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Chapter 13&14

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  • 1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 13 Meiosis and Sexual Life Cycles
  • 2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Overview: Hereditary Similarity and Variation • Living organisms – Are distinguished by their ability to reproduce their own kind
  • 3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Heredity – Is the transmission of traits from one generation to the next • Variation – Shows that offspring differ somewhat in appearance from parents and siblings Figure 13.1
  • 4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Genetics – Is the scientific study of heredity and hereditary variation
  • 5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes
  • 6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance of Genes • Genes – Are the units of heredity – Are segments of DNA
  • 7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Each gene in an organism’s DNA – Has a specific locus on a certain chromosome • We inherit – One set of chromosomes from our mother and one set from our father
  • 8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Comparison of Asexual and Sexual Reproduction • In asexual reproduction – One parent produces genetically identical offspring by mitosis Figure 13.2 Parent Bud 0.5 mm
  • 9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In sexual reproduction – Two parents give rise to offspring that have unique combinations of genes inherited from the two parents
  • 10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 13.2: Fertilization and meiosis alternate in sexual life cycles • A life cycle – Is the generation-to-generation sequence of stages in the reproductive history of an organism
  • 11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sets of Chromosomes in Human Cells • In humans – Each somatic cell has 46 chromosomes, made up of two sets – One set of chromosomes comes from each parent
  • 12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 5 µm Pair of homologous chromosomes Centromere Sister chromatids Figure 13.3 • A karyotype – Is an ordered, visual representation of the chromosomes in a cell
  • 13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Homologous chromosomes – Are the two chromosomes composing a pair – Have the same characteristics – May also be called autosomes
  • 14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Sex chromosomes – Are distinct from each other in their characteristics – Are represented as X and Y – Determine the sex of the individual, XX being female, XY being male
  • 15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A diploid cell – Has two sets of each of its chromosomes – In a human has 46 chromosomes (2n = 46)
  • 16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In a cell in which DNA synthesis has occurred – All the chromosomes are duplicated and thus each consists of two identical sister chromatids Figure 13.4 Key Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) 2n = 6 Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Centromere
  • 17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Unlike somatic cells – Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes
  • 18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Behavior of Chromosome Sets in the Human Life Cycle • At sexual maturity – The ovaries and testes produce haploid gametes by meiosis
  • 19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • During fertilization – These gametes, sperm and ovum, fuse, forming a diploid zygote • The zygote – Develops into an adult organism
  • 20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.5 Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46) • The human life cycle
  • 21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Variety of Sexual Life Cycles • The three main types of sexual life cycles – Differ in the timing of meiosis and fertilization
  • 22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In animals – Meiosis occurs during gamete formation – Gametes are the only haploid cells Gametes Figure 13.6 A Diploid multicellular organism Key MEIOSIS FERTILIZATION n n n 2n2n Zygote Haploid Diploid Mitosis (a) Animals
  • 23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings MEIOSIS FERTILIZATION n n n n n 2n 2n Haploid multicellular organism (gametophyte) Mitosis Mitosis Spores Gametes Mitosis Zygote Diploid multicellular organism (sporophyte) (b) Plants and some algaeFigure 13.6 B • Plants and some algae – Exhibit an alternation of generations – The life cycle includes both diploid and haploid multicellular stages
  • 24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings MEIOSIS FERTILIZATION n n n n n 2n Haploid multicellular organism Mitosis Mitosis Gametes Zygote (c) Most fungi and some protistsFigure 13.6 C • In most fungi and some protists – Meiosis produces haploid cells that give rise to a haploid multicellular adult organism – The haploid adult carries out mitosis, producing cells that will become gametes
  • 25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid • Meiosis – Takes place in two sets of divisions, meiosis I and meiosis II
  • 26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Stages of Meiosis • An overview of meiosis Figure 13.7 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 1 2 Homologous chromosomes separate Haploid cells with replicated chromosomes Sister chromatids separate Haploid cells with unreplicated chromosomes Meiosis I Meiosis II
  • 27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Meiosis I – Reduces the number of chromosomes from diploid to haploid • Meiosis II – Produces four haploid daughter cells
  • 28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Centrosomes (with centriole pairs) Sister chromatids Chiasmata Spindle Tetrad Nuclear envelope Chromatin Centromere (with kinetochore) Microtubule attached to kinetochore Tertads line up Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Pairs of homologous chromosomes split up Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example INTERPHASE MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I • Interphase and meiosis I Figure 13.8
