Lector 2 Topic II Transmission Genetics I 1-6-10

Lector 2 Topic II Transmission Genetics I 1-6-10 -...

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Unformatted text preview: BIS101-001 Genes and Gene Expression Lecture 2 Transmission Genetics I Mendel’s Principles: Meiosis, Chromosomes and Probabilities January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 1 Last Lecture: The Study of Biological Information ® Biological information, fundamental to life, is encoded in DNA. ® Biological function emerges primarily from proteins and RNAs. ® Complex biological systems emerge from regulatory networks that specify gene action. ® All life forms are closely related ® The modular nature of the genome facilitates rapid evolution ® Powerful analytical tools and philosophy of investigation, permitting the dissection of the genetic complexities for the good of humankind January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 2 This Lecture: Mendel and Chromosomes ® Mendel’s 1st and 2nd Laws of Inheritance. ® Definitions. ® Using crosses to infer the nature of inheritance. ® Test Crosses and recessive traits. ® Mitosis and Meiosis. ® Mendelian inheritance and Chromosome Theory. ® Cell cycle and chromosome replication. ® Product and Sum Rules. ® Application of Product and Sum Rules in genetic crosses. January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 3 Definitions ® ® ® ® ® ® ® ® ® ® ® ® ® ® ® ® ® ® ® ® Allele: An alternate form of the same gene, (e.g., the seed color gene in peas can be yellow or green). Character: Any specific, true-breeding property of an organism. Synonymous with "trait or "phenotype." true"phenotype." Dihybrid Cross: A cross between two individuals with different alleles at two gene loci. A dihybrid is a heterozygous individual with different alleles at two gene loci. Diploid: A eukaryote cell or organism having two sets of chromosomes (or a pair of genes at each locus). (or Dominant: An allele or trait that expresses its phenotype when heterozygous with a recessive allele, (e.g., A is dominant to heterozygous a because the phenotypes AA and Aa are the same). AA Aa Eukaryote: An organism or cell containing two sets of genes (diploid) for each character, where the entire genetic complement is contained within a well-defined nucleus. Prokaryotes are 1N and have no defined nucleus (e.g.,bacteria) welle.g.,bacteria) F1: First filial generation. The progeny of a cross between two parental types that differ in one or more genes. parental Gene: The basic unit of heredity that occupies a fixed chromosomal location (locus). The gene contains the information for location tRNAs, rRNAs and mRNA (i.e., proteins). tRNAs, Genotype: The specific allelic composition of a cell or set of genes. Haploid: A cell or organism having only one set of chromosomes (i.e., one gene at each locus or haploid). one Gamete: A haploid reproductive cell such as a sperm (or pollen) and egg (or ova). egg Heterozygous: A diploid with dissimilar alleles at one or more loci and therefore not true-breeding for the trait(s)at that therefore truelocus or loci. Homozygous: A diploid having the same allele at a given locus, therefore, true-breeding for the trait at that locus. trueLocus: The region along the length of a chromosome where the alleles (pair of genes) for a particular trait resides. Monohybrid Cross: A cross between two individuals with different alleles at one gene locus. A monohybrid is a heterozygous individual with different alleles at one gene locus. locus. N: The haploid chromosome number. Prokaryotes are 1N while most eukaryotes are 2N. eukaryotes Phenotype: The detectable outward biochemical or morphological manifestation of a specific genotype. The form a manifestation character or trait takes in a specific individual. Punnett Square: A cross-multiplication table used for determining the expected genetic outcomes of a mating. crossoutcomes Recessive: An allele or trait that does not express its phenotype in the heterozygous condition (e.g., a is recessive to A when the phenotype of Aa is the same as AA). Aa AA Testcross: A cross between an F1 individual (unknown genotype) and the homozygous recessive parent of the F1, the homozygous "tester." Because the genotype of the tester is known, the test cross reveals the genotype of the F1. BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 4 January 6, 2010 Patterns of Inheritance: Main Concepts ® The existence of genes can be inferred by observing standard inferred progeny ratios in the offspring of mating between two different phenotypes ® Discrete phenotypic differences in a trait can sometimes be due phenotypic to differences in a single gene ® Each type of gene is represented twice in each cell, one on twice each member of a chromosome pair. ® Inheritance patterns are based on chromosome behavior at meiosis meiosis ® During gamete formation, each member of a gene pair segregate producing two types of gametes segregate producing ® In gamete formation, gene pairs on different chromosome pairs behave independently of one another (unless they’re linked). independently January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 5 Milestones of Modern Genetics (p.14-16) ® 1853 Mendel begins pea breeding experiments Mendel's First Law: The Principle of Equal