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 14 Mendel and the Gene Idea

2. 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?

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1 poss explanation of heredity is a “blending” hypothesis – genetic material contributed by 2 parents mixes the way blue & yellow paints blend to make green

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings “particulate” hypothesis of inheritance is an alternative view – Parents pass on discrete heritable units, genes

5. 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

6. 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

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel’s Experimental, Quantitative Approach Mendel chose to work with peas – B/c they are available in many varieties – B/c he could strictly control which plants mated with which

8. 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 (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 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

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Some genetic vocabulary – Character- heritable feature (flower color) – Trait- variant of a character, (such as purple or white flowers)

10. 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”

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings typical breeding exp: – Mendel mated 2 contrasting, true-breeding varieties, a process called hybridization true-breeding parents are: P generation

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings hybrid offspring of P generation- F 1 generation F 1 indiv;s self-pollinate- F 2 generation is produced

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Law of Segregation When Mendel crossed contrasting, true- breeding white & purple flowered pea plants – All of the offspring were purple When Mendel crossed the F 1 plants – Many of the plants had purple flowers, but some had white flowers

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel discovered – A ratio of about 3:1, purple to white flowers, in the F 2 generation 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.

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel reasoned that – In the F 1 plants, only the purple flower factor was affecting flower color in these hybrids – Purple flower color was dominant, & white flower color was recessive

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel observed the same pattern – In many other pea plant characters Table 14.1

17. 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 F 2 offspring Four related concepts make up this model Law of segregation: Each allele in a pair separates into diff gamete formation

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1 st, alternative versions of genes – Account for variations in inherited char’s, are now called alleles Figure 14.4 Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 nd, for each character – An organism inherits 2 alleles, 1 from each parent – genetic locus is represented twice

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 3 rd, if the 2 alleles at a locus differ – Then, the dom allele, determines the organism’s appearance – the recessive allele, has no noticeable effect on the organism’s appearance (phenotype)

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 4 th, the law of segregation – The 2 alleles for a heritable char separate (segregate) during gamete formation & end up in diff gametes

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Does Mendel’s segregation model account for the 3:1 ratio he observed in the F 2 generation of his numerous crosses? – We can answer this question using a Punnett square

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel’s law of segregation, probability and the Punnett square Figure 14.5 P Generation F 1 Generation F 2 Generation P p P p P p P p PpPp PP pp Pp Appearance: Genetic makeup: Purple flowers PP White flowers pp Purple flowers Pp Appearance: Genetic makeup: Gametes: F 1 sperm F 1 eggs 1/21/2 1/21/2  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.

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Useful Genetic Vocabulary An organism homozygous for a particular gene – Has a pair of identical alleles for that gene – shows true-breeding An organism heterozygous for a gene – Has a pair of alleles that are diff for that gene

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings phenotype- physical appearance genotype- genetic makeup

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phenotype versus genotype Figure 14.6 3 1 1 2 1 Phenotype Purple White Genotype PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Ratio 3:1 Ratio 1:2:1

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Testcross In pea plants w/ purple flowers – The genotype is not immediately obvious

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings testcross – finds genotype of an organism w/ the dom phenotype, but unknown genotype – Crosses an indiv w/ dom phenotype w/ an indiv that is homozygous recessive for a trait

29. 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 Pp P p pp 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

30. 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 F 1 offspring produced in this cross – Were monohybrids, heterozygous for one character

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel identified his 2 nd law of inheritance – By following 2 characters at the same time xing 2, true-breeding parents differing in 2 characters – Produces dihybrids in the F 1 generation, heterozygous for both characters

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How are 2 characters transmitted from parents to offspring? – As a package? – Independently?

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings YYRR P Generation GametesYRyr  yyrr YyRr Hypothesis of dependent assortment Hypothesis of independent assortment F 2 Generation (predicted offspring) 1⁄21⁄2 YR yr 1 ⁄ 2 1⁄21⁄2 yr YYRRYyRr 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 1 ⁄ 16 YYRR YYRr YyRR YyRr YyrrYyRr YYrr YyRR YyRr yyRRyyRr yyrr yyRr Yyrr YyRr Phenotypic ratio 9:3:3:1 315108 101 32 Phenotypic ratio approximately 9:3:3:1 F 1 Generation Eggs YR Yr yRyr 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. A dihybrid cross – Illustrates the inheritance of 2 characters Produces four phenotypes in the F 2 generation Figure 14.8

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Using the information from a dihybrid cross, Mendel developed law of independent assortment – Each pair of alleles segregates independently during gamete formation this law applies only to genes on diff, nonhomologous chromo’s Genes located near each other on the same chromo tend to be inherited together

35. 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

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Multiplication and Addition Rules Applied to Monohybrid Crosses multiplication rule – probability that 2 or more independent events will occur together is the product of their individual probabilities

37. 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 R 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 1⁄21⁄2 r r R r r Sperm  Eggs Figure 14.9

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings rule of addition – the probability that any 1 of 2 or more exclusive events will occur is calculated by adding together their indiv probabilities

39. 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

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A dihybrid or multicharacter cross – same as to 2 or more independent monohybrid crosses occurring simultaneously to calculate the chances for various genotypes from such x’s – Each char 1 st is considered separately & then the indiv probabilities are multiplied together