  • 29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS MEIOSIS II: Separates sister chromatids Cleavage furrow Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Two haploid cells form; chromosomes are still doubleFigure 13.8 • Telophase I, cytokinesis, and meiosis II
  • 30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Comparison of Mitosis and Meiosis • Meiosis and mitosis can be distinguished from mitosis – By three events in Meiosis l
  • 31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Synapsis and crossing over – Homologous chromosomes physically connect and exchange genetic information
  • 32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Tetrads on the metaphase plate – At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates
  • 33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Separation of homologues – At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell – In anaphase II of meiosis, the sister chromatids separate
  • 34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.9 MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads positioned at the metaphase plate Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n n n n Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n 2n Daughter cells of mitosis 2n = 6 • A comparison of mitosis and meiosis
  • 35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution • Reshuffling of genetic material in meiosis – Produces genetic variation
  • 36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Origins of Genetic Variation Among Offspring • In species that produce sexually – The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation
  • 37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Independent Assortment of Chromosomes • Homologous pairs of chromosomes – Orient randomly at metaphase I of meiosis
  • 38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In independent assortment – Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs Figure 13.10 Key Maternal set of chromosomes Paternal set of chromosomes Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Possibility 2 Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4
  • 39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Crossing Over • Crossing over – Produces recombinant chromosomes that carry genes derived from two different parents Figure 13.11 Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes
  • 40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Random Fertilization • The fusion of gametes – Will produce a zygote with any of about 64 trillion diploid combinations
  • 41. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evolutionary Significance of Genetic Variation Within Populations • Genetic variation – Is the raw material for evolution by natural selection
  • 42. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Mutations – Are the original source of genetic variation • Sexual reproduction – Produces new combinations of variant genes, adding more genetic diversity
  • 43. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 14 Mendel and the Gene Idea
  • 44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Overview: Drawing from the Deck of Genes • What genetic principles account for the transmission of traits from parents to offspring?
  • 45. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • One possible explanation of heredity is a “blending” hypothesis – The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green
  • 46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea – Parents pass on discrete heritable units, genes
  • 47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Gregor Mendel – Documented a particulate mechanism of inheritance through his experiments with garden peas Figure 14.1
  • 48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance • Mendel discovered the basic principles of heredity – By breeding garden peas in carefully planned experiments
  • 49. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel’s Experimental, Quantitative Approach • Mendel chose to work with peas – Because they are available in many varieties – Because he could strictly control which plants mated with which
  • 50. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Crossing pea plants Figure 14.2 1 5 4 3 2 Removed stamens from purple flower Transferred sperm- bearing pollen from stamens of white flower to egg- bearing carpel of purple flower Parental generation (P) Pollinated carpel matured into pod Carpel (female) Stamens (male) Planted seeds from pod Examined offspring: all purple flowers First generation offspring (F1) 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. TECHNIQUETECHNIQUE When pollen from a white flower fertilizes eggs of a purple flower, the first-generation hybrids all have purple flowers. The result is the same for the reciprocal cross, the transfer of pollen from purple flowers to white flowers. TECHNIQUERESULTS
  • 51. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Some genetic vocabulary – Character: a heritable feature, such as flower color – Trait: a variant of a character, such as purple or white flowers
  • 52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Mendel chose to track – Only those characters that varied in an “either- or” manner • Mendel also made sure that – He started his experiments with varieties that were “true-breeding”
  • 53. 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
  • 54. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The hybrid offspring of the P generation – Are called the F1 generation • When F1 individuals self-pollinate – The F2 generation is produced
  • 55. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Law of Segregation • When Mendel crossed contrasting, true- breeding 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
  • 56. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Mendel discovered – A ratio of about three to one, purple to white flowers, in the F2 generation Figure 14.3 P Generation (true-breeding parents) Purple flowers White flowers  F1 Generation (hybrids) All plants had purple flowers 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 cross- pollinated with other F1 hybrids. Flower color was then observed in the F2 generation. RESULTS Both purple-flowered plants and white- flowered 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.
