1. Chapter 14 Mendelian Genetics

2. I. Mendel’s Approach Advantages of pea plants for genetic study: – There are many varieties with distinct heritable features, or characters (flower color); character variants (purple or white flowers) are called traits – Mating of plants can be controlled – Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels) – Cross-pollination (fertilization between different plants) accomplished by dusting one plant with pollen from another

3. Fig. 14-2a Stamens Carpel Parental generation (P) TECHNIQUE 1 2 3 4

4. Fig. 14-2b First filial gener- ation offspring (F 1 ) RESULTS 5

5. Mendel tracked characters that varied in an either-or manner True-breeding = plants produce offspring of same variety when they self-pollinate

6. Mendel mated two contrasting, true-breeding varieties (hybridization) P generation - true-breeding parents F 1 generation - hybrid offspring of the P generation When F 1 individuals self-pollinate, the F 2 generation is produced

7. II. Law of Segregation Crossing true-breeding white and purple flowered pea plants, all F 1 hybrids were purple Crossing F 1 hybrids, many of the F 2 plants had purple flowers, but some had white Mendel discovered a ratio of about three to one, purple to white flowers, in the F 2 generation

8. Fig. 14-3-1 EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers 

9. Fig. 14-3-2 EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers  F 1 Generation (hybrids) All plants had purple flowers

10. Fig. 14-3-3 EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers  F 1 Generation (hybrids) All plants had purple flowers F 2 Generation 705 purple-flowered plants 224 white-flowered plants

11. Mendel reasoned only the purple flower factor was affecting flower color in F 1 hybrids (purple color = dominant trait and white color = recessive trait Mendel observed same pattern of inheritance in six other characters, each represented by two traits

12. Table 14-1

13. A. Mendel’s Model First concept = different versions of genes give variations in inherited characters one gene for purple flowers and another for white flowers Alleles = These alternative versions of a gene Each gene resides at a specific locus on a specific chromosome

14. Fig. 14-4 Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers

15. Second concept = an organism inherits two alleles, one from each parent Mendel made this deduction without knowing about the role of chromosomes The two alleles at a locus on a chromosome may be identical TT or tt (P generation) or two alleles at a locus may differ Tt (F 1 hybrids)

16. Third concept = two alleles at a locus differ, then one (dominant allele) determines the organism’s appearance, and the other (recessive allele) has no effect on appearance In the flower-color example, the F 1 plants had purple flowers because the allele for that trait is dominant

17. Fourth concept (law of segregation) = the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes egg or sperm gets only one of two alleles that are in the somatic cells of an organism Distribution of homologous chroms during meiosis

18. Fig. 14-5-3 P Generation Appearance: Genetic makeup: Gametes: Purple flowers White flowers PP P pp p F 1 Generation Gametes: Genetic makeup: Appearance: Purple flowers Pp P p 1/21/2 1/21/2 F 2 Generation Sperm Eggs P P PPPp p p pp 31

19. B. Vocabulary Homozygous = two identical alleles for a character (TT or tt) Heterozygous = two different alleles for a gene (Tt) heterozygotes are not true-breeding

20. Phenotype = physical appearance Genotype = genetic makeup In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes

21. Fig. 14-6 Phenotype Purple 3 Genotype 1 White Ratio 3:1 (homozygous) (heterozygous) PP Pp pp Ratio 1:2:1 1 1 2

22. C. The Testcross How can we tell the genotype of an individual with the dominant phenotype? Individual could be either homozygous dominant (PP) or heterozygous (Pp) Testcross: breeding the mystery individual with a homozygous recessive (pp) individual If any offspring display the recessive phenotype (pp), mystery parent must be heterozygous (Pp)

23. Fig. 14-7 TECHNIQUE RESULTS Dominant phenotype, unknown genotype: PP or Pp? Predictions Recessive phenotype, known genotype: pp  If PPIf Pp or Sperm ppp p P P P p Eggs Pp pp or All offspring purple 1 / 2 offspring purple and 1 / 2 offspring white

24. III. Law of Independent Assortment In Mendel’s experiment, F 1 offspring were monohybrids, individuals that are heterozygous for one character – A cross between such heterozygotes is called a monohybrid cross

25. Mendel identified his second law of inheritance by following two characters at the same time Crossing two true-breeding parents differing in two characters (YYRR x yyrr) produces dihybrids in the F 1 generation, heterozygous for both characters (YyRr) Dihybrid cross, a cross between F 1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

