1. Chapter 14: Mendel and the Gene Idea

2. Essential Knowledge
3.a.3 – The chromosomal basis of inheritance provides an understanding of the pattern of passage (transmission) of genes from parent to offspring ( ).
4.c.2 – Environmental factors influence the expression of the genotype in an organism – (14.3).
4.c.4 – The diversity of species within an ecosystem may influence the stability of the ecosystem (14.3).

3. Past/Present Genetic Hypotheses
1. Blending Theory - traits were like paints and mixed evenly from both parents 2. Incubation Theory - only one parent controlled the traits of the children Ex: Spermists and Ovists 3. Particulate Model - parents pass on traits as discrete units that retain their identities in the offspring

4. Gregor Mendel Father of Modern Genetics
Mendel’s paper published in 1866, but was not recognized by science until the early 1900’s
Died prior to his “fame”

5. Reasons for Mendel's Success
Used an experimental approach (scientific method)
Applied mathematics to the study of natural phenomena
Ratios and probability
Kept good records and observations
Large test sample/size

6. Why Use Peas? Short life span Bisexual Many traits known
*Both sexes in one flower/plant
*Stamens and carpels
Many traits known
*Easy to see/observe traits
Cross- and self-pollinating
*Easy to control reproduction
You can eat the failures 

8. Cross-pollination Cross between two different parents
Results in hybrid offspring
The offspring may be different than the parents.

10. Self-pollination Cross with only one flower
Stamens/carpels fertilize each other!
Naturally occurring event in pea plants
Results in pure-bred offspring where the offspring are identical to the parents
Is this asexual reproduction???
NO…you still have gametes

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

12. Monohybrid or Mendelian Crosses
Mono = one
Crosses that work with a single character at a time
Example - Tall X short

13. 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

14. Offspring F1 - first filial generation F2 - second filial generation,
Filial – Latin for “son”
F2 - second filial generation,
Bred by crossing two F1 plants together or allowing a F1 to self-pollinate

15. Notice: TWO P1 plants shown (cross fertilz.)
Notice: only ONE plant shown (self-fertilz.)

16. 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)
Tall
Short

17. Results - Summary Mendel observed SAME pattern in ALL 7 characters
F1 generation showed only one of the traits (regardless of sex)
The other trait reappeared in the F2 at ~25%
3:1 ratio; 3 dominant – 1 recessive
Remember: the % are estimates (still have mutations that could change %)

18. Mendel's Hypothesis Genes can have alternate versions called alleles
Each offspring inherits two alleles, one from each parent
He made this conclusion without having knowledge of chromosomes/DNA makeup

19. Homologous chromosomes
** Remember: Each diploid cell has a pair of homologous chromosomes
-Therefore, any gene has 2 loci
*one on maternal chromo
*one on paternal chromo

20. Mendel's Hypothesis
If the two alleles differ, the dominant allele is expressed
The recessive allele remains “