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14 mendel text

  1. 1. Mendel and the Gene Idea <ul><li>What genetic principles account for the transmission of traits from parents to offspring? </li></ul><ul><li>One possible explanation of heredity is a “blending” hypothesis </li></ul><ul><ul><li>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 </li></ul></ul>
  2. 2. <ul><li>An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea </li></ul><ul><ul><li>Parents pass on discrete heritable units, genes </li></ul></ul>
  3. 3. <ul><li>Gregor Mendel </li></ul><ul><ul><li>Documented a particulate mechanism of inheritance through his experiments with garden peas </li></ul></ul>Figure 14.1
  4. 4. Mendel’s Experimental, Quantitative Approach <ul><li>Mendel chose to work with peas </li></ul><ul><ul><li>Because they are available in many varieties </li></ul></ul><ul><ul><li>Because he could strictly control which plants mated with which </li></ul></ul>
  5. 5. <ul><li>Crossing pea plants </li></ul>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 (F 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 TECHNIQUE 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. TECHNIQUE RESULTS
  6. 6. <ul><li>Some genetic vocabulary </li></ul><ul><ul><li>Character: a heritable feature, such as flower color </li></ul></ul><ul><ul><li>Trait: a variant of a character, such as purple or white flowers </li></ul></ul>
  7. 7. <ul><li>Mendel chose to track </li></ul><ul><ul><li>Only those characters that varied in an “either-or” manner </li></ul></ul><ul><li>Mendel also made sure that </li></ul><ul><ul><li>He started his experiments with varieties that were “true-breeding” </li></ul></ul>
  8. 8. <ul><li>In a typical breeding experiment </li></ul><ul><ul><li>Mendel mated two contrasting, true-breeding varieties, a process called hybridization </li></ul></ul><ul><li>The true-breeding parents </li></ul><ul><ul><li>Are called the P generation </li></ul></ul>
  9. 9. <ul><li>The hybrid offspring of the P generation </li></ul><ul><ul><li>Are called the F 1 generation </li></ul></ul><ul><li>When F 1 individuals self-pollinate </li></ul><ul><ul><li>The F 2 generation is produced </li></ul></ul>
  10. 10. The Law of Segregation <ul><li>When Mendel crossed contrasting, true-breeding white and purple flowered pea plants </li></ul><ul><ul><li>All of the offspring were purple </li></ul></ul><ul><li>When Mendel crossed the F 1 plants </li></ul><ul><ul><li>Many of the plants had purple flowers, but some had white flowers </li></ul></ul>
  11. 11. <ul><li>Mendel discovered </li></ul><ul><ul><li>A ratio of about three to one, purple to white flowers, in the F 2 generation </li></ul></ul>Figure 14.3 P Generation (true-breeding parents) Purple flowers White flowers  F 1 Generation (hybrids) All plants had purple flowers F 2 Generation EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by ). The resulting F 1 hybrids were allowed to self-pollinate or were cross- pollinated with other F 1 hybrids. Flower color was then observed in the F 2 generation. RESULTS Both purple-flowered plants and white- flowered plants appeared in the F 2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white.
  12. 12. <ul><li>Mendel reasoned that </li></ul><ul><ul><li>In the F 1 plants, only the purple flower factor was affecting flower color in these hybrids </li></ul></ul><ul><ul><li>Purple flower color was dominant, and white flower color was recessive </li></ul></ul>
  13. 13. <ul><li>Mendel observed the same pattern </li></ul><ul><ul><li>In many other pea plant characters </li></ul></ul>Table 14.1
  14. 14. Mendel’s Model <ul><li>Mendel developed a hypothesis </li></ul><ul><ul><li>To explain the 3:1 inheritance pattern that he observed among the F 2 offspring </li></ul></ul><ul><li>Four related concepts make up this model </li></ul>
  15. 15. <ul><li>First, alternative versions of genes </li></ul><ul><ul><li>Account for variations in inherited characters, which are now called alleles </li></ul></ul>Figure 14.4 Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers
  16. 16. <ul><li>Second, for each character </li></ul><ul><ul><li>An organism inherits two alleles, one from each parent </li></ul></ul><ul><ul><li>A genetic locus is actually represented twice </li></ul></ul>
  17. 17. <ul><li>Third, if the two alleles at a locus differ </li></ul><ul><ul><li>Then one, the dominant allele, determines the organism’s appearance </li></ul></ul><ul><ul><li>The other allele, the recessive allele, has no noticeable effect on the organism’s appearance </li></ul></ul>
  18. 18. <ul><li>Fourth, the law of segregation </li></ul><ul><ul><li>The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes </li></ul></ul>
  19. 19. <ul><li>Does Mendel’s segregation model account for the 3:1 ratio he observed in the F 2 generation of his numerous crosses? </li></ul><ul><ul><li>We can answer this question using a Punnett square </li></ul></ul>
  20. 20. <ul><li>Mendel’s law of segregation, probability and the Punnett square </li></ul>Figure 14.5 P Generation F 1 Generation F 2 Generation P p P p P p P p Pp PP pp Pp Appearance: Genetic makeup: Purple flowers PP White flowers pp Purple flowers Pp Appearance: Genetic makeup: Gametes: Gametes: F 1 sperm F 1 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 F 1 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 F 2 generation. This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F 1  F 1 ( 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.