26. Fig. 14-8 EXPERIMENT RESULTS P Generation F 1 Generation Predictions Gametes Hypothesis of dependent assortment YYRRyyrr YR yr YyRr  Hypothesis of independent assortment or Predicted offspring of F 2 generation Sperm YR yr Yr YR yR Yr yR yr YR YYRR YyRr YYRr YyRR YYrr Yyrr yyRR yyRr yyrr Phenotypic ratio 3:1 Eggs Phenotypic ratio 9:3:3:1 1/21/2 1/21/2 1/21/2 1/21/2 1/41/4 yr 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 3/43/4 9 / 16 3 / 16 1 / 16 Phenotypic ratio approximately 9:3:3:1 31510810132

27. Law of independent assortment = each pair of alleles segregates independently of other pairs of alleles during gamete formation Applies only to genes on different, nonhomologous chromosomes Genes located near each other on the same chromosome tend to be inherited together Mendelian Genetics - Bozeman (16 min)

28. IV. Solving Problems w/ Probability Apply the multiplication (simultaneous events) and addition (separate events) rules to predict the outcome

29. V. Inheritance Patterns Inheritance of characters by a single gene are not always dominant / recessive

30. A. Degrees of Dominance Complete dominance = phenotypes of heterozygote and dominant homozygote are same Incomplete dominance = phenotype of hybrids is between phenotypes of the two parents (RR x WW = RW, pink) Codominance = two dominant alleles affect the phenotype in separate, distinguishable ways (RR x WW = RW, red and white hair)

31. Fig. 14-10-3 Red P Generation Gametes White CRCRCRCR CWCWCWCW CRCR CWCW F 1 Generation Pink CRCWCRCW CRCR CWCW Gametes 1/21/2 1/21/2 F 2 Generation Sperm Eggs CRCR CRCR CWCW CWCW CRCRCRCR CRCWCRCW CRCWCRCW CWCWCWCW 1/21/2 1/21/2 1/21/2 1/21/2

33. Dominant allele does not subdue a recessive allele; alleles don’t interact Alleles are variations in a gene’s nucleotide sequence Dominance/recessiveness of alleles depend on the level at which we examine the phenotype Dominance and Phenotype

34. Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain – Organismal level, allele is recessive – Biochemical level, phenotype (enzyme activity level) is incompletely dominant – Molecular level, alleles are codominant

35. Frequency of Dominant Alleles Dominant alleles are not necessarily more common in populations than recessive alleles 1 in 400 babies in the US is born with extra fingers or toes (polydactyl) Gene is dominant, recessive is more frequent

36. B. Multiple Alleles Most genes have more than 2 alleles Four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: I A, I B, and i. (I A and I B are codominant but dominant over i)

37. Fig. 14-11 IAIA IBIB i A B none (a) The three alleles for the ABO blood groups and their associated carbohydrates Allele Carbohydrate Genotype Red blood cell appearance Phenotype (blood group) I A I A or I A i A B I B I B or I B i IAIBIAIB AB iiO (b) Blood group genotypes and phenotypes

38. C. Pleiotropy Most genes have multiple phenotypic effects, a property called pleiotropy For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

39. D. Polygenic Inheritance Quantitative characters are those that vary in the population along a continuum Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype Ex. Skin color in humans

40. Fig. 14-13 Eggs Sperm Phenotypes: Number of dark-skin alleles: 0 1 2 345 6 1 / 64 6 / 64 15 / 64 20 / 64 15 / 64 6 / 64 1 / 64 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 AaBbCc 

41. VI. Environmental Impact on Phenotype Some phenotypes for a character depends on environment as well as genotype Norm of reaction = phenotypic range of a genotype influenced by the environment Ex. hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity

42. Fig. 14-14

43. Norms of reaction are generally broadest for polygenic characters – called multifactorial because genetic and environmental factors collectively influence phenotype

44. Concept 14.4: Many human traits follow Mendelian patterns of inheritance Humans are not good subjects for genetic research –Generation time is too long –Parents produce relatively few offspring –Breeding experiments are unacceptable However, basic Mendelian genetics endures as the foundation of human genetics Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

45. Pedigree Analysis A pedigree is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can be traced and described using pedigrees Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

