Genetics mendelian.ppt
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Gregor Mendel conducted experiments breeding pea plants that varied in traits like flower color. By carefully tracking the inheritance of traits over generations, he discovered two laws of heredity: segregation and independent assortment. Mendel found that traits separate, or segregate, so offspring receive one allele for each trait from each parent. He also found that different traits assort independently, resulting in unpredictable combinations in offspring. Mendel's laws explained the patterns of inheritance he observed, like the 3:1 ratio of dominant to recessive traits in the second filial generation. His work established the foundations of classical genetics.
Genetics mendelian.ppt
- 1. Genetics • Gregor Mendel – Documented a particulate mechanism of inheritance through his experiments with garden peas PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Figure 14.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1
- 2. • Mendel used the scientific approach to identify two laws of inheritance • Mendel discovered the basic principles of heredity • Crossing pea plants 1 APPLICATION By crossing (mating) two true-breeding varieties of an organism, scientists can study patterns of inheritance. In this example, Mendel crossed pea plants that varied in flower color. TECHNIQUE Removed stamens from purple flower 2 Transferred sperm- bearing pollen from stamens of white flower to eggbearing carpel of purple flower Parental generation (P) – By breeding garden peas in carefully planned experiments 3 Pollinated carpel Stamens Carpel (male) (female) matured into pod 4 Planted seeds from pod When pollen from a white flower fertilizes TECHNIQUE RESULTS eggs of a purple flower, the first-generation hybrids all have purple flowers. The result is the same for the reciprocal cross, the transfer First generation of pollen from purple flowers to white flowers. offspring (F1) 5 Examined offspring: all purple flowers Figure 14.2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2
- 3. • Some genetic vocabulary – Character: a heritable feature, such as flower color – Trait: a variant of a character, such as purple or white flowers Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In a typical breeding experiment – Mendel mated two contrasting, true-breeding varieties, a process called hybridization • The true-breeding parents – Are called the P generation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 3
- 4. The Law of Segregation • The hybrid offspring of the P generation – Are called the F1 generation • When F1 individuals self-pollinate – The F2 generation is produced Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • When Mendel crossed contrasting, truebreeding white and purple flowered pea plants – All of the offspring were purple • When Mendel crossed the F1 plants – Many of the plants had purple flowers, but some had white flowers Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 4
- 5. • Mendel discovered • Mendel reasoned that – A ratio of about three to one, purple to white flowers, in the F2 generation EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by ×). The resulting F1 hybrids were allowed to self-pollinate or were crosspollinated with other F1 hybrids. Flower color was then observed in the F2 generation. P Generation (true-breeding parents) × Purple flowers White flowers – In the F1 plants, only the purple flower factor was affecting flower color in these hybrids – Purple flower color was dominant, and white flower color was recessive F1 Generation (hybrids) All plants had purple flowers RESULTS Both purple-flowered plants and whiteflowered plants appeared in the F2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white. F2 Generation Figure 14.3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 5
- 6. Mendel’s Model • Mendel observed the same pattern – In many other pea plant characters • Mendel developed a hypothesis – To explain the 3:1 inheritance pattern that he observed among the F2 offspring • Four related concepts make up this model Table 14.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 6
- 7. • First, alternative versions of genes • Second, for each character – Account for variations in inherited characters, which are now called alleles – A genetic locus is actually represented twice Allele for purple flowers Locus for flower-color gene Figure 14.4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings – An organism inherits two alleles, one from each parent Homologous pair of chromosomes Allele for white flowers Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 7
