1. Chapter 14 Mendel and the Gene Idea

2. Inheritance The passing of traits from parents to offspring. Humans have known about inheritance for thousands of years.

3. Genetics The scientific study of the inheritance. Genetics is a relatively “new” science (about 150 years).

4. Genetic Theories 1. Blending Theory - traits were like paints and mixed evenly from both parents. What happen over time if this were true? This would lead to a homogenous population over time. Can’t explain traits that skip a generation

5. 2. Particulate Model - parents pass on traits as discrete units that retain their identities in the offspring. -like a deck of cards

6. Gregor Mendel Father of Modern Genetics.

8. Reasons for Mendel's Success Used an experimental approach. Applied mathematics to the study of natural phenomena. Kept good records.

9. Mendel was a pea picker. He used peas as his study organism.

10. Why Use Peas? Short life span. Bisexual. Many traits known. Cross- and self-pollinating. (You can eat the failures).

11. Cross-pollination Two parents. Results in hybrid offspring where the offspring may be different than the parents.

13. Self-pollination One flower as both parents. Natural event in peas. Results in pure-bred offspring where the offspring are identical to the parents.

14. Mendel's Work Used seven characters, each with two traits or expressions. Example: Character - height Traits - tall or short.

15. Monohybrid Crosses Crosses that work with a single character at a time. Example - Purple X White

16. P Generation The Parental generation or the first two individuals used in a cross. Example - Purple X White Mendel used reciprocal crosses, where the parents alternated for the trait.

17. Offspring F1 - first filial generation. F2 - second filial generation, bred by crossing two F1 plants together or allowing a F1 to self-pollinate.

21. Results - Summary In all crosses, the F1 generation showed only one of the traits regardless of which color was male or female. The other trait reappeared in the F2 at ~25% (3:1 ratio).

23. Mendel's Hypothesis 1. Genes can have alternate versions called alleles. 2. Each offspring inherits two alleles, one from each parent.

24. Mendel's Hypothesis 3. If the two alleles differ, the dominant allele is expressed. The recessive allele remains hidden unless the dominant allele is absent. Comment - do not use the terms “strongest” to describe the dominant allele.

25. Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers Allele Purple White

26. Mendel's Hypothesis 4. The two alleles for each trait separate during gamete formation and end up in different games. This now called: Mendel's Law of Segregation

27. Law of Segregation

28. Mendel’s Experiments Showed that the Particulate Model best fit the results.

29. Vocabulary Phenotype - the physical appearance of the organism. Genotype - the genetic makeup of the organism, usually shown in a code. P = purple p = white

31. Helpful Vocabulary Homozygous - When the two alleles are the same (PP or pp). Heterozygous- When the two alleles are different (Pp).

32. 6 Mendelian Crosses are Possible Cross Genotype Phenotype TT X tt all Tt all Dom Tt X Tt 1TT:2Tt:1tt 3 Dom: 1 Res TT X TT all TT all Dom tt X tt all tt all Res TT X Tt 1TT:1Tt all Dom Tt X tt 1Tt:1tt 1 Dom: 1 Res

33. Test Cross Cross of a dominant phenotype of unknown genotype with a recessive phenotype. Ex: P? X pp If PP - all dominant phenotype If Pp - 1 Dominant: 1 Recessive

35. Dihybrid Cross Cross with two genetic traits. Need 4 letters to code for the cross. Ex: TtRr Each Gamete - Must get 1 letter for each trait. Ex. TR, Tr, etc.

36. Number of Kinds of Gametes Critical to calculating the results of higher level crosses. Look for the number of heterozygous traits.

37. Equation The formula 2 n can be used, where “n” = the number of heterozygous traits. Ex: TtRr, n=2 2 2 or 4 different kinds of gametes are possible. TR, tR, Tr, tr

38. Dihybrid Cross TtRr X TtRr Each parent can produce 4 types of gametes. TR, Tr, tR, tr Cross is a 4 X 4 with 16 possible offspring.

39. Results 9 Tall, Red flowered 3 Tall, white flowered 3 short, Red flowered 1 short, white flowered Or: 9:3:3:1

40. Law of Independent Assortment The inheritance of 1st genetic trait is NOT dependent on the inheritance of the 2 nd trait. Inheritance of height is independent of the inheritance of flower color.

41. Comment Ratio of Tall to short is 3:1 Ratio of Red to white is 3:1 The cross is really a product of the ratio of each trait multiplied together. (3:1) X (3:1)

43. Probability Genetics is a specific application of the rules of probability. Probability - the chance that an event will occur out of the total number of possible events.

45. Genetic Ratios The monohybrid “ratios” are actually the “probabilities” of the results of random fertilization. Ex: 3:1 75% chance of the dominant 25% chance of the recessive

46. Rule of Multiplication or Product Rule The probability that two alleles will come together at fertilization, is equal to the product of their separate probabilities.

47. Example: TtRr X TtRr The probability of getting a tall offspring is ¾. The probability of getting a red offspring is ¾. The probability of getting a tall red offspring is ¾ x ¾ = 9/16

48. Comment Use the Product Rule to calculate the results of complex crosses rather than work out the Punnett Squares. Ex: TtrrGG X TtRrgg

49. Solution “T’s” = Tt X Tt = 3:1 “R’s” = rr X Rr = 1:1 “G’s” = GG x gg = 1:0 Product is: (3:1) X (1:1) X (1:0 ) = 3:3:1:1

50. Dominance vs Phenotype A dominant allele does not subdue a recessive allele; alleles don’t interact. Alleles are simply variations in a gene’s nucleotide sequence.