  21. 21. Useful Genetic Vocabulary <ul><li>An organism that is homozygous for a particular gene </li></ul><ul><ul><li>Has a pair of identical alleles for that gene </li></ul></ul><ul><ul><li>Exhibits true-breeding </li></ul></ul><ul><li>An organism that is heterozygous for a particular gene </li></ul><ul><ul><li>Has a pair of alleles that are different for that gene </li></ul></ul>
  22. 22. <ul><li>An organism’s phenotype </li></ul><ul><ul><li>Is its physical appearance </li></ul></ul><ul><li>An organism’s genotype </li></ul><ul><ul><li>Is its genetic makeup </li></ul></ul>
  23. 23. <ul><li>Phenotype versus genotype </li></ul>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
  24. 24. The Testcross <ul><li>In pea plants with purple flowers </li></ul><ul><ul><li>The genotype is not immediately obvious </li></ul></ul><ul><li>A testcross </li></ul><ul><ul><li>Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype </li></ul></ul><ul><ul><li>Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait </li></ul></ul>
  25. 25. <ul><li>The testcross </li></ul>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 Pp Pp pp pp Pp Pp 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
  26. 26. The Law of Independent Assortment <ul><li>Mendel derived the law of segregation </li></ul><ul><ul><li>By following a single trait </li></ul></ul><ul><li>The F 1 offspring produced in this cross </li></ul><ul><ul><li>Were monohybrids, heterozygous for one character </li></ul></ul>
  27. 27. <ul><li>Mendel identified his second law of inheritance </li></ul><ul><ul><li>By following two characters at the same time </li></ul></ul><ul><li>Crossing two, true-breeding parents differing in two characters </li></ul><ul><ul><li>Produces dihybrids in the F 1 generation, heterozygous for both characters </li></ul></ul>
  28. 28. <ul><li>How are two characters transmitted from parents to offspring? </li></ul><ul><ul><li>As a package? </li></ul></ul><ul><ul><li>Independently? </li></ul></ul>
  29. 29. <ul><li>A dihybrid cross </li></ul><ul><ul><li>Illustrates the inheritance of two characters </li></ul></ul><ul><li>Produces four phenotypes in the F 2 generation </li></ul>Figure 14.8 YYRR P Generation Gametes YR yr  yyrr YyRr Hypothesis of dependent assortment Hypothesis of independent assortment F 2 Generation (predicted offspring) 1 ⁄ 2 YR YR yr 1 ⁄ 2 1 ⁄ 2 1 ⁄ 2 yr YYRR YyRr yyrr YyRr 3 ⁄ 4 1 ⁄ 4 Sperm Eggs Phenotypic ratio 3:1 YR 1 ⁄ 4 Yr 1 ⁄ 4 yR 1 ⁄ 4 yr 1 ⁄ 4 9 ⁄ 16 3 ⁄ 16 3 ⁄ 16 1 ⁄ 16 YYRR YYRr YyRR YyRr Yyrr YyRr YYrr YYrr YyRR YyRr yyRR yyRr yyrr yyRr Yyrr YyRr Phenotypic ratio 9:3:3:1 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 F 1 Generation Eggs YR Yr yR yr 1 ⁄ 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 F 1 plants. Self-pollination of the F 1 dihybrids, which are heterozygous for both characters, produced the F 2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color ( Y ) and round shape ( R ) are dominant.
  30. 30. <ul><li>Using the information from a dihybrid cross, Mendel developed the law of independent assortment </li></ul><ul><ul><li>Each pair of alleles segregates independently during gamete formation </li></ul></ul>
  31. 31. <ul><li>The laws of probability govern Mendelian inheritance </li></ul><ul><li>Mendel’s laws of segregation and independent assortment </li></ul><ul><ul><li>Reflect the rules of probability </li></ul></ul>
  32. 32. <ul><li>Probability in a monohybrid cross </li></ul><ul><ul><li>Can be determined using this rule </li></ul></ul> Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm R r r R R R R 1 ⁄ 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
  33. 33. Solving Complex Genetics Problems with the Rules of Probability <ul><li>We can apply the rules of probability </li></ul><ul><ul><li>To predict the outcome of crosses involving multiple characters </li></ul></ul>
  34. 34. <ul><li>A dihybrid or other multicharacter cross </li></ul><ul><ul><li>Is equivalent to two or more independent monohybrid crosses occurring simultaneously </li></ul></ul><ul><li>In calculating the chances for various genotypes from such crosses </li></ul><ul><ul><li>Each character first is considered separately and then the individual probabilities are multiplied together </li></ul></ul>