- 8. • Third, if the two alleles at a locus differ – Then one, the dominant allele, determines the organism’s appearance – The other allele, the recessive allele, has no noticeable effect on the organism’s appearance Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Fourth, the law of segregation – The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 8
- 9. • Does Mendel’s segregation model account for the 3:1 ratio he observed in the F2 generation of his numerous crosses? • Mendel’s law of segregation, probability and the Punnett square Each true-breeding plant of the parental generation has identical alleles, PP or pp. Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele. – We can answer this question using a Punnett square P Generation × Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: p P Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purpleflower allele is dominant, all these hybrids have purple flowers. F1 Generation When the hybrid plants produce gametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele. Gametes: This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F1 × F1 (Pp × Pp) cross. Each square represents an equally probable product of fertilization. For example, the bottom left box shows the genetic combination resulting from a p egg fertilized by a P sperm. Appearance: Genetic makeup: Purple flowers Pp 1/ p 2 1/ 2 P F1 sperm P p PP Pp F2 Generation P F1 eggs p pp Pp Figure 14.5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Random combination of the gametes results in the 3:1 ratio that Mendel observed in the F2 generation. 3 :1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 9
- 10. Useful Genetic Vocabulary • An organism that is homozygous for a particular gene – Has a pair of identical alleles for that gene – Exhibits true-breeding • An organism’s phenotype – Is its physical appearance • An organism’s genotype – Is its genetic makeup • An organism that is heterozygous for a particular gene – Has a pair of alleles that are different for that gene Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 10
- 11. The Testcross • Phenotype versus genotype Phenotype Purple 3 Purple • In pea plants with purple flowers Genotype PP (homozygous) – The genotype is not immediately obvious 1 Pp (heterozygous) 2 Pp (heterozygous) Purple 1 Figure 14.6 White pp (homozygous) Ratio 3:1 Ratio 1:2:1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 11
- 12. • A testcross – Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype – Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait • The testcross APPLICATION An organism that exhibits a dominant trait, such as purple flowers in pea plants, can be either homozygous for the dominant allele or heterozygous. To determine the organism’s genotype, geneticists can perform a testcross. × Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp If PP, then all offspring purple: TECHNIQUE In a testcross, the individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent. If Pp, then 1⁄2 offspring purple and 1⁄2 offspring white: p p p p Pp Pp pp pp RESULTS P Pp P Pp P p Pp Pp Figure 14.7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 12
- 13. The Law of Independent Assortment • Mendel derived the law of segregation – By following a single trait • The F1 offspring produced in this cross – Were monohybrids, heterozygous for one character Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Mendel identified his second law of inheritance – By following two characters at the same time • Crossing two, true-breeding parents differing in two characters – Produces dihybrids in the F1 generation, heterozygous for both characters Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 13
- 14. • How are two characters transmitted from parents to offspring? – As a package? – Independently? • A dihybrid cross – Illustrates the inheritance of two characters • Produces four phenotypes in the F2 generation EXPERIMENT Two true-breeding pea plants— one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F1 plants. Self-pollination of the F1 dihybrids, which are heterozygous for both characters, produced the F2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant. P Generation YYRR yyrr Gametes F1 Generation YR × Hypothesis of dependent assortment yr YyRr Hypothesis of independent assortment Sperm RESULTS 1⁄ YR 2 CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. Sperm yr 1⁄ 2 Eggs 1 F2 Generation ⁄2YR YYRR YyRr (predicted offspring) 1 ⁄ yr 2 YyRr yyrr 3⁄ 4 1⁄ 4 1⁄ 4 Yr 1 ⁄ yR 4 1⁄ 4 Phenotypic ratio 3:1 YR 1 ⁄ Yr 4 1 ⁄ yR 4 Eggs 1 ⁄ YR 4 1⁄ 4 yr 9⁄ 16 1⁄ 4 yr YYRR YYRr YyRR YyRr YYrr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr 3⁄ 16 Yyrr yyRr 3⁄ 16 yyrr 1⁄ 16 Phenotypic ratio 9:3:3:1 Figure 14.8 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 14