41. 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 b/w genotype & phenotype is rarely simple

42. 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

43. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Spectrum of Dominance Complete dominance – phenotypes of the heterozygote & dom homozygote are identical

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings codominance – 2 dom alleles affect the phenotype in separate, distinguishable ways Ex: blood type

45. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings incomplete dominance – phenotype of F 1 hybrid is somewhere b/w the phenotypes of the 2 parental varieties Figure 14.10 P Generation F 1 Generation F 2 Generation Red C R Gametes CRCR CWCW  White C W Pink C R C W Sperm CRCR CRCR CRCR CwCw CRCR CRCR Gametes 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 Eggs 1⁄21⁄2 C R C R C W C W C R C W

46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Relation B/w Dominance & Phenotype Dominant & recessive alleles – Do not really “interact” – Lead to synthesis of diff proteins that make a phenotype

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Frequency of Dominant Alleles Dom alleles are not necessarily more common in pop’s than recessive alleles For ex, 1 baby out of 400 in the USA is born with extra fingers or toes The allele for this unusual trait is dom to the allele for the more common trait of 5 digits per appendage In this ex, the recessive allele is far more prevalent than the dominant allele in the pop

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Frequency of Dominant Alleles Dominant alleles – Are not necessarily more common in pop’s than recessive alleles

49. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Multiple Alleles Most genes exist in pop’s – In more than 2 allelic forms

50. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ABO blood group in humans – determined by multiple alleles Table 14.2

51. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pleiotropy pleiotropy – gene has multiple phenotypic effects

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pleiotropy For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

53. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Extending Mendelian Genetics for Two or More Genes Some traits – May be determined by 2 or more genes

54. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Epistasis epistasis – gene at 1 locus alters the phenotypic expression of a gene at a 2 nd locus

55. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An example of epistasis Figure 14.11 BC bCBc bc 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 BC bC Bc bc 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 BBCcBbCc BBcc Bbcc bbcc bbCc BbCc BbCC bbCC BbCc bbCc BBCCBbCC BBCc BbCc 9 ⁄ 16 3 ⁄ 16 4 ⁄ 16 BbCc  Sperm Eggs

56. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Polygenic Inheritance Many human characters – Vary in the pop along a continuum & are called quantitative char’s

57. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  AaBbCc aabbcc Aabbcc AaBbccAaBbCcAABbCc AABBCcAABBCC 20 ⁄ 64 15 ⁄ 64 6 ⁄ 64 1 ⁄ 64 Fraction of progeny Quantitative variation usually indicates polygenic inheritance – An additive effect of 2 or more genes on a single phenotype Figure 14.12

58. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Nature and Nurture: The Environmental Impact on Phenotype simple Mendelian genetics don’t apply… – When the phenotype for a char depends on environment as well as on genotype

59. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The norm of rxn – Is the phenotypic range of a particular genotype that is influenced by the environment Figure 14.13

60. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Multifactorial char’s – influenced by both genetic & environmental factors

61. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Integrating a Mendelian View of Heredity and Variation organism’s phenotype – Includes- physical appearance, internal anatomy, physiology, & behavior – Reflects its overall genotype & environ history

62. 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

63. 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

64. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pedigree Analysis pedigree – family tree that describes the interrelationships of parents & children across generations

65. 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 wwWw ww Ww WW or Ww ww First generation (grandparents) Second generation (parents plus aunts and uncles) Third generation (two sisters) Ff ffFf 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)

66. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pedigrees – Can also be used to make predictions about future offspring

67. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recessively Inherited Disorders Many genetic disorders are inherited in a recessive manner 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

68. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recessively inherited disorders – Show up only in indiv’ls homozygous for the allele Carriers – Are heterozygous individuals who carry the recessive allele but are phenotypically normal

69. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cystic Fibrosis Cystic fibrosis: – Mucus buildup in some internal organs – Abnormal absorption of nutrients in the small intestine

70. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sickle-Cell Disease Sickle-cell disease – 1 of 400 African-Americans – caused by the substitution of a single aa in the hemoglobin protein in rbc’s Symptoms include – Physical weakness, pain, organ damage, & even paralysis

71. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mating of Close Relatives Matings b/w relatives – Can ↑ the probab of the appearance of a genetic disease – called consanguineous matings

72. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Dominantly Inherited Disorders Some human disorders – Are due to dominant alleles

73. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ex: achondroplasia – A form of dwarfism that is lethal when homo for the dom allele Figure 14.15

74. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Huntington’s disease – degenerative disease of the nervous system – Has no obvious phenotypic effects until about 35 to 40 years of age Figure 14.16

75. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Multifactorial Disorders Many human diseases – Have both genetic & environ components Ex: – Heart disease & cancer

76. 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

77. 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

78. 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

79. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fetal Testing amniocentesis - liquid that bathes the fetus is removed & tested chorionic villus sampling (CVS) – sample of the placenta is removed & tested

80. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fetal testing Figure 14.17 A, B (a) Amniocentesis Amniotic fluid withdrawn Fetus PlacentaUterus 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

81. 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