Segregation. During the formation of gametes, the paired characters (alleles) segregate (or separate) equally so that half of the gametes receive one allele and half of the gametes receive the other member of the pair. Mendel's Five Assumptions: (1) Hereditary determinants (genes) are particulate in nature (therefore there is no blending of characters). (2) Each adult pea plant has a pair of genes (alleles) for each character (as can be seen in the F1, one member can be dominant and one recessive) (3) Members of each gene pair segregate equally into the gametes (pollen/ovule, sperm/egg) (4) Consequently, each gamete carries one member of each gene pair (5) The union of gametes from each parent (to form the zygote) is random. January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 6 Mendel’s 1st Law: Segregation of round, wrinkled (see Fig. 2-12) (see Parents Gametes YY Y only Yy zygote X yy y only Points 1&2 Points 4 1st Filial Generation (F1) 1 Ovule produced 1/2Y 1/2Y Pollen produced 1/2y 1/4YY 1/4Yy 1/2y 1/4Yy 1/4yy Points 3 Points 5 Overall F2 Genotypic Ratio — 1YY : 2Yy : 1yy or 3:1 Phenotypic ratio 2 January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 7 Mendel’s Seven Characters ® Mendel’s Seven Characters: 7 phenotype pairs January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 8 Cross-pollination and selfing are two types of crosses January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 9 Mendel’s crosses resulted in specific phenotypic crosses January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 10 Pea Hybridization Techniques: Monohybrid Cross pollen placed on stigma ® Monohybrid Cross of peas with purple and white flowers anthers removed P purple white F1 All purple January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 11 Dominance and Recessive Traits ® The dominance or recessive nature of a character, trait or gene can be determine by a “test cross” involving the F1 and the homozygous recessive parent. ® Test crosses typically give 1:1 ratios for a monohybrid 1:1 cross involving a recessive trait. January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 12 Test cross is at right (F1 Y/y x F2 y/y) Test cross January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 13 Mendel’s strong inference based on large numbers ® Counting traits from his monohybrid crosses, Mendel inferred his famous 3:1 ratio, characteristic of his law of Equal Segregation. During the formation of gametes, the paired characters (alleles) segregate (or separate) equally so that half of the gametes receive one allele and half of the gametes receive the other member of the pair. January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 14 A single-gene model explains Mendel’s ratios January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 15 Mendel’s Second Law ® Independent Assortment: Gene pairs on separate chromosomes pairs, assort themselves independently at meiosis. ® This can be seen from a dihybrid cross between two dihybrid different characters (seed shape and seed color) on different chromosomes. ® 9:3:3:1 ratios are always characteristic of a dihybrid cross. January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 16 Mendel’s Law of Independent Assortment ® In the case of the 9:3:3:1 ratio produced from a dihybrid cross, Mendel discovered what looked like dihybrid an exception to the 3:1 ratio. The 9:3:3:1 ratio is, in fact, a mixture of two 3:1 ratios. ® In the case of two characters, the members of both pairs of alleles assort themselves independently into gametes. In the previous slide figure, the 16 genotypes can be broken down into 12:4 round-towrinkled phenotypes and 12:4 yellow-to-green phenotypes. In both instance we can see a 3:1 ratio. January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 17 Dihybrid Cross (2 monohybrid crosses) ® The result of a dihybrid cross are simply the combination of two, monohybrid crosses ® In this example, the ratio of “round” to “wrinkled” seeds is 12:4 or 3:1. January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 18 Dihybrid Cross (2 monohybrid crosses) ® The result of a dihybrid cross are simply the combination of two, monohybrid crosses ® In this example, the ratio of “yellow” to “green” seeds is 12:4 or 3:1. January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 19 Law of Independent Assortment: Summary ® Although undiscovered for nearly 40 years, Mendel's experiments laid the foundation for all future genetic analysis. His work made it possible to infer the existence and nature of hereditary material without ever seeing it. ® Furthermore, his laws of equal segregation and independent assortment anticipated what would later be found by microscopic observation in the nuclei of cells. January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 20 Mendel’s Second Law ® Mendel’s Law of Independent Assortment is also known as “interchromosomal recombination” and is observed when dominant and recessive gene pairs are located on separate chromosomes pairs or “unlinked.” ® As with the monohybrid cross, a “test cross” can be use to demonstrate the presence of recessive alleles in the F1 progeny January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 21 Dihybrid Test Cross ® Mendel’s crosses always produced an F1 that