- 15. • Using the information from a dihybrid cross, Mendel developed the law of independent assortment – Each pair of alleles segregates independently during gamete formation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The laws of probability govern Mendelian inheritance • Mendel’s laws of segregation and independent assortment – Reflect the rules of probability Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 15
- 16. The Multiplication and Addition Rules Applied to Monohybrid Crosses • The multiplication rule • Probability in a monohybrid cross – Can be determined using this rule – States that the probability that two or more independent events will occur together is the product of their individual probabilities Rr Rr × × Segregation of alleles into eggs Segregation of alleles into sperm Sperm R 1⁄ 2 R R 1⁄ 2 r Figure 14.9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1⁄ 4 1⁄ 4 r r R R Eggs 1⁄ 2 r 1⁄ 2 R 1⁄ 4 r r 1⁄ 4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 16
- 17. • The rule of addition – States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Solving Complex Genetics Problems with the Rules of Probability • We can apply the rules of probability – To predict the outcome of crosses involving multiple characters Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 17
- 18. • A dihybrid or other multicharacter cross – Is equivalent to two or more independent monohybrid crosses occurring simultaneously • In calculating the chances for various genotypes from such crosses • Inheritance patterns are often more complex than predicted by simple Mendelian genetics • The relationship between genotype and phenotype is rarely simple – Each character first is considered separately and then the individual probabilities are multiplied together Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 18
- 19. Extending Mendelian Genetics for a Single Gene The Spectrum of Dominance • The inheritance of characters by a single gene • Complete dominance – May deviate from simple Mendelian patterns Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings – Occurs when the phenotypes of the heterozygote and dominant homozygote are identical Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 19
- 20. • In codominance – Two dominant alleles affect the phenotype in separate, distinguishable ways • In incomplete dominance – The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties P Generation Red C RC R • The human blood group MN White CWCW × Gametes CR CW – Is an example of codominance Pink C RC W F1 Generation Gametes Eggs F2 Generation Figure 14.10 CR 1⁄ 2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1⁄ 2 Cw 1⁄ 2 1⁄ 2 CR 1⁄ 2 CR CR 1⁄2 CR Sperm CR CR CR CW CR CW CW CW Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 20
- 21. Multiple Alleles • The Relation Between Dominance and Phenotype • Most genes exist in populations – In more than two allelic forms • Dominant and recessive alleles – Do not really “interact” – Lead to synthesis of different proteins that produce a phenotype Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 21
- 22. • The ABO blood group in humans – Is determined by multiple alleles pleiotropy- single gene controls more than one character eg. one gene affects corolla, anther, calyx, leaf and capsule of tobacco Table 14.2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 22
- 23. Epistasis • In epistasis Epistasis – Cucurbita pepo – A gene at one locus alters the phenotypic expression of a gene at a second locus Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 23
- 24. Extending Mendelian Genetics for Two or More Genes • An example of epistasis BbCc • Some traits × – May be determined by two or more genes BbCc Sperm 1⁄ 4 BC 1⁄ 4 bC 1⁄ 4 Bc 1⁄ 4 bc Eggs 1⁄ 4 BC BBCC BbCC BBCc BbCc 1⁄ 4 bC BbCC bbCC BbCc bbCc 1⁄ 4 Bc BBCc BbCc BBcc 1⁄ 4 bc BbCc bbCc Bbcc 9⁄ 16 3⁄ 16 Bbcc bbcc 4⁄ 16 Figure 14.11 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 24
- 25. Polygenic Inheritance Polygenic inheritance • Many human characters – Vary in the population along a continuum and are called quantitative characters Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 25
- 26. • Quantitative variation usually indicates polygenic inheritance – An additive effect of two or more genes on a single phenotype AaBbCc × AaBbCc aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC Fraction of progeny 20⁄ 64 15⁄ 64 Figure 14.12 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 6⁄ 64 1⁄ 64 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 26
- 27. Linked genes- genes are on the same chromosome • Eukaryotic cells store hereditary information in the nucleus • Nucleid acids- DNA Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Transfer of nucleic acids à transfer of hereditary traits Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 27
