PRELIM_GENETICS.pdf

Principle of Independent Assortment - Inheritance of one trait has no effect on the LESSON 1: inheritance of another trait I...

Principle of Independent Assortment - Inheritance of one trait has no effect on the LESSON 1: inheritance of another trait INTRODUCTION TO TRAITS GENETICS Genetics - study of how traits are passed from parent to offspring Genetics: The Science of Heredity THEORIES and Variation Theory of Pangesis by Aristotle (384-322 Genetics - branch of biology that deals with BC) - Semen was formed everywhere in a principles of heredity and variation in all living man's body and such semen reflected that things. characteristics of the body part from where it Fundamental properties and problems was form of life and living, thus imprinting on all aspects of biology. Theory of Inheritance and Acquired Characteristics by Jean Baptiste de Lamarck Genetics was derived from the Greek word (1744-1829) - This proposed to be the gen, meaning to become or to grow into fundamental mechanism of evolutionary something. The term was coined by William change Baterson in 1906 Body modification acquired by use or disuse could be transmitted to the offspring because semen formed Beginning of Genetics Gregor Mendel (1822-1884) - Discovered Germ Plasm Theory by August Weisman that hereditary characteristics were (1834-1914) - Germplasm/sex cells determined by elementary "factors" that are perpetuated themselves in reproduction transmitted between generations in uniform generation after generation. predictable fashion. Somatoplasm was produced by the germplasm only as a means to protect Attributes of Gene and reproduce itself. 1. lt is inherited from generation to generation Kolreuter (1733-1806) - although hybrids 2. It provides information regarding the between species would produce considerable structure, function, and other diversity Gartner (1772-1850), Naudin Who is Gregor Mendel? - "Father of Genetics" (1815-1899) Gregor Mendel - Father of Genetics Carl Corens (Germany) and Erick Von The choice of Psium sativum L. as - Ischmermark and Hugo de Vries experimental material. (Netherlands (1900) Mendel concentrated on a single trait. - Duplicated Mendel's work on different He made sure that strains or varieties plants; provided follow-ups works used in hybridization work were true - Rediscovery of Mendel breeding by growing them for two years. He then selected as parental materials only those that produced progenies that William Bateson, Saunders and resembled them. Cuenot Mendel introduced the quantitative - Applied principles in animals approach of classified hybrid progenies and determined their respective frequencies. Walter Sutton (USA) and Theodore Mendel formulated theories that explained his experimental results. Boveri (Germany (1903) He formulated theories that explained his Segregation of pairs of factors that Mendel experimental results. postulated during gamete formation is paralleled by the separation of homologous chromosomes during meiosis. Thomas Hunt (1910) and Calvin Bridge (1916) - Discovery of sex chromosomes and association between specific genes and chromosomes. - Each chromosome contained one but many genes. Oswald Avery, Collin McLeod and Maclyn McCarty - Identified DNA as the hereditary material James Watson and Francis Crick LESSON 2: (1953) - Identify DNA's chemical properties CHROMOSOMAL BASIS OF HEREDITY SCOPE OF GENETICS Individual - The development and maintenance of its own unique, inherent pattern in dynamic interplay with the environment are the central problems of life. Species - the ability to transfer these systems to the other generations is the primary requirement for continued existence. Living forms - the orderly variety of patterns and their changes with time on geological Cytoplasm scale constitute the accomplishment of organic evolution. Mitochondria Golgi apparatus - netlike staining bodies The definition of similarities and differences in commonly found in cells engaged in patterns encountered within human species secretion. and the degree of plasticity in these systems, Endoplasmic reticulum - double walled are basic to human understanding and membrane folded in layers that appear to important to human welfare. be connected with cell membrane. Ribosomes - small particles floating in cytoplasm Applications of Genetics Centrosome and accompanying centriole - organelle that duplicates itself and 1. Plant, Animal and Microbial shows continuous inheritance between environment cell generations. 2. Medicine Chloroplast 3. Genetic counselling 4. Legal applications. Nucleus Primary director of cellular activity and inheritance.  Surrounded by a double membrane that appears in contact with the endoplasmic Acrocentric chromosomes have a centromere reticulum and cell membrane. which is severe v oftset from the center Chromatin - chromosomes leading to one very long and one very short Nucleoli may be found attached to section. specific chromosome regions. Telocentric chromosomes have the centromere at the very end of the chromosome. 