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. Gregor Mendel
Father of Modern Genetics.

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

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

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

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

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

11. Monohybrid or Mendelian Crosses
Crosses that work with a single character at a time.
Example - Tall X short

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

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

17. Another Sample Cross P1 Tall X short (TT x tt) F1 all Tall (Tt)
F tall to 1 short
(1 TT: 2 Tt: 1 tt)

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

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

20. 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.

21. Mendel's Hypothesis
4. The two alleles for each trait separate during gamete formation. This now called: Mendel's Law of Segregation

22. Law of Segregation

23. Vocabulary Phenotype - the physical appearance of the organism.
Genotype - the genetic makeup of the organism, usually shown in a code.
T = tall
t = short

25. Helpful Vocabulary
Homozygous - When the two alleles are the same (TT/tt).
Heterozygous- When the two alleles are different (Tt).

26. 6 Mendelian Crosses are Possible
Cross Genotype Phenotype
TT X tt all Tt all Dom
Tt X Tt TT:2Tt:1tt Dom: 1 Res
TT X TT all TT all Dom
tt X tt all tt all Res
TT X Tt TT:1Tt all Dom
Tt X tt Tt:1tt Dom: 1 Res

27. Test Cross
Cross of a suspected heterozygote with a homozygous recessive.
Ex: T_ X tt
If TT - all dominant
If Tt - 1 Dominant: 1 Recessive

29. 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.

30. 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.

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

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

34. 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)

35. 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.

37. 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

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

39. 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

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

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

42. Incomplete Dominance
When the F1 hybrids show a phenotype somewhere between the phenotypes of the two parents.(blending)
Ex. Red X White snapdragons
F1 = all pink
F2 = 1 red: 2 pink: 1 white

44. Result No hidden Recessive. 3 phenotypes and 3 genotypes Red = CR CR
Pink = CRCW
White = CWCW

45. Another example

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

47. Result
No hidden Recessive.
3 phenotypes and genotypes

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

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

50. Alleles and Blood Types
Type Genotypes
A IA IA or IAi
B IB IB or IBi
AB IAIB
O ii

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

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

55. Gerbils

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

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

58. Epistasis in Mice

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

60. 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.

62. 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.

63. 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.

64. Pedigree Chart Symbols
Male
Female
Person with trait

65. Sample Pedigree

66. Recessive Trait
Dominant Trait

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

68. Sickle-cell Disease
Most common inherited disease among African-Americans.
Single amino acid substitution results in malformed hemoglobin.
Reduced O2 carrying capacity.
Codominant inheritance.

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

71. 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.

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

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

74. 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.

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

76. General Formal 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.

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

78. 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

79. 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 every child will be albino.

80. 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.

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

82. Fetal Testing
Biochemical Tests
Chromosome Analysis

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

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

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

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

89. Multifactorial Diseases
Where Genetic and Environment Factors interact to cause the Disease.

90. Ex. Heart Disease
Genetic
Diet
Exercise
Bacterial Infection