looked like only one of the two parents. For example, in the monohybrid cross RR x rr, the F1 RR progeny Rr looks like the round parent. The “test cross” (Fig. 2.18) can be Rr used to reveal the presence of the recessive allele in the F1. The test cross always involves crossing the F1 progeny with the homozygous recessive parent. For a monohybrid cross, the test cross progeny exhibit a 1:1 ratio of Rr and rr. In the dihybrid cross shown above, a 1:1:1:1 ratio of RrYy; Rr rr RrYy; Rryy; rrYy; rryy RY ry Ry rY ry RrYy Rryy rrYy rryy 1: 1:1:1 (these ratios occur only with “unlinked genes) January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 22 Chromosome Behavior (mitosis versus meiosis) ® The dihybrid cross reflects the behavior of chromosomes during meiosis. Chromosome mechanics during mitosis and meiosis was unknown to Mendel. Interphase Prophase Prometaphase Metaphase Cytokinesis Telophase Anaphase January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 23 Chromosome Behavior (mitosis-meiosis see Figs. 2-15) January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 24 Stages of the asexual cell cycle (Fig. 3-13) January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 25 Cell Cycle Control (p. 89 Fig. 4.7) G0 January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 26 S-Phase is DNA replication of chromosomes January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 27 Review of Chromosomes During Mitosis January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 28 Review of Chromosomes During Meiosis January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 29 Chromosome Theory of Inheritance: Chapter 4 ®Chromosome continuity. The chromosomes one observes during one metaphase are exactly the same as those observed in the next metaphase (in the nucleus of the daughter cell). ®During mitosis and meiosis, chromosomes divide along the length of the chromosomes. ®The pairing between chromosomes during prophase of meiosis is specific, not random. ®Chromosome individuality. Different chromosome pairs are qualitatively different (i.e., carry different genes). ®During meiosis, different chromosome pairs assort themselves independently (Mendel's dihybrid cross). ®A pair of characters (like those described in Mendel’s dihybrid cross) can assort themselves independently during anaphase I of meiosis. January 6, 2010 ® 1902: Sutton and Boveri propose the chromosome theory of inheritance. 30 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ Comparison of Mitosis and Meiosis Mitosis Interphase invisible replication invisible Prophase (4N) (4N) — — — — — Metaphase Meiosis Interphase invisible replication invisible Prophase I (4N) (4N) (leptotene: begining of condensation) (zygotene: invisible pairing) (pachytene: visible pairing) (diplotene: repulsion begins) (diakinesis: movement to poles) Metaphase I (visible bivalents) (visible January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 31 Comparison of Mitosis and Meiosis Mitosis Anaphase Telophase — — — — — 2 daughter cell with 2N complement of chromosomes Meiosis Anaphase I (early & late) (early Telophase (nucleus formation) (nucleus Interphase II (2N, no replication) (2N, Prophase II Metaphase II (single chromosomes (single at the metaphase plate) Anaphase II (centromeres separate) (centromeres Telophase II (nucleus forms) (nucleus 4 monads with (1N) haploid complement of chromosomes January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 32 Comparison of Mitosis and Meiosis:10 Points ®The products of mitosis are two daughter cells, each 2N. The products of meiosis are four gametes, each 1N. ®During mitosis “N” goes from 2N to 4N to 2N as a result of 1 replication or S phase and 1 cell division. In meiosis, “N” goes for 2N to 4N to 2N to 1N as a result of 1 replication or S phase but 2 cell division cycles. ®Unlike mitotic prophase, the separate chromosomes of meiosis zygotene prophase I are not distributed throughout the nucleus but paired into bivalents (paired homologs). The number of bivalents seen in this stage equal the haploid number of the organism. ®There is no pairing of homologous chromosomes in mitosis. In meiosis pairing of homologous chromosomes starts in zygotene and becomes visible in pachytene. ®In meiosis, the homologous chromosome pairs align themselves at the metaphase plate of metaphase I while in mitosis, chromosomes align at the metaphase plate separately, not in pairs. January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 33 Comparison of Mitosis and Meiosis ® In mitosis, centromeres divide at anaphase, In meiosis, centromeres divide during anaphase II, not anaphase I. ® S phase in cells about to undergo meiosis is longer than for mitosis. In the newt, Triturus, it takes 12 hours to go through S prior to mitosis while it takes 10 days in pre-meiotic cells. ® In meiosis, the fibers coming from the centromeres of the two sister chromatids of each chromosome are oriented toward the same pole in meiosis. In mitosis, fibers attached to each centromere for each chromatid are oriented to opposite poles. ® Prophase I of meiosis is much longer than prophase of mitosis. Depending on the organism, the pachytene and diplotene stages of prophase I may last weeks or years depending on the organism. ® Crossing over during