2. Secondary Constriction - Pinching off a small chromosomal section forms the satellite. This is often associated with regions where the nucleolus is formed or attached. Chromosome structure 3. Nucleolus-Organizing region 1. Centrosome or Primary Constriction - The organization of nucleolus is the function of a specific point on a Permanent well defined region on the particular chromosome. When the chromosome where kinetochore proteins nucleolus is visible. it can be seen to be are attached to. attached to this region. The The spindle fibers bind to kinetochore chromosome where this region is and the depolymerisation of the spindle located is known as the nucleolus fibers enables the chromosome to move organizer. to opposite poles. The position of the centromere along the length of the chromosome during cell 4. Chromomeres and knobs division: - string characteristic particles or Metacentric Chromosomes - have the unequal size and unequal sizes are centromere in the center, such that both unequal distance apart. sections are of equal length - The smaller "bead of string" are called chromomeres and larger ones are Submetacentric chromosomes have the called "knobs" centromere slightly offset from the center leading to a slight asymmetry in the length or the two sections Cell division - Shorter than G-1 and S phases. Mitosis Mechanism of cell division bv which the M-Phase genetic and chromosome composition of a cell is faithfully reproduced in each - Structural changes of chromosomes daughter cell. are visible Growing cells undergo cell cycle which - Subdivided into stages - prophase, consists of 4 distinct phases: metaphase, anaphase, telophase G1, S, G2 and M Interphase - non mitotic stage Cell Division Prophase Chromosomes condense enough to be seen with a light microscope. Spindle forms between the 2 centrioles. Spindle fibers attach to kinetochores. G-1 phase - First gap period and longest phase of cell cycle - The cell increases in volume and building new protoplasm and organelles; secretion granules and cewa mareras are aço enorared S-phase - DNA replication Metaphase G-2 Phase - of alignment chromosomes in the cell - Synthesis of RNA and proteins plate along of necessary for chromosome synthesis - Fibers attached to kinetochores on and for mitotic spindle. both sides of each chromosome.  - Chromosomes unwind. Cytokinesis Anaphase - Division of the cytoplasm. - Separation of the sister chromatids. - Two complete diploid cells that are identical to the original cell Centromere - DurIng cytokinesis in animal cells. the Chromatids moves apart toward the cell pinches in two. opposite poles - A cleavage furrow produced by Disassembly of the tubulin subunits microfilaments deepens until the cell shortens the microtubules. splits. - In plant cells, cytokinesis is accomplished by formation of a cell plate. Telophase - Re-formation of the nuclei once the chromosomes are at opposite poles.  Consequences of Mitosis Chromosomes are reproduced and transmitted equally to daughter cells so that these are identical to each other and to their parent cell. Gene arises only by replication of pre-existing genes. A different allele may arise from a gene only by mutation. Both genes and chromosomes are capable of mutation and involved in replicating new The gene retains its individuality regardless of the nature of its allele. The chromosomes also retain their individuality as indicated by the following proofs: ○ Chromosomes hold the same relative position to late anaphase to telophase as they enter the next prophase in Meiosis succeeding mitosis. Chromosome number of the cells is ○ The parts of the chromosome reduced to half its usual number (Sexually associated with the nucleolus remain reproducing organism). associated during that interval This is also preceded by G-1, S and G-2 phases of the cell cycle. Homologues that have mutated retain their This consists of 2 nuclear divisions separate difference, mitosis after following each other in rapid mitosis. consequence: Meiosis I (reduction division); and Meiosis II ( equation division). Cytokinesis During cytokinesis in animal cells, the cell pinches in two. Meiosis I (Reduction Division) A cleavage furrow produced by Prophase I, Metaphase I, Anaphase I microfilaments deepens until the cell and Telophase I are observed in splits. Meiosis. Prophase I - substage of prophase I are leptotene, zygotene, pachytene diplotene and diakinesis.  