- 28. DNA STRUCTURE Chargaff’s rule • Polymer of nucleotide • nucleotide – Sugar + phophate group + nitrogen containing bases • Nitrogen containing bases – PURINE- G and A PYRIMIDINE- C and U (RNA) or T (DNA) ¡ Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A= T; G=C Equal proportion of purines and pyrimidines Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ¡ 28
- 29. DNA DNA REPLICATION • Double helix structure • During S-phase • Anti parallel • Semi-conservative ¡ 2 chains of nucleotides held by H-bond Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 29
- 30. DNA polymerase Okazaki • Need primers (RNA) – Will be replaced later Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Synthesis is discontinuous Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 30
- 31. THE CENTRAL DOGMA TRACES THE FLOW OF GENE-ENCODED INFORMATION Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 31
- 32. Central Dogma • Most hereditary traits reflect the action of enzymes • Info for the structure of an enzyme à DNA • GENE – Specific region in the DNA that codes for an enzyme Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 32
- 33. The Products of Gene Expression: A Developing Story Basic Principles of Transcription and Translation • As researchers learned more about proteins • Transcription – The made minor revision to the one gene–one enzyme hypothesis • Genes code for polypeptide chains or for RNA molecules – Is the synthesis of RNA under the direction of DNA – Produces messenger RNA (mRNA) • Translation – Is the actual synthesis of a polypeptide, which occurs under the direction of mRNA – Occurs on ribosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 33
- 34. • In eukaryotes • In prokaryotes – Transcription and translation occur together – RNA transcripts are modified before becoming true mRNA Nuclear envelope TRANSCRIPTION DNA DNA TRANSCRIPTION mRNA Ribosome Pre-mRNA RNA PROCESSING TRANSLATION mRNA Polypeptide Ribosome (a) Prokaryotic cell. In a cell lacking a nucleus, mRNA produced by transcription is immediately translated without additional processing. Figure 17.3a Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings TRANSLATION Polypeptide Figure 17.3b (b) Eukaryotic cell. The nucleus provides a separate compartment for transcription. The original RNA transcript, called pre-mRNA, is processed in various ways before leaving the nucleus as mRNA. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 34
- 35. TRANSCRIPTION Heterogeneous nuclear RNA (hnRNA) • DNA sequence in the gene is transcribed into an RNA sequence • RNA polymerase • promoter Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 35
- 36. Ribonucleic acid (RNA) Codons • Messenger RNA (mRNA) • DNA encodes for sequence of a.a. in proteins – Transcripts of gene used to direct a.a. assembly into proteins • Ribosomal RNA (rRNA) • DNA à mRNA transcripts • Ribosomes read sequence in increments of 3 nucleotides à CODON – Combine with proteins to make up the ribosomes • Transfer RNA (tRNA) – Transport a.a. to ribosomes • GATTACA A A (DNA) • CUAAUGU U U (mRNA) • CUA-AUG-UUU Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 36
- 37. TRANSLATION • The ribosome has three binding sites for tRNA • mRNA transcript is translated into a.a. – The P site • mRNA binds with rRNA in ribosomes – The A site • One codon is exposed at a time – The E site P site (Peptidyl-tRNA binding site) A site (AminoacyltRNA binding site) E site (Exit site) Large subunit E mRNA binding site Figure 17.16b Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings P A Small subunit (b) Schematic model showing binding sites. A ribosome has an mRNA binding site and three tRNA binding sites, known as the A, P, and E sites. This schematic ribosome will appear in later diagrams. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 37
- 38. tRNA The GENETIC code • Carries a particular a.a. • Anticodon Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 38
- 39. Translocation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 39
- 40. Termination • When nonsense codon is exposed Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 40
- 41. • The norm of reaction – Is the phenotypic range of a particular genotype that is influenced by the environment Figure 14.13 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 41
- 42. Integrating a Mendelian View of Heredity and Variation • Multifactorial characters – Are those that are influenced by both genetic and environmental factors • An organism’s phenotype – Includes its physical appearance, internal anatomy, physiology, and behavior – Reflects its overall genotype and unique environmental history Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 42