  • 57. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Mendel reasoned that – 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
  • 58. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Mendel observed the same pattern – In many other pea plant characters Table 14.1
  • 59. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel’s Model • 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
  • 60. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • First, alternative versions of genes – Account for variations in inherited characters, which are now called alleles Figure 14.4 Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers
  • 61. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Second, for each character – An organism inherits two alleles, one from each parent – A genetic locus is actually represented twice
  • 62. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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
  • 63. 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
  • 64. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Does Mendel’s segregation model account for the 3:1 ratio he observed in the F2 generation of his numerous crosses? – We can answer this question using a Punnett square
  • 65. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Mendel’s law of segregation, probability and the Punnett square Figure 14.5 P Generation F1 Generation F2 Generation P p P p P p P p PpPP ppPp Appearance: Genetic makeup: Purple flowers PP White flowers pp Purple flowers Pp Appearance: Genetic makeup: Gametes: Gametes: F1 sperm F1 eggs 1 /2 1 /2  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. Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purple- flower allele is dominant, all these hybrids have purple flowers. When the hybrid plants produce gametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele. 3 : 1 Random combination of the gametes results in the 3:1 ratio that Mendel observed in the F2 generation. 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.
  • 66. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 that is heterozygous for a particular gene – Has a pair of alleles that are different for that gene
  • 67. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • An organism’s phenotype – Is its physical appearance • An organism’s genotype – Is its genetic makeup
  • 68. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Phenotype versus genotype Figure 14.6 3 1 1 2 1 Phenotype Purple Purple Purple White Genotype PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Ratio 3:1 Ratio 1:2:1
  • 69. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Testcross • In pea plants with purple flowers – The genotype is not immediately obvious
  • 70. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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
  • 71. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The testcross Figure 14.7  Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp If PP, then all offspring purple: If Pp, then 1 ⁄2 offspring purple and 1 ⁄2 offspring white: p p P P Pp Pp PpPp pp pp PpPp P p p p 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. 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. RESULTS
  • 72. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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
  • 73. 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
  • 74. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • How are two characters transmitted from parents to offspring? – As a package? – Independently?
  • 75. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings YYRRP Generation Gametes YR yr yyrr YyRr Hypothesis of dependent assortment Hypothesis of independent assortment F2 Generation (predicted offspring) 1 ⁄2 YR YR yr 1 ⁄2 1 ⁄2 1 ⁄2 yr YYRR YyRr yyrrYyRr 3 ⁄4 1 ⁄4 Sperm Eggs Phenotypic ratio 3:1 YR1 ⁄4 Yr1 ⁄4 yR1 ⁄4 yr1 ⁄4 9 ⁄16 3 ⁄16 3 ⁄16 1 ⁄16 YYRR YYRr YyRR YyRr YyrrYyRrYYrrYYrr YyRR YyRr yyRR yyRr yyrryyRrYyrrYyRr Phenotypic ratio 9:3:3:1 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 F1 Generation Eggs YR Yr yR yr1 ⁄4 1 ⁄4 1 ⁄4 1 ⁄4 Sperm RESULTS CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. 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. • A dihybrid cross – Illustrates the inheritance of two characters • Produces four phenotypes in the F2 generation Figure 14.8
  • 76. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Using the information from a dihybrid cross, Mendel developed the law of independent assortment – Each pair of alleles segregates independently during gamete formation
  • 77. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 14.2: The laws of probability govern Mendelian inheritance • Mendel’s laws of segregation and independent assortment – Reflect the rules of probability
  • 78. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Multiplication and Addition Rules Applied to Monohybrid Crosses • The multiplication rule – States that the probability that two or more independent events will occur together is the product of their individual probabilities
  • 79. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Probability in a monohybrid cross – Can be determined using this rule  Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm R r r R R R R1 ⁄2 1 ⁄2 1 ⁄2 1 ⁄4 1 ⁄4 1 ⁄4 1 ⁄4 1 ⁄2 r r R r r Sperm  Eggs Figure 14.9
  • 80. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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
  • 81. 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
  • 82. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 – Each character first is considered separately and then the individual probabilities are multiplied together
  • 83. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics • The relationship between genotype and phenotype is rarely simple
  • 84. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Extending Mendelian Genetics for a Single Gene • The inheritance of characters by a single gene – May deviate from simple Mendelian patterns
  • 85. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Spectrum of Dominance • Complete dominance – Occurs when the phenotypes of the heterozygote and dominant homozygote are identical
  • 86. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In codominance – Two dominant alleles affect the phenotype in separate, distinguishable ways • The human blood group MN – Is an example of codominance
  • 87. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In incomplete dominance – The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties Figure 14.10 P Generation F1 Generation F2 Generation Red CR CR Gametes CR CW  White CW CW Pink CR CW Sperm CR CR CR Cw CR CRGametes 1 ⁄2 1 ⁄2 1 ⁄2 1 ⁄2 1 ⁄2 Eggs 1 ⁄2 CR CR CR CW CW CW CR CW
  • 88. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The Relation Between Dominance and Phenotype • Dominant and recessive alleles – Do not really “interact” – Lead to synthesis of different proteins that produce a phenotype
  • 89. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Frequency of Dominant Alleles • Dominant alleles – Are not necessarily more common in populations than recessive alleles
  • 90. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Multiple Alleles • Most genes exist in populations – In more than two allelic forms
  • 91. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The ABO blood group in humans – Is determined by multiple alleles Table 14.2
  • 92. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pleiotropy • In pleiotropy – A gene has multiple phenotypic effects
  • 93. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Extending Mendelian Genetics for Two or More Genes • Some traits – May be determined by two or more genes
  • 94. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Epistasis • In epistasis – A gene at one locus alters the phenotypic expression of a gene at a second locus
  • 95. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • An example of epistasis Figure 14.11 BC bC Bc bc1 ⁄4 1 ⁄4 1 ⁄4 1 ⁄4 BC bC Bc bc 1 ⁄4 1 ⁄4 1 ⁄4 1 ⁄4 BBCc BbCc BBcc Bbcc Bbcc bbccbbCcBbCc BbCC bbCC BbCc bbCc BBCC BbCC BBCc BbCc 9 ⁄16 3 ⁄16 4 ⁄16 BbCc BbCc Sperm Eggs
  • 96. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Polygenic Inheritance • Many human characters – Vary in the population along a continuum and are called quantitative characters
  • 97. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  AaBbCc AaBbCc aabbcc Aabbcc AaBbcc AaBbCc AABbCcAABBCcAABBCC 20 ⁄64 15 ⁄64 6 ⁄64 1 ⁄64 Fractionofprogeny • Quantitative variation usually indicates polygenic inheritance – An additive effect of two or more genes on a single phenotype Figure 14.12
  • 98. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Nature and Nurture: The Environmental Impact on Phenotype • Another departure from simple Mendelian genetics arises – When the phenotype for a character depends on environment as well as on genotype
  • 99. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The norm of reaction – Is the phenotypic range of a particular genotype that is influenced by the environment Figure 14.13
  • 100. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Multifactorial characters – Are those that are influenced by both genetic and environmental factors
  • 101. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Integrating a Mendelian View of Heredity and Variation • An organism’s phenotype – Includes its physical appearance, internal anatomy, physiology, and behavior – Reflects its overall genotype and unique environmental history
  • 102. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Even in more complex inheritance patterns – Mendel’s fundamental laws of segregation and independent assortment still apply
  • 103. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 14.4: Many human traits follow Mendelian patterns of inheritance • Humans are not convenient subjects for genetic research – However, the study of human genetics continues to advance
  • 104. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pedigree Analysis • A pedigree – Is a family tree that describes the interrelationships of parents and children across generations
  • 105. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Inheritance patterns of particular traits – Can be traced and described using pedigrees Figure 14.14 A, B Ww ww ww Ww wwWwWwwwwwWw WW or Ww ww First generation (grandparents) Second generation (parents plus aunts and uncles) Third generation (two sisters) Ff Ff ff Ff ffFfFfffFfFF or Ff ff FF or Ff Widow’s peak No Widow’s peak Attached earlobe Free earlobe (a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)
  • 106. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Pedigrees – Can also be used to make predictions about future offspring
  • 107. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recessively Inherited Disorders • Many genetic disorders – Are inherited in a recessive manner
  • 108. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Recessively inherited disorders – Show up only in individuals homozygous for the allele • Carriers – Are heterozygous individuals who carry the recessive allele but are phenotypically normal
  • 109. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cystic Fibrosis • Symptoms of cystic fibrosis include – Mucus buildup in the some internal organs – Abnormal absorption of nutrients in the small intestine
  • 110. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sickle-Cell Disease • Sickle-cell disease – Affects one out of 400 African-Americans – Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells • Symptoms include – Physical weakness, pain, organ damage, and even paralysis
  • 111. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mating of Close Relatives • Matings between relatives – Can increase the probability of the appearance of a genetic disease – Are called consanguineous matings
  • 112. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Dominantly Inherited Disorders • Some human disorders – Are due to dominant alleles
  • 113. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • One example is achondroplasia – A form of dwarfism that is lethal when homozygous for the dominant allele Figure 14.15
  • 114. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Huntington’s disease – Is a degenerative disease of the nervous system – Has no obvious phenotypic effects until about 35 to 40 years of age Figure 14.16
  • 115. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Multifactorial Disorders • Many human diseases – Have both genetic and environment components • Examples include – Heart disease and cancer
  • 116. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Testing and Counseling • Genetic counselors – Can provide information to prospective parents concerned about a family history for a specific disease
  • 117. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Counseling Based on Mendelian Genetics and Probability Rules • Using family histories – Genetic counselors help couples determine the odds that their children will have genetic disorders
  • 118. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Tests for Identifying Carriers • For a growing number of diseases – Tests are available that identify carriers and help define the odds more accurately
  • 119. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fetal Testing • In amniocentesis – The liquid that bathes the fetus is removed and tested • In chorionic villus sampling (CVS) – A sample of the placenta is removed and tested
  • 120. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Fetal testing Figure 14.17 A, B (a) Amniocentesis Amniotic fluid withdrawn Fetus Placenta Uterus Cervix Centrifugation A sample of amniotic fluid can be taken starting at the 14th to 16th week of pregnancy. (b) Chorionic villus sampling (CVS) Fluid Fetal cells Biochemical tests can be Performed immediately on the amniotic fluid or later on the cultured cells. Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping. Several weeks Biochemical tests Several hours Fetal cells Placenta Chorionic viIIi A sample of chorionic villus tissue can be taken as early as the 8th to 10th week of pregnancy. Suction tube Inserted through cervix Fetus Karyotyping and biochemical tests can be performed on the fetal cells immediately, providing results within a day or so. Karyotyping
  • 121. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Newborn Screening • Some genetic disorders can be detected at birth – By simple tests that are now routinely performed in most hospitals in the United States

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