46. Fig. 14-15a Key Male Female Affected male Affected female Mating Offspring, in birth order (first-born on left)

47. Fig. 14-15b 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) 3rd generation (two sisters) Widow’s peakNo widow’s peak (a) Is a widow’s peak a dominant or recessive trait? Wwww Ww ww Ww wwWW Ww or

48. Fig. 14-15c Attached earlobe 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) 3rd generation (two sisters) Free earlobe (b) Is an attached earlobe a dominant or recessive trait? Ff ffFf ff FFor FF Ff

49. Pedigrees can also be used to make predictions about future offspring We can use the multiplication and addition rules to predict the probability of specific phenotypes

50. Recessively Inherited Disorders Many genetic disorders are inherited in a recessive manner Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

51. The Behavior of Recessive Alleles 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 (i.e., pigmented) Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

52. Fig. 14-16 Parents Normal Sperm Eggs Normal (carrier) Normal (carrier) Albino Aa A A AA Aa a aa a 

53. If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele Most societies and cultures have laws or taboos against marriages between close relatives Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

54. Cystic Fibrosis Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

55. Sickle-Cell Disease Sickle-cell disease affects one out of 400 African-Americans The disease 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

56. Dominantly Inherited Disorders Some human disorders are caused by dominant alleles Dominant alleles that cause a lethal disease are rare and arise by mutation Achondroplasia is a form of dwarfism caused by a rare dominant allele Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

57. Fig. 14-17 Eggs Parents Dwarf Normal Dwarf Sperm Dd  dd d D Dd dd Dd d d

58. Huntington’s disease is a degenerative disease of the nervous system The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age Huntington’s Disease Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

59. Multifactorial Disorders Many diseases, such as heart disease and cancer, have both genetic and environmental components Little is understood about the genetic contribution to most multifactorial diseases Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

60. Genetic Testing and Counseling Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

61. 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

62. Tests for Identifying Carriers For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

63. 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 Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Video: Ultrasound of Human Fetus I Video: Ultrasound of Human Fetus I

64. Fig. 14-18 Amniotic fluid withdrawn Fetus Placenta Uterus Cervix Centrifugation Fluid Fetal cells Several hours Several weeks Several weeks (a) Amniocentesis (b) Chorionic villus sampling (CVS) Several hours Several hours Fetal cells Bio- chemical tests Karyotyping Placenta Chorionic villi Fetus Suction tube inserted through cervix

65. Fig. 14-18a Fetus Amniotic fluid withdrawn Placenta Uterus Cervix Centrifugation Fluid Fetal cells Several hours Several weeks Several weeks Bio- chemical tests Karyotyping (a) Amniocentesis

66. Fig. 14-18b (b) Chorionic villus sampling (CVS) Bio- chemical tests PlacentaChorionic villi Fetus Suction tube inserted through cervix Fetal cells Several hours Several hours Karyotyping

67. 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

68. Fig. 14-UN2 Degree of dominance Complete dominance of one allele Incomplete dominance of either allele Codominance Description Heterozygous phenotype same as that of homo- zygous dominant Heterozygous phenotype intermediate between the two homozygous phenotypes Heterozygotes: Both phenotypes expressed Multiple alleles Pleiotropy In the whole population, some genes have more than two alleles One gene is able to affect multiple phenotypic characters CRCRCRCR CRCWCRCW CWCWCWCW IAIBIAIB I A, I B, i ABO blood group alleles Sickle-cell disease PP Pp Example

69. Fig. 14-UN3 Description Relationship among genes EpistasisOne gene affects the expression of another Example Polygenic inheritance A single phenotypic character is affected by two or more genes BbCc BC bC Bc bc 9 : 3: 4 AaBbCc

70. Fig. 14-UN4

71. Fig. 14-UN5 George Sandra TomSam Arlene Wilma AnnMichael Carla DanielAlan Tina Christopher

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78. You should now be able to: 1.Define the following terms: true breeding, hybridization, monohybrid cross, P generation, F 1 generation, F 2 generation 2.Distinguish between the following pairs of terms: dominant and recessive; heterozygous and homozygous; genotype and phenotype 3.Use a Punnett square to predict the results of a cross and to state the phenotypic and genotypic ratios of the F 2 generation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

79. 4.Explain how phenotypic expression in the heterozygote differs with complete dominance, incomplete dominance, and codominance 5.Define and give examples of pleiotropy and epistasis 6.Explain why lethal dominant genes are much rarer than lethal recessive genes 7.Explain how carrier recognition, fetal testing, and newborn screening can be used in genetic screening and counseling Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings