codes for the synthesis of a single polypeptide. Genotype All of the alleles present at the locus (or closely linked loci) of a blood group system, indicating chromosomal alignment if appropriate, e.g., AO in the ABO BGS, CDe/cde in the Rh BGS, or MS/Ns in the MNSs BGS. Genotypes are indicated by superscripts, underlining, or italics. Haploid number of chromosomes the number of chromosomes found in sex cells, which in humans is. HeterozygousThe situation in which allelic genes are different, e.g. the Kk genotype in the Kell BGS or the Fya Fyb genotype in the Duffy BGS. Homologous chromosomes A matched pair of chromosomes, one from each parent, e.g., two #6 chromosomes. HomozygousThe situation in which allelic genes are identical, e.g., the KK genotype or the Fya Fya genotype. LocusThe location of allelic genes on the chromosome, e.g., A, B, and O genes occur at the ABO locus. (Plural. RecessiveGenes are recessive if the phenotype that they code for is only expressed when the genes are homozygous, e.g., le le genes, in the Lewis system or h h genes in the ABO BGS. = loci) The Mendelian Concept of a Gene In the 1860’s, an Austrian monk named Gregor Mendel introduced a new theory of inheritance based on his experimental work with pea plants. Mendel instead believed that heredity is the result of discrete units of inheritance, and every single unit (or gene) was independent in its actions in an individual’s genome. For any given trait, an individual inherits one gene from each parent so that the individual has a pairing of two genes. Mendel observed seven pea plant traits that are easily recognized in one of two forms: 1. 2. 3. 4. 5. 6. 7. Flower color: purple or white Flower position: axial or terminal Stem length: long or short Seed shape: round or wrinkled Seed color: yellow or green Pod shape: inflated or constricted Pod color: green or yellow Mendel's Laws are as follows 1. The Law of Dominance 2. The Law of Segregation 3. The Law of Independent Assortment The Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype. Law of Segregation The pair of alleles of each parent separate and only one allele passes from each parent on to an offspring which allele in a parent's pair of alleles is inherited is a matter of chance Segregation of alleles occurs during the process of gamete formation (meiosis) randomly unite at fertilization Law of independent assortment law of independent assortment. It states that the alleles of one gene sort into the gametes independently of the alleles of another gene. Since we're talking about a cross with double heterozygotes, what we're monitoring here in the F1 generation is called a dihybrid cross. That is a cross between individuals that areheterozygous at two different loci. Adrian expects that his F1 cross will produce a 3:1 ratio between the dominant and recessive traits. However, he's perplexed to observe that instead he sees a 9:3:3:1 ratio among four different phenotypes. Chromosomes X and Y and Sex Determination In a human, the normal chromosomes complement is 46, 44 of which are autosomes while 2 distinct chromosomes are deemed sex chromosomes, which determine the sex of an organism and various sex linked characteristics. In most animals, those who possess XX chromosomes are female while male animals possess an X and a Y chromosome. However, this is not true of all organisms, as it can be reversed in some species. Sex Chromosomes A humans' sex is predetermined in the sperm gamete. The egg gamete mother cell is said to be homogametic, because all its cell possess the XX sex chromosomes. sperm gametes are deemed heterogametic because around half of them contain the X chromosome and others possess the Y chromosome to compliment the first X chromosome. In light of this, there are two possibilities that can occur during fertilisation between male and female gametes, XX and XY. Since sperm are the variable factor (i.e. which sperm fertilises the egg) they are responsible for determining sex. Chromosomes X and Y Chromosomes X and Y do not truly make up a homologous pair. They act similarly in their roles, but they are not homologous (the same). The X chromosome in humans is much longer than the Y chromosome and also contains many more genes. These genes are said to be sex linked, due to the fact they are present in one of the sex chromosomes. During fertilisation, when the opposing homologous chromosomes come together, the smaller Y chromosome offers no dominance against the 'extra' X chromosomes as indicated below. Chromosomes X and Y do not truly make up a homologous pair. They act similarly in their roles, but they are not homologous (the same). The X chromosome in humans is much longer than the Y chromosome and also contains many more genes. These genes are said to be sex linked, due to the fact they are present in one of the sex chromosomes. During fertilisation, when the opposing homologous chromosomes come together, the smaller Y chromosome offers no dominance against the 'extra' X chromosomes . The arrows indicate sex linked genes in the X chromosome. In this homologous pairing, all those genes are dominant, because there are no opposing genes in the Y chromosome to offer dominance. So when the organism has an XY chromosome compliment (i.e. a male), these sex linked genes are freely expressed in the organisms phenotype, an