  35. 35. The Spectrum of Dominance <ul><li>Complete dominance </li></ul><ul><ul><li>Occurs when the phenotypes of the heterozygote and dominant homozygote are identical </li></ul></ul><ul><li>In codominance </li></ul><ul><ul><li>Two dominant alleles affect the phenotype in separate, distinguishable ways </li></ul></ul><ul><li>The human blood group </li></ul><ul><li>- Is an example of codominance </li></ul>
  36. 36. <ul><li>In incomplete dominance </li></ul><ul><ul><li>The phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties </li></ul></ul>Figure 14.10 P Generation F 1 Generation F 2 Generation Red C R C R Gametes C R C W  White C W C W Pink C R C W Sperm C R C R C R C w C R C R Gametes 1 ⁄ 2 1 ⁄ 2 1 ⁄ 2 1 ⁄ 2 1 ⁄ 2 Eggs 1 ⁄ 2 C R C R C R C W C W C W C R C W
  37. 37. <ul><li>Frequency of Dominant Alleles </li></ul><ul><li>Dominant alleles </li></ul><ul><ul><li>Are not necessarily more common in populations than recessive alleles </li></ul></ul>
  38. 38. Multiple Alleles <ul><li>The ABO blood group in humans </li></ul><ul><ul><li>Is determined by multiple alleles </li></ul></ul>Table 14.2
  39. 39. Pleiotropy <ul><li>In pleiotropy </li></ul><ul><ul><li>A gene has multiple phenotypic effects </li></ul></ul><ul><li>Some traits (polygenic) </li></ul><ul><ul><li>May be determined by two or more genes </li></ul></ul><ul><li>In epistasis </li></ul><ul><ul><li>A gene at one locus alters the phenotypic expression of a gene at a second locus </li></ul></ul>
  40. 40. <ul><li>An example of epistasis </li></ul>Figure 14.11 BC bC Bc bc 1 ⁄ 4 1 ⁄ 4 1 ⁄ 4 1 ⁄ 4 BC bC Bc bc 1 ⁄ 4 1 ⁄ 4 1 ⁄ 4 1 ⁄ 4 BBCc BbCc BBcc Bbcc Bbcc bbcc bbCc BbCc BbCC bbCC BbCc bbCc BBCC BbCC BBCc BbCc 9 ⁄ 16 3 ⁄ 16 4 ⁄ 16 BbCc BbCc  Sperm Eggs
  41. 41. <ul><li>Quantitative variation usually indicates polygenic inheritance </li></ul><ul><ul><li>An additive effect of two or more genes on a single phenotype </li></ul></ul>Figure 14.12  AaBbCc AaBbCc aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC 20 ⁄ 64 15 ⁄ 64 6 ⁄ 64 1 ⁄ 64 Fraction of progeny
  42. 42. Nature and Nurture: The Environmental Impact on Phenotype <ul><li>Another departure from simple Mendelian genetics arises </li></ul><ul><ul><li>When the phenotype for a character depends on environment as well as on genotype </li></ul></ul>
  43. 43. <ul><li>The norm of reaction </li></ul><ul><ul><li>Is the phenotypic range of a particular genotype that is influenced by the environment </li></ul></ul>Figure 14.13
  44. 44. <ul><li>Multifactorial characters </li></ul><ul><ul><li>Are those that are influenced by both genetic and environmental factors </li></ul></ul><ul><li>An organism’s phenotype </li></ul><ul><ul><li>Includes its physical appearance, internal anatomy, physiology, and behavior </li></ul></ul><ul><ul><li>Reflects its overall genotype and unique environmental history </li></ul></ul>
  45. 45. <ul><li>Even in more complex inheritance patterns </li></ul><ul><ul><li>Mendel’s fundamental laws of segregation and independent assortment still apply </li></ul></ul>
  46. 46. <ul><li>Many human traits follow Mendelian patterns of inheritance </li></ul><ul><li>Humans are not convenient subjects for genetic research </li></ul><ul><ul><li>However, the study of human genetics continues to advance </li></ul></ul>
  47. 47. Different Colors?
  48. 48. Shades skin color
  49. 49. Shades skin color
  50. 50. A&E skin color
  51. 51. A&E skin color
  52. 52. A&E skin color
  53. 53. One Generation
  54. 54. Skin tones – Nat. Geo. ‘ Yet with the effects of human migrations and cultural habits, people in one place can show tremendous variation in skin tone – like students from the Washington International Primary School.” ‘Unmasking Skin,’ Joel L. Swerdlow, National Geographic , Nov. 2002 p46-47.
  55. 55.
  56. 56. Tower of Babel
  57. 57. Punnit Square - grays
  58. 58. Punnit Square - grays
  59. 59. Punnit Square - grays
  60. 60. Eye Shapes
  61. 61. Q 664 abcnews www.abcnews.com , Science page, &quot;We're all the same,&quot; 9/10/98 What the facts show is that there are differences among us, but they stem from culture, not race. Q 664
  62. 62. Biological Fact
  63. 63. Acts 17:26 Acts 17:26 And hath made of one blood all nations of men for to dwell on all the face of the earth, and hath determined the times before appointed, and the bounds of their habitation;
  64. 64. Races?
  65. 65. Biblical View Acts 17:26
  66. 66. Biblical View Acts 17:26
  67. 67. Biblical View Acts 17:26
  68. 68. Pedigree Analysis <ul><li>A pedigree </li></ul><ul><ul><li>Is a family tree that describes the interrelationships of parents and children across generations </li></ul></ul>
  69. 69. <ul><li>Inheritance patterns of particular traits </li></ul><ul><ul><li>Can be traced and described using pedigrees </li></ul></ul>Figure 14.14 A, B <ul><li>Pedigrees </li></ul><ul><ul><li>Can also be used to make predictions about future offspring </li></ul></ul>Ww ww ww Ww ww Ww Ww ww ww Ww WW or Ww ww First generation (grandparents) Second generation (parents plus aunts and uncles) Third generation (two sisters) Ff Ff ff Ff ff Ff Ff ff Ff FF 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)
  70. 70. Recessively Inherited Disorders <ul><li>Many genetic disorders </li></ul><ul><ul><li>Are inherited in a recessive manner </li></ul></ul><ul><li>Recessively inherited disorders </li></ul><ul><ul><li>Show up only in individuals homozygous for the allele </li></ul></ul><ul><li>Carriers </li></ul><ul><ul><li>Are heterozygous individuals who carry the recessive allele but are phenotypically normal </li></ul></ul>
  71. 71. Cystic Fibrosis <ul><li>Symptoms of cystic fibrosis include </li></ul><ul><ul><li>Mucus buildup in the some internal organs </li></ul></ul><ul><ul><li>Abnormal absorption of nutrients in the small intestine </li></ul></ul>
  72. 72. Sickle-Cell Disease <ul><li>Sickle-cell disease </li></ul><ul><ul><li>Affects one out of 400 African-Americans </li></ul></ul><ul><ul><li>Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells </li></ul></ul><ul><li>Symptoms include </li></ul><ul><ul><li>Physical weakness, pain, organ damage, and even paralysis </li></ul></ul>
  73. 73. Mating of Close Relatives <ul><li>Matings between relatives </li></ul><ul><ul><li>Can increase the probability of the appearance of a genetic disease </li></ul></ul><ul><ul><li>Are called consanguineous matings </li></ul></ul>
  74. 74. Dominantly Inherited Disorders <ul><li>Some human disorders </li></ul><ul><ul><li>Are due to dominant alleles </li></ul></ul>
  75. 75. <ul><li>One example is achondroplasia </li></ul><ul><ul><li>A form of dwarfism that is lethal when homozygous for the dominant allele </li></ul></ul>Figure 14.15
  76. 76. <ul><li>Huntington’s disease </li></ul><ul><ul><li>Is a degenerative disease of the nervous system </li></ul></ul><ul><ul><li>Has no obvious phenotypic effects until about 35 to 40 years of age </li></ul></ul>Figure 14.16
  77. 77. Multifactorial Disorders <ul><li>Many human diseases </li></ul><ul><ul><li>Have both genetic and environment components </li></ul></ul><ul><li>Examples include </li></ul><ul><ul><li>Heart disease and cancer </li></ul></ul>
  78. 78. Genetic Testing and Counseling <ul><li>Genetic counselors </li></ul><ul><ul><li>Can provide information to prospective parents concerned about a family history for a specific disease </li></ul></ul>
  79. 79. Counseling Based on Mendelian Genetics and Probability Rules <ul><li>Using family histories </li></ul><ul><ul><li>Genetic counselors help couples determine the odds that their children will have genetic disorders </li></ul></ul>
  80. 80. Tests for Identifying Carriers <ul><li>For a growing number of diseases </li></ul><ul><ul><li>Tests are available that identify carriers and help define the odds more accurately </li></ul></ul><ul><li>In amniocentesis </li></ul><ul><ul><li>The liquid that bathes the fetus is removed and tested </li></ul></ul><ul><li>In chorionic villus sampling (CVS) </li></ul><ul><ul><li>A sample of the placenta is removed and tested </li></ul></ul>
  81. 81. <ul><li>Fetal testing </li></ul>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
  82. 82. Newborn Screening <ul><li>Some genetic disorders can be detected at birth </li></ul><ul><ul><li>By simple tests that are now routinely performed in most hospitals in the United States </li></ul></ul>
  83. 83. <ul><li>Overview: Locating Genes on Chromosomes </li></ul><ul><li>Genes </li></ul><ul><ul><li>Are located on chromosomes </li></ul></ul><ul><ul><li>Can be visualized using certain techniques </li></ul></ul>Figure 15.1
  84. 84. <ul><li>Mendelian inheritance has its physical basis in the behavior of chromosomes </li></ul><ul><li>Several researchers proposed in the early 1900s that genes are located on chromosomes </li></ul><ul><li>The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment </li></ul>
  85. 85. <ul><li>The chromosome theory of inheritance states that </li></ul><ul><ul><li>Mendelian genes have specific loci on chromosomes </li></ul></ul><ul><ul><li>Chromosomes undergo segregation and independent assortment </li></ul></ul>