and their coiled nature is very Leptotene Stage - The chromosomes are apparent. Later the chiasmata appear slender, long with many bead-like structures to move toward the end, in the process along their length known as terminalisation. Zygotene Stage - The pairing of homologous chromosomes (or chromosomes identical in Diakinesis their genetic loci and their visible structure) - The chromosomes become more begins at this stage. contracted and assume unique The paired homologous chromosomes configurations due to the repulsion of from bivalent (II). the chromatid pairs. Synapsis is very precise so that it is - During this stage, the nucleolus begins homologous by chromosome and to disintegrate and the spindle proceeds in a zipper-like fashion. formation begins. The bivalents are Synaptinemal complex = Think distributed evenly in the nucleus. crossing over Pachytene Stage - The chromosomes are thickened owing to coiling and are closely appressed that they may not be resolved. Each chromosome or bivalent consists of four chromatids. During this stage chromatid breaks occur and are repaired - formation of chiasma at the point of exchange. During this stage, the nucleolus is particularly evident and certain Metaphase I - The nuclear membrane chromosomes are attached to it: disintegrates and the spindle appears. The nucleolus organizer. bivalent moves to the metaphase plate where they become oriented properly. Diplotene Stage - The longitudinal separation of bivalents initiates diplonema. The Anaphase I - The chromosome move from the homologues separate starting from the metaphase plate to the poles during centromere and proceed toward both ends Anaphase I except at the chiasmata. Accounts for the reductional phase of At this stage, the synaptinemal Meiosis I complex is no longer functional, hence the separation strands. The Telophase I - The chromosomes regroup and chromosomes are actively shortening their coiled structure begin to relax.  Cytokinesis In some species, a cross wall is developed at Meiosis (Equational Division) the Metaphase I plate at the end of Meiosis I Prophase II - This stage is similar to mitotic and a second wall at right angle to the first prophase except that it has the chromosome develops after Meoisis, two walls develop number. The chromosomes appear as double simultaneously after Meiosis II structure. The nucleolus and the nuclear membrane disintegrate during Consequences of Meiosis Prophase I. Meiosis II is equational division. Each of the Metaphase II - Two spindles are formed in the two daughter nuclei of Meiosis I undergo position of the nuclei and the chromosomes mitosis, producing four haploid: align on the equational plate of their respective spindle. Because of reductional division in meiosis I, meiosis makes possible the conservation of Anaphase II - The daughter chromosomes the chromosome number from generation to move toward the opposite poles. generation in the sexually reproducing The chromosomes at this stage are organisms. The haploid gametes restore the more like the chromosomes at mitotic diploid number after fertilization. anaphase. At Telophase I, each paternal and maternal Telophase II - The chromosomes uncoil and chromosome has equal probability of being lengthen. The nuclear membrane membrane located at one or other daughter nucleus. and the nucleolus reappear. consequently paternal and maternal Each of the four daughter cells is a chromosomes may be combined in each haploid. gamete. At the end of meiosis, each of the four daughter cells contains one representative of each pair of chromosomes present in the nucleus at the start of meiosis Crossing-over between non-sister chromatids is the mechanism by which genes maybe shuffled and exchanged. Lifecycles Intermediary or sporic meiosis The general pattern of eukaryotic life cycle 1. Diploid phase, which is characterized by a series of mitotic divisions, followed by.. 2. Meiosis, the first step in gamete formation, which may be followed by.. 3. A series of haploid mitotic divisions and then, 4. Fertilization or fusion of 2 gametes or haploid nuclei, thus restoring the diploid prase Initial or zygotic meiosis Terminal or gametic meiosis  during gamete formation in the F1, so LESSON 3: GENE that gametes carry R and others r. SEGREGATION AND These type of gametes occur at equal frequencies in the and pollen grains. INTERACTION Self fertilization of the F1 causes Phenotype - Appearance of an organism - random combinations of the male and morphology, physiology and behavior female gametes to for F2. genotypic ratio Genotype - Genetic constitution that an Phenotypic ratio individual inherits The inheritance of seed characteristics follow complete dominance. The homozygous Law of Segregation Dominant (RR) and heterozygous (Rr) The In Mendel's work on garden peas: same phenotype. The parental plants (P generation) were form pure breeding lines or varieties At formation of gametes, the two (Homozygous) chromosomes or each pair separate Cross between round-seeded plants and (segregate) into two different cell which form wrinkled seeded once yielded a first the gametes. This