51. Variations on Mendel 1. Incomplete Dominance 2. Codominance 3. Multiple Alleles 4. Epistasis 5. Polygenic Inheritance

52. Incomplete Dominance When the F1 hybrids show a phenotype somewhere between the phenotypes of the two parents. Often a “dose” effect Ex. Red X White snapdragons F1 = all pink F2 = 1 red: 2 pink: 1 white

54. Result No hidden Recessive 3 phenotypes and 3 genotypes (Hint! – often a “dose” effect) Red = C R C R Pink = C R C W White = C W C W

55. Another example

56. Codominance Both alleles are expressed equally in the phenotype. Ex. MN blood group MM MN NN

57. Result No hidden Recessive 3 phenotypes and 3 genotypes (but not a “dose” effect)

58. Multiple Alleles When there are more than 2 alleles for a trait Ex. ABO blood group I A - A type antigen I B - B type antigen i - no antigen

59. Result Multiple genotypes and phenotypes. Very common event in many traits.

60. Alleles and Blood Types Type Genotypes A I A I A or I A i B I B I B or I B i AB I A I B O ii

64. Comment Rh blood factor is a separate factor from the ABO blood group. Rh+ = dominant Rh- = recessive A+ blood = dihybrid trait

65. Epistasis When 1 gene locus alters the expression of a second locus. Ex: 1 st gene: C = color, c = albino 2 nd gene: B = Brown, b = black

66. Gerbils

67. In Gerbils CcBb X CcBb Brown X Brown F1 = 9 brown (C_B_) 3 black (C_bb) 4 albino (cc__)

68. Result Ratios often altered from the expected. One trait may act as a recessive because it is “hidden” by the second trait.

69. Polygenic Inheritance Factors that are expressed as continuous variation. Lack clear boundaries between the phenotype classes. Ex: skin color, height

70. Genetic Basis Several genes govern the inheritance of the trait. Ex: Skin color is likely controlled by at least 4 genes. Each dominant gives a darker skin.

72. Result Mendelian ratios fail. Traits tend to "run" in families. Offspring often intermediate between the parental types. Trait shows a “bell-curve” or continuous variation.

73. Genetic Studies in Humans Often done by Pedigree charts. Why? Can’t do controlled breeding studies in humans. Small number of offspring. Long life span.

74. Pedigree Chart Symbols Male Female Person with trait

75. Sample Pedigree

76. Dominant Trait Recessive Trait

77. Human Recessive Disorders Several thousand known: Albinism Sickle Cell Anemia Tay-Sachs Disease Cystic Fibrosis PKU Galactosemia

78. Sickle-cell Disease Most common inherited disease among African-Americans. Single amino acid substitution results in malformed hemoglobin. Reduced O 2 carrying capacity. Codominant inheritance.

80. Tay-Sachs Eastern European Jews. Brain cells unable to metabolize type of lipid, accumulation of causes brain damage. Death in infancy or early childhood.

81. Dominance vs Phenotype For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype.

82. Example -Tay-Sachs Disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain. At the organismal level, the allele is recessive. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

83. Tay-Sachs At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant. At the molecular level, the alleles are codominant.

84. Cystic Fibrosis Most common lethal genetic disease in the U.S. Most frequent in Caucasian populations (1/20 a carrier). Produces defective chloride channels in membranes.

85. Recessive Pattern Usually rare. Skips generations. Occurrence increases with consaguineous matings. Often an enzyme defect.

86. Human Dominant Disorders Less common then recessives. Ex: Huntington’s disease Achondroplasia Familial Hypercholsterolemia

87. Inheritance Pattern Each affected individual had one affected parent. Doesn’t skip generations. Homozygous cases show worse phenotype symptoms. May have post-maturity onset of symptoms.

89. Homework Read Chapter 14 (Hillis – 8) Chapter 14 – Mon. 1/28

90. Genetic Screening Risk assessment for an individual inheriting a trait. Uses probability to calculate the risk.

91. General Formula R = F X M X D R = risk F = probability that the female carries the gene. M = probability that the male carries the gene. D = Disease risk under best conditions.

92. Example Wife has an albino parent. Husband has no albinism in his pedigree. Risk for an albino child?

93. Risk Calculation Wife = probability is 1.0 that she has the allele. Husband = with no family record, probability is near 0. Disease = this is a recessive trait, so risk is Aa X Aa =.25 R = 1 X 0 X.25 R = 0

94. Risk Calculation Assume husband is a carrier, then the risk is: R = 1 X 1 X.25 R =.25 There is a.25 chance that any child will be albino.

96. Common Mistake If risk is.25, then as long as we don’t have 4 kids, we won’t get any with the trait. Risk is.25 for each child. It is not dependent on what happens to other children.

97. Carrier Recognition Fetal Testing Amniocentesis Chorionic villi sampling Newborn Screening

98. Fetal Testing Biochemical Tests Chromosome Analysis

99. Amniocentesis Administered between 11 - 14 weeks. Extract amnionic fluid = cells and fluid. Biochemical tests and karyotype. Requires culture time for cells.

101. Chorionic Villi Sampling Administered between 8 - 10 weeks. Extract tissue from chorion (placenta). Slightly greater risk but no culture time required.

104. Newborn Screening Blood tests for recessive conditions that can have the phenotypes treated to avoid damage. Genotypes are NOT changed. Ex. PKU

105. Newborn Screening Required by law in all states. Tests 1- 6 conditions. Required of “home” births too.

106. Multifactorial Diseases Where Genetic and Environment Factors interact to cause the Disease. Becoming more widely recognized in medicine.

107. Ex. Heart Disease Genetic Diet Exercise Bacterial Infection

108. Genes & Environment

109. Summary Know the Mendelian crosses and their patterns. Be able to work simple genetic problems (practice). Watch genetic vocabulary. Be able to read pedigree charts.

110. Summary Be able to recognize and work with some of the “common” human trait examples.