  86. 86. <ul><li>The chromosomal basis of Mendel’s laws </li></ul>Figure 15.2 Yellow-round seeds ( YYRR ) Green-wrinkled seeds ( yyrr ) Meiosis Fertilization Gametes All F 1 plants produce yellow-round seeds ( YyRr ) P Generation F 1 Generation Meiosis Two equally probable arrangements of chromosomes at metaphase I LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT Anaphase I Metaphase II Fertilization among the F 1 plants 9 : 3 : 3 : 1 1 4 1 4 1 4 1 4 YR yr yr yR Gametes Y R R Y y r r y R Y y r R y Y r R y Y r R Y r y r R Y y R Y r y R Y Y R R Y r y r y R y r Y r Y r Y r Y R y R y R y r Y F 2 Generation Starting with two true-breeding pea plants, we follow two genes through the F 1 and F 2 generations. The two genes specify seed color (allele Y for yellow and allele y for green) and seed shape (allele R for round and allele r for wrinkled). These two genes are on different chromosomes. (Peas have seven chromosome pairs, but only two pairs are illustrated here.) The R and r alleles segregate at anaphase I, yielding two types of daughter cells for this locus. 1 Each gamete gets one long chromosome with either the R or r allele. 2 Fertilization recombines the R and r alleles at random. 3 Alleles at both loci segregate in anaphase I, yielding four types of daughter cells depending on the chromosome arrangement at metaphase I. Compare the arrangement of the R and r alleles in the cells on the left and right 1 Each gamete gets a long and a short chromosome in one of four allele combinations. 2 Fertilization results in the 9:3:3:1 phenotypic ratio in the F 2 generation. 3
  87. 87. Morgan’s Experimental Evidence: Scientific Inquiry <ul><li>Thomas Hunt Morgan </li></ul><ul><ul><li>Provided convincing evidence that chromosomes are the location of Mendel’s heritable factors </li></ul></ul>
  88. 88. Morgan’s Choice of Experimental Organism <ul><li>Morgan worked with fruit flies </li></ul><ul><ul><li>Because they breed at a high rate </li></ul></ul><ul><ul><li>A new generation can be bred every two weeks </li></ul></ul><ul><ul><li>They have only four pairs of chromosomes </li></ul></ul>
  89. 89. <ul><li>Morgan first observed and noted </li></ul><ul><ul><li>Wild type, or normal, phenotypes that were common in the fly populations </li></ul></ul><ul><li>Traits alternative to the wild type </li></ul><ul><ul><li>Are called mutant phenotypes </li></ul></ul>Figure 15.3
  90. 90. Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair <ul><li>In one experiment Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) </li></ul><ul><ul><li>The F 1 generation all had red eyes </li></ul></ul><ul><ul><li>The F 2 generation showed the 3:1 red:white eye ratio, but only males had white eyes </li></ul></ul>
  91. 91. <ul><li>Morgan determined </li></ul><ul><ul><li>That the white-eye mutant allele must be located on the X chromosome </li></ul></ul>Figure 15.4 The F 2 generation showed a typical Mendelian 3:1 ratio of red eyes to white eyes. However, no females displayed the white-eye trait; they all had red eyes. Half the males had white eyes, and half had red eyes. Morgan then bred an F 1 red-eyed female to an F 1 red-eyed male to produce the F 2 generation. RESULTS P Generation F 1 Generation X F 2 Generation Morgan mated a wild-type (red-eyed) female with a mutant white-eyed male. The F 1 offspring all had red eyes. EXPERIMENT
  92. 92. CONCLUSION Since all F 1 offspring had red eyes, the mutant white-eye trait ( w ) must be recessive to the wild-type red-eye trait ( w + ). Since the recessive trait—white eyes—was expressed only in males in the F 2 generation, Morgan hypothesized that the eye-color gene is located on the X chromosome and that there is no corresponding locus on the Y chromosome, as diagrammed here. P Generation F 1 Generation F 2 Generation Ova (eggs) Ova (eggs) Sperm Sperm X X X X Y W W + W + W W + W + W + W + W + W + W + W + W W + W W W
  93. 93. <ul><li>Morgan’s discovery that transmission of the X chromosome in fruit flies correlates with inheritance of the eye-color trait </li></ul><ul><ul><li>Was the first solid evidence indicating that a specific gene is associated with a specific chromosome </li></ul></ul>
  94. 94. <ul><li>Linked genes tend to be inherited together because they are located near each other on the same chromosome </li></ul><ul><li>Each chromosome </li></ul><ul><ul><li>Has hundreds or thousands of genes </li></ul></ul>
  95. 95. How Linkage Affects Inheritance: Scientific Inquiry <ul><li>Morgan did other experiments with fruit flies </li></ul><ul><ul><li>To see how linkage affects the inheritance of two different characters </li></ul></ul>
  96. 96. <ul><li>Morgan crossed flies </li></ul><ul><ul><li>That differed in traits of two different characters </li></ul></ul>Double mutant (black body, vestigial wings) Double mutant (black body, vestigial wings) Wild type (gray body, normal wings) P Generation (homozygous) b + b + vg + vg + x b b vg vg F 1 dihybrid (wild type) (gray body, normal wings) b + b vg + vg b b vg vg TESTCROSS x b + vg + b vg b + vg b vg + b vg b + b vg + vg b b vg vg b + b vg vg b b vg + vg 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal Sperm Parental-type offspring Recombinant (nonparental-type) offspring RESULTS EXPERIMENT Morgan first mated true-breeding wild-type flies with black, vestigial-winged flies to produce heterozygous F 1 dihybrids, all of which are wild-type in appearance. He then mated wild-type F 1 dihybrid females with black, vestigial-winged males, producing 2,300 F 2 offspring, which he “scored” (classified according to phenotype). CONCLUSION If these two genes were on different chromosomes, the alleles from the F 1 dihybrid would sort into gametes independently, and we would expect to see equal numbers of the four types of offspring. If these two genes were on the same chromosome, we would expect each allele combination, B + vg + and b vg , to stay together as gametes formed. In this case, only offspring with parental phenotypes would be produced. Since most offspring had a parental phenotype, Morgan concluded that the genes for body color and wing size are located on the same chromosome. However, the production of a small number of offspring with nonparental phenotypes indicated that some mechanism occasionally breaks the linkage between genes on the same chromosome. Figure 15.5 Double mutant (black body, vestigial wings) Double mutant (black body, vestigial wings)
  97. 97. <ul><li>Morgan determined that </li></ul><ul><ul><li>Genes that are close together on the same chromosome are linked and do not assort independently </li></ul></ul><ul><ul><li>Unlinked genes are either on separate chromosomes or are far apart on the same chromosome and assort independently </li></ul></ul>Parents in testcross b + vg + b vg b + vg + b vg b vg b vg b vg b vg Most offspring X or
  98. 98. Recombination of Unlinked Genes: Independent Assortment of Chromosomes <ul><li>When Mendel followed the inheritance of two characters </li></ul><ul><ul><li>He observed that some offspring have combinations of traits that do not match either parent in the P generation </li></ul></ul>Gametes from green- wrinkled homozygous recessive parent ( yyrr ) Gametes from yellow-round heterozygous parent ( YyRr ) Parental- type offspring Recombinant offspring YyRr yyrr Yyrr yyRr YR yr Yr yR yr
  99. 99. <ul><li>Recombinant offspring </li></ul><ul><ul><li>Are those that show new combinations of the parental traits </li></ul></ul><ul><li>When 50% of all offspring are recombinants </li></ul><ul><ul><li>Geneticists say that there is a 50% frequency of recombination </li></ul></ul>
  100. 100. Recombination of Linked Genes: Crossing Over <ul><li>Morgan discovered that genes can be linked </li></ul><ul><ul><li>But due to the appearance of recombinant phenotypes, the linkage appeared incomplete </li></ul></ul>
  101. 101. <ul><li>Morgan proposed that </li></ul><ul><ul><li>Some process must occasionally break the physical connection between genes on the same chromosome </li></ul></ul><ul><ul><li>Crossing over of homologous chromosomes was the mechanism </li></ul></ul>
  102. 102. <ul><li>Linked genes </li></ul><ul><ul><li>Exhibit recombination frequencies less than 50% </li></ul></ul>Figure 15.6 Testcross parents Gray body, normal wings (F 1 dihybrid) b + vg + b vg Replication of chromosomes b + vg b + vg + b vg vg Meiosis I: Crossing over between b and vg loci produces new allele combinations. Meiosis II: Segregation of chromatids produces recombinant gametes with the new allele combinations.  Recombinant chromosome b + vg + b    vg b + vg b vg + b vg Sperm b    vg b    vg Replication of chromosomes vg vg b b b vg b    vg Meiosis I and II: Even if crossing over occurs, no new allele combinations are produced. Ova Gametes Testcross offspring Sperm b +   vg + b   vg b +   vg b   vg + 965 Wild type (gray-normal) b +   vg + b   vg b   vg b   vg b   vg b   vg + b +   vg + b   vg + 944 Black- vestigial 206 Gray- vestigial 185 Black- normal Recombination frequency = 391 recombinants 2,300 total offspring  100 = 17% Parental-type offspring Recombinant offspring Ova b vg Black body, vestigial wings (double mutant) b
  103. 103. Linkage Mapping: Using Recombination Data: Scientific Inquiry <ul><li>A genetic map </li></ul><ul><ul><li>Is an ordered list of the genetic loci along a particular chromosome </li></ul></ul><ul><ul><li>Can be developed using recombination frequencies </li></ul></ul>
  104. 104. <ul><li>A linkage map </li></ul><ul><ul><li>Is the actual map of a chromosome based on recombination frequencies </li></ul></ul>Recombination frequencies 9% 9.5% 17% b cn vg Chromosome The b–vg recombination frequency is slightly less than the sum of the b–cn and cn–vg frequencies because double crossovers are fairly likely to occur between b and vg in matings tracking these two genes. A second crossover would “cancel out” the first and thus reduce the observed b–vg recombination frequency. In this example, the observed recombination frequencies between three Drosophila gene pairs ( b–cn 9%, cn–vg 9.5%, and b–vg 17%) best fit a linear order in which cn is positioned about halfway between the other two genes: RESULTS A linkage map shows the relative locations of genes along a chromosome. APPLICATION TECHNIQUE A linkage map is based on the assumption that the probability of a crossover between two genetic loci is proportional to the distance separating the loci. The recombination frequencies used to construct a linkage map for a particular chromosome are obtained from experimental crosses, such as the cross depicted in Figure 15.6. The distances between genes are expressed as map units (centimorgans), with one map unit equivalent to a 1% recombination frequency. Genes are arranged on the chromosome in the order that best fits the data. Figure 15.7
  105. 105. <ul><li>The farther apart genes are on a chromosome </li></ul><ul><ul><li>The more likely they are to be separated during crossing over </li></ul></ul>
  106. 106. <ul><li>Many fruit fly genes </li></ul><ul><ul><li>Were mapped initially using recombination frequencies </li></ul></ul>Figure 15.8 Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial wings Brown eyes Long aristae (appendages on head) Gray body Red eyes Normal wings Red eyes Wild-type phenotypes II Y I X IV III 0 48.5 57.5 67.0 104.5
  107. 107. <ul><li>Sex-linked genes exhibit unique patterns of inheritance </li></ul><ul><li>An organism’s sex </li></ul><ul><ul><li>Is an inherited phenotypic character determined by the presence or absence of certain chromosomes </li></ul></ul>
  108. 108. <ul><li>In humans and other mammals </li></ul><ul><ul><li>There are two varieties of sex chromosomes, X and Y </li></ul></ul>Figure 15.9a (a) The X-Y system 44 + XY 44 + XX Parents 22 + X 22 + Y 22 + XY Sperm Ova 44 + XX 44 + XY Zygotes (offspring)
  109. 109. <ul><li>Different systems of sex determination </li></ul><ul><ul><li>Are found in other organisms </li></ul></ul>Figure 15.9b–d 22 + XX 22 + X 76 + ZZ 76 + ZW 16 (Haploid) 16 (Diploid) (b) The X–0 system (c) The Z–W system (d) The haplo-diploid system
  110. 110. Inheritance of Sex-Linked Genes <ul><li>The sex chromosomes </li></ul><ul><ul><li>Have genes for many characters unrelated to sex </li></ul></ul><ul><li>A gene located on either sex chromosome </li></ul><ul><ul><li>Is called a sex-linked gene </li></ul></ul>
  111. 111. <ul><li>Sex-linked genes </li></ul><ul><ul><li>Follow specific patterns of inheritance </li></ul></ul>Figure 15.10a–c X A X A X a Y X a Y X A X a X A Y X A Y X A Y a X A X A Ova Sperm X A X a X A Y Ova X A X a X A X A X A Y X a Y X a Y A X A Y Sperm X A X a X a Y   Ova X a Y X A X a X A Y X a Y X a Y a X A X a A father with the disorder will transmit the mutant allele to all daughters but to no sons. When the mother is a dominant homozygote, the daughters will have the normal phenotype but will be carriers of the mutation. If a carrier mates with a male of normal phenotype, there is a 50% chance that each daughter will be a carrier like her mother, and a 50% chance that each son will have the disorder. If a carrier mates with a male who has the disorder, there is a 50% chance that each child born to them will have the disorder, regardless of sex. Daughters who do not have the disorder will be carriers, where as males without the disorder will be completely free of the recessive allele. (a) (b) (c) Sperm
  112. 112. <ul><li>Some recessive alleles found on the X chromosome in humans cause certain types of disorders </li></ul><ul><ul><li>Color blindness </li></ul></ul><ul><ul><li>Duchenne muscular dystrophy </li></ul></ul><ul><ul><li>Hemophilia </li></ul></ul>
  113. 113. X inactivation in Female Mammals <ul><li>In mammalian females </li></ul><ul><ul><li>One of the two X chromosomes in each cell is randomly inactivated during embryonic development </li></ul></ul>
  114. 114. <ul><li>If a female is heterozygous for a particular gene located on the X chromosome </li></ul><ul><ul><li>She will be a mosaic for that character </li></ul></ul>Two cell populations in adult cat: Active X Orange fur Inactive X Early embryo: X chromosomes Allele for black fur Cell division and X chromosome inactivation Active X Black fur Inactive X Figure 15.11
  115. 115. <ul><li>Alterations of chromosome number or structure cause some genetic disorders </li></ul><ul><li>Large-scale chromosomal alterations </li></ul><ul><ul><li>Often lead to spontaneous abortions or cause a variety of developmental disorders </li></ul></ul>
  116. 116. Abnormal Chromosome Number <ul><li>When nondisjunction occurs </li></ul><ul><ul><li>Pairs of homologous chromosomes do not separate normally during meiosis </li></ul></ul><ul><ul><li>Gametes contain two copies or no copies of a particular chromosome </li></ul></ul>Figure 15.12a, b Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n + 1 n + 1 n  1 n – 1 n + 1 n – 1 n n Number of chromosomes Nondisjunction of homologous chromosomes in meiosis I Nondisjunction of sister chromatids in meiosis II (a) (b)
  117. 117. <ul><li>Aneuploidy </li></ul><ul><ul><li>Results from the fertilization of gametes in which nondisjunction occurred </li></ul></ul><ul><ul><li>Is a condition in which offspring have an abnormal number of a particular chromosome </li></ul></ul>
  118. 118. <ul><li>If a zygote is trisomic </li></ul><ul><ul><li>It has three copies of a particular chromosome </li></ul></ul><ul><li>If a zygote is monosomic </li></ul><ul><ul><li>It has only one copy of a particular chromosome </li></ul></ul>
  119. 119. <ul><li>Polyploidy </li></ul><ul><ul><li>Is a condition in which there are more than two complete sets of chromosomes in an organism </li></ul></ul>Figure 15.13
  120. 120. Alterations of Chromosome Structure <ul><li>Breakage of a chromosome can lead to four types of changes in chromosome structure </li></ul><ul><ul><li>Deletion </li></ul></ul><ul><ul><li>Duplication </li></ul></ul><ul><ul><li>Inversion </li></ul></ul><ul><ul><li>Translocation </li></ul></ul>
  121. 121. <ul><li>Alterations of chromosome structure </li></ul>Figure 15.14a–d A B C D E F G H Deletion A B C E G H F A B C D E F G H Duplication A B C B D E C F G H A A M N O P Q R B C D E F G H B C D E F G H Inversion Reciprocal translocation A B P Q R M N O C D E F G H A D C B E F H G (a) A deletion removes a chromosomal segment. (b) A duplication repeats a segment. (c) An inversion reverses a segment within a chromosome. (d) A translocation moves a segment from one chromosome to another, nonhomologous one. In a reciprocal   translocation, the most common type, nonhomologous chromosomes exchange fragments. Nonreciprocal translocations also occur, in which a chromosome transfers a fragment without receiving a fragment in return.
  122. 122. Human Disorders Due to Chromosomal Alterations <ul><li>Alterations of chromosome number and structure </li></ul><ul><ul><li>Are associated with a number of serious human disorders </li></ul></ul>
  123. 123. Down Syndrome <ul><li>Down syndrome </li></ul><ul><ul><li>Is usually the result of an extra chromosome 21, trisomy 21 </li></ul></ul>Figure 15.15
  124. 124. Aneuploidy of Sex Chromosomes <ul><li>Nondisjunction of sex chromosomes </li></ul><ul><ul><li>Produces a variety of aneuploid conditions </li></ul></ul>
  125. 125. <ul><li>Klinefelter syndrome </li></ul><ul><ul><li>Is the result of an extra chromosome in a male, producing XXY individuals </li></ul></ul><ul><li>Turner syndrome </li></ul><ul><ul><li>Is the result of monosomy X, producing an X0 karyotype </li></ul></ul>
  126. 126. Disorders Caused by Structurally Altered Chromosomes <ul><li>Cri du chat </li></ul><ul><ul><li>Is a disorder caused by a deletion in a chromosome </li></ul></ul>
  127. 127. <ul><li>Certain cancers </li></ul><ul><ul><li>Are caused by translocations of chromosomes </li></ul></ul>Figure 15.16 Normal chromosome 9 Reciprocal translocation Translocated chromosome 9 Philadelphia chromosome Normal chromosome 22 Translocated chromosome 22
  128. 128. <ul><li>Some inheritance patterns are exceptions to the standard chromosome theory </li></ul><ul><li>Two normal exceptions to Mendelian genetics include </li></ul><ul><ul><li>Genes located in the nucleus </li></ul></ul><ul><ul><li>Genes located outside the nucleus </li></ul></ul>
  129. 129. Genomic Imprinting <ul><li>In mammals </li></ul><ul><ul><li>The phenotypic effects of certain genes depend on which allele is inherited from the mother and which is inherited from the father </li></ul></ul>
  130. 130. <ul><li>Genomic imprinting </li></ul><ul><ul><li>Involves the silencing of certain genes that are “stamped” with an imprint during gamete production </li></ul></ul>Figure 15.17a, b (a) A wild-type mouse is homozygous for the normal igf2 allele. Normal Igf2 allele (expressed) Normal Igf2 allele with imprint (not expressed) Paternal chromosome Maternal chromosome Wild-type mouse (normal size) Normal Igf2 allele Paternal Maternal Mutant lgf2 allele Mutant lgf2 allele Paternal Maternal Dwarf mouse Normal Igf2 allele with imprint Normal size mouse (b) When a normal Igf2 allele is inherited from the father, heterozygous mice grow to normal size. But when a mutant allele is inherited from the father, heterozygous mice have the dwarf phenotype.
  131. 131. Inheritance of Organelle Genes <ul><li>Extranuclear genes </li></ul><ul><ul><li>Are genes found in organelles in the cytoplasm </li></ul></ul>
  132. 132. <ul><li>The inheritance of traits controlled by genes present in the chloroplasts or mitochondria </li></ul><ul><ul><li>Depends solely on the maternal parent because the zygote’s cytoplasm comes from the egg </li></ul></ul>Figure 15.18
  133. 133. <ul><li>Some diseases affecting the muscular and nervous systems </li></ul><ul><ul><li>Are caused by defects in mitochondrial genes that prevent cells from making enough ATP </li></ul></ul>