is a universal law and generation progeny (r_ = round seeds. always during gamete formation in all sexually When F1 plants were self fertilized - reproducing organisms, the two factors of a examples of round and wrinkled seeds pair pass into different gametes. Each gamete appear in the F2 receives one member of a pair of factors and that gametes are pure. Summary of Mendel's hybridization works: For any character. the F1 showed one alternative trait. Such character that was shown was dominant and a character that was hidden was recessive. reciprocal crosses gave the same results. The trait did not appear in the F1 reappeared in the F2, but in a frequency of ¼ of the total number Parents Contributed equally to the progeny. F2-3:1 segregation of dominant and recessive trait F1 contains r alternative factors or is heterozygous. Rr. These 2 factors (alleles) segregate from each other Law of Independent Assortment When 2 or more genes/pairs of alleles are considered simultaneously, 2nd law applies. States that genes for different characters are inherited independently of one another or alleles of different gene pairs separate independently from each other and random combine during meiosis. Chromosomal Basis of Mendelian Laws Walter Sutton and Theodor Boyeri (1900) found correlations between the behaviour of the allele in a gene pair and homologous chromosomes during meiosis. The alleles exist in pair, the homologous chromosome also exist in pairs.; if alleles in a gene pair separate, the homologous chromosome also separate at anaphase I or meiosis: and alee and the chromosome are in pairs immediately after fertilization.  When there are 2 gene differences, an independent assortment of genes can also be observed. However, the ratios will not be the same as those of the diploid organisms. Segregation and Assortment in Haploid Organisms One advantage of using microorganisms like Chlamydomonas and Neurospora is In some microorganisms, the major portion of their ability to recover all haploid meiotic the lifecycle is in the haploid stage. They products of an individual zygote. become diploid during sexual conjugation, which is immediately followed by meiosis and the consequent formation of new haploid clones. Phenotypically, each allele is masked Dominance Relationships by an allelic mate and segregation and ~ Incomplete dominance or mo dominance assortment between genes give different Dominance is absent and the progeny does phenotypic ratios from those diploid not resemble any of its parent. The F1 are organisms. intermediate between 2 parents. - Example: Chlamydomonas reinhardi In the four O'clock plant Mirabilis jalapa and Snapdragon or Antirrhinum law of dominance is not followed. Overdominance - The heterozygote exceeds the phenotypic measurements of the homozygous parents. Co-dominance Lethal Genes - When each allele of a gene is Lethality may exist as recessive lethals or associated with a specific substance, dominant lethals. codominance Recessive Lethals - those that are lethal will occur when both substances appear when in homozygous recessive together in the heterozygote. condition. These genes mav have dominant and recessive phenotypic effect. Lethal genes, which have dominant phenotypic effects in the heterozygote, are lethal homozygous recessive conditions. Heavy freckling in humans. Multiple alleles Dominant lethals - genes whose lethal effects occur when a dominant allele is In his system, the alleles act within the same present in homozygous or heterozygous phenotypic range and are called isoalleles. conditions Many or such have been discovered - mutant isoalleles; normal alleles. ○ Epiloia ○ Huntington's Disease Penetrance of lethal gene - Recessive or dominant penetrance may vary. Some lethal genes have high degree of penetrance and expression, allowing little or no survival among affected genotypes beyond embryonic stage. Others are called semI-lethals permit a large proportion of affected genotypes to survive. Environmental influence on lethal genes - Genes may be influenced by the environment that the organism is able to survive under permissive conditions and cause lethality under restrictive conditions. Several nattems: - Occurrence of lethality by exposure of the organism to restrictive condition  during specific period of development (monophasic) - Lethality because of exposure to restrictive conditions during 2 or more (polyphasic) developmental stages. - PossessIon by letha-bearing males and females or different temperature-sensitive stages. - Occurrence of lethality by exposure to restrictive conditions at any stages of development Recessive Epistasis - There is complete dominance in both Modifier Genes gene pairs, but one gene, Modifiers are genes that change the - When homozygous recessive hides or phenotypic effects of other genes in masks the effect of the other. quantitative fashion. As a rule, modification is achieved by either dilution or enhancement of the effects of maior genes. Modifier of mutant genes also exist and in some cases, they completely suppress the phenotypic expression of mutant gene - Suppressors. Dominant epistasis Gene Interactions - There is complete dominance at both gene pairs but one gene, when Novel phenotypes dominant, masks the effect of the - There is complete dominance in both other. gene pairs. New phenotypes results in interaction between dominants and also interaction between homozygous recessives  - There is dominance at both gene pairs, Example: Seed capsule of Shepherd's but one gene, when dominant, is purse. epistatic to the second. The second gene, when homozygous recessive, is epistatic to the first. Pseudoalleles Complementary Genes There is complete dominance in both gene pairs, but either recessive homozygote is epistatic to the effects of the other gene. Flower color in Pea. Genes P and C are required for pigment production. Environmental Influence on Gene Duplicate Genes Expression - There is complete dominance in both Variable interactions between a genotype and gene pairs but either gene when different environment dominant is epistatic to the other.  Genotype determines the range Internal environment phenotypes that mav develop - Range Effects that lead to phenotypic changes that or reaction appear correlated primarily to changes within the organism. Penetrance - refers to the proportion Age or genotypes that show an expected Age of onset of different genetic traits phenotype. ic is a statistical concept or in humans the regularity which gene is expressed. Sex Expressivity - degree to which a Some traits are sex limited - appear in particular phenotypic effect is one sex and not in the other expressed by the individual. Sex controlled/sex influenced traits - harelip, gout (men); spina bifida Pleiotropy - a situation in which one gene has (women) multiple phenotypic effects. Substrates - Synthesized by the Phenylketonuria organism and their presence or Phenocopy - environmental mimic of absence maybe genetically controlled gene action. The environmental Diabetes mellitus influence is sufficiently strong so that the resulting phenotype simulates the effects of certain genes External Environment Temperature - close correlation between the rate of chemical reactions and temperature. Red color of primrose at room temperature and its white color at over 30°C. Twin Studies Light - Nutrition Yellow fat in rabbits depends on One of the ways to measure similarities and genotype y and green vegetables eaten. differences is to determine whether the Maternal relations - Blood group character is present or absent in one or both incompatibilities between mother and members. If both members show the offspring produce special effects on the character. the pair is concordant. If only one survival of particular genotypes. member of a pair shows the trait, then it is phenotypically dissimilar or discordant. The extent of concordance of twins may be used to measure the roles of environment and heredity expression or a certain phenotype. If the trait has high concordance in identical twins and low concordance in fraternal twins, then the trait may be considered to have strong hereditary elements. More equal concordance and discordance When 2 events are mutually exclusive, the ratios between the identical and fraternal probability that the individual will have one of groups would signify less hereditary influence the characteristics is equal to the sum of their and more environmental influence on probabilities. expression of certain traits ½ (A) + ½ (a) = 2/2 or 1 Level of Significance Sampling Errors - chance deviations from the expected values. In deciding whether to accept or reject the hypothesis. size of the discrepancy between the observed and expected ratios must be evaluated. one should decide how large discrepancy can be permitted before he suspects that something other than chance alone is involved in the deviations observed. If deviations can be shown to occur more often Probability and Statistical Testing than once in 20 trials, the observations are When the probability of an event is conventionally accepted as satisfactory fit to independent of that of the other event, so that the expected the occurrence of one doesn't interfere with the occurrence of the other, 1 the probability of both events occurring together is the product of their separate probabilities.  Normal Distribution The curve is high in the center, representing higher frequencies for the most common combinations: it tapers off equally at both extremes for the rarer combinations. Chi-square Test Binomial Distribution Probability that a certain combination can be described by binomial coefficient for this combination relative to the total number of possible combinations.