example being hairy ears developing in old age. Sex Linked Characteristics These sex linked genes on the X chromosome display a number of characteristics. The following are just some examples of phenotypes as a result of these genes in expression; Red-Green colour blindness Haemophilia - A condition which prevents the clotting of the blood Hairy ears in men through advancing age More information on sex linked characteristics and how they are passed on from generation to generation will be available in new areas of the site soon. The next page looks at genetic mutations and the consequences as a result of them. X-linked recessive condition Red-green color blindness Red-green color blindness simply means that a person cannot distinguish shades of red and green (usually blue-green). Their visual acuity (ability to see) is normal. There are no serious complications; however, affected individuals may not be considered for certain occupations involving transportation or the Armed Forces where color recognition is required. Males are affected 16 times more often than females, because the gene is located on the X chromosome. Hemophilia A Hemophilia A is a disorder where the blood cannot clot properly due to a deficiency of a clotting factor called Factor VIII. This results in abnormally heavy bleeding that will not stop, even from a small cut. People with hemophilia A bruise easily and can have internal bleeding into their joints and muscles. Hemophilia A is seen in one in 10,000 live male births. Treatment is available by infusion of Factor VIII (blood transfusion). Female carriers of the gene may show some mild signs of Factor VIII deficiency such as bruising easily or taking longer than usual to stop bleeding when cut. However, not all female carriers present these symptoms. One third of all cases are thought to be new mutations in the family (not inherited from the mother). Sickle-cell Anemia Sickle-cell anemia, also called sickle-cell disease, is a hereditary dis order in which abnormal hemoglobin * within the red blood cells (RBCs) causes the cells to take on abnormal sickle (crescent) shapes. This decreases the ability of the hemoglobin to transport oxygen throughout the body. The sickled cells tend to bunch up and clog the blood vessels, and they tend to break apart more easily than normal RBCs. This may cause inflammation, pain, tissue damage, and anemia Diabetes Diabetes is a chronic condition associated with abnormally high levels of sugar (glucose) in the blood. Insulin produced by the pancreas lowers blood glucose. Genetic Engineering Genetic engineering is the process of manually adding new DNA to an organism. The goal is to add one or more new traits that are not already found in that organism. Examples of genetically engineered (transgenic) organisms currently on the market include plants with resistance to some insects, plants that can tolerate herbicides, and crops with modified oil content. Genetic Engineering Genetic engineering is the process by which scientists modify the genome of an organism. Creation of genetically modified organisms requires recombinant DNA. Recombinant DNA is a combination of DNA from different organisms or different locations in a given genome that would not normally be found in nature. In most cases, use of recombinant DNA means that you have added an extra gene to an organism to alter a trait or add a new trait. Some uses of genetic engineering include improving the nutritional quality of food, creating pest-resistant crops, and creating infection. Genetic Engineering Uses of Transgenic plants: In order to improve the quality and quantity of plants, traditional method of plant breeding is replaced by the creation of transgenic plants. The transgenic plants are plants carrying foreign genes introduced deliberately into them to develop a new character useful for the plant. The infection of plants by microorganism mostly viruses, poor production and decline in quality of plants due to attack by insects and the plants inability to withstand the pesticide or the weedicide used in the agriculture process welcomed the genetic engineering technology to develop transgenic plants with new characters like resistance to infections, defensive against the attacking insects and resistance to pesticides or weedicide(wild plant). Genetic Engineering Uses Transgenic animals: Transgenic animals are animals carrying foreign genes deliberately introduced into them and exhibiting the characteristics of the introduced gene. Animals are suitable for various research activities trying to help mankind. In that way transgenic animals are created to study human diseases to derive appropriate treatment methods and to develop and identify the drug useful to treat the disease. The presence of human proteins in milk of animals is made possible by genetic engineering. Gene transfer is done in animals to increase the milk production and to increase the growth. Genetic Engineering The recombinant proteins produced in the industry using the techniques of genetic engineering acts as drugs for various human diseases. To name a few, insulin produced for diabetes, alpha 1- antitrypsin in treating emphysema, calcitonin(a hormone secreted by the thyroid that has the effect of lowering blood calcium). to treat rickets(a disease of children caused by vitamin D deficiency).