the pachytene stage of prophase I of meiosis can result in some exchanges of genetic material between homologs. January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 34 Independent Assortment and Meiosis R r R r RY R y r y rY BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 35 January 6, 2010 Independent Assortment (Interchromosomal Recombination) January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 36 Selfing of Mendel’s dihybrid F1 produces F2 progeny in the ratio 9:3:3:1. January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 37 Mendel’s Cross Reveals a Probabilistic Reality ® The Punnett square at the Punnett left is just a graphical representation of probabilistic outcomes for different genotypes and phenotypes. ® The utility of Punnett squares breaks down, however, with the addition of each new trait to the cross (e.g., monhydrid = 4 squares, dihybrids = 16 squares, trihybrids = 64 squares or (2n)2 where n = the number of traits in the cross). January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 38 Mendel’s Cross Reveals a Probabilistic Reality ® The utility of Punnett squares breaks down, however, with the addition of each new trait to the cross (e.g., monhydrid = 4 squares, dihybrids = 16 squares, trihybrids = 64 squares or (2n)2 where n = the number of traits in the cross). January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 39 Mendel’s Cross Reveals a Statistical Reality (p. 21, 27) ® According to Mendel’s Laws, the segregation of alleles into gametes are independent events. ® Therefore, since the union of gametes is random, predicting the expected outcome for progeny can be determined using the product rule and the product the sum rule. January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 40 Product and Sum Rule ® The product rule states that the probability of independent product events occurring together is the product of the probabilities of the individual events themselves (also called the “and” rule). ® The sum rule states that the probability of either of two sum mutually exclusive events occurring is the sum of their individual probabilities (also called the “or” rule). ® The product rule is useful for calculating the expected frequency of a particular genotype while the sum rule is useful in calculating the expected frequency of a phenotype. January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 41 Product Rule ® The possible outcomes of rolling dice follow the product rule because the outcome on each separate die is independent of the others. For example, the probability of rolling a pair of 4s is calculate based on the probability of rolling one 4 on one die, which is 1/6 (because the die has six sides and only one side carries the 4). This probability is written as follows: p (of a 4) = 1/6 ® Therefore the probability of rolling a pair of 4s can be written: p (of two 4s) = 1/6 x 1/6 = 1/36 As you can see, the product rule deals with one independent event and another independent event occurring together (4 and and 5). and January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 42 Sum Rule ® The sum rule deals with the probability of one independent event or another occurring (A or B). We have already or determined that the probability of two 4s is 1/36. By the same way we can use the product rule to calculate that the probability of two 5s is also 1/36 (1/6 x 1/6). ® Now we can calculate the probability of either two 4s or two 5s occurring. Because these outcomes are mutually exclusive, the sum rule tell us that the probability of two 4s and two 5s is 1/18 (1/36 + 1/36). This probability can be written as: p (of two 4s or two 5s) = 1/36 + 1/36 = 1/18 As you can see, the sum rule deals with the probability of one independent event or another independent event occurring. or January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 43 Mendel’s Cross Reveals a Statistical Reality (Fig. 3-4) ® F1 RrYy (round, yellow) RrYy ® Gametes: RY; Ry; rY; ry p of R = 1/2 p of r = 1/2 p of Y = 1/2 p of y = 1/2 p of RY = 1/2 x 1/2 = 1/4 p of Ry = 1/2 x 1/2 = 1/4 p of ry = 1/2 x 1/2 = 1/4 p of rY = 1/2 x 1/2 = 1/4 p of RRYY = 1/4 x 1/4 = 1/16 p of wrinkled = 1/16+ 1/16+ 1/16+ 1/16 = 4/16 or 1/4 January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 44 Product Rule Example ® To create at homozygous tester plant line for five traits, a researcher had to make the following cross: A /a ; b /b ; C /c ; D /d ; E /e X A /a ; B /b ; C /c ; d /d ; E /e What is the probability of finding a plant with the following genotype: a /a ; b /b ; c /c ; d /d ; e /e A/a x A/a b/b x B/b C/c x C/c D/d x d/d E/e x E/e January 6, 2010 a/a = 1/4 a/a b/b = 1/2 b/b c/c = 1/4 c/c d/d = 1/2 d/d e/e = 1/4 e/e BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 45 Product Rule = 1/256 Product This Lecture: Mendel and Chromosomes ® Mendel’s 1st and 2nd Laws of Inheritance. ® Definitions ® Using crosses to infer the nature of inheritance. ® Test Crosses and recessive traits ® Mitosis and Meiosis. ® Mendelian inheritance and Chromosome Theory ® Cell cycle and chromosome replication ® Product and Sum Rules ® Application of Product and Sum Rules in genetic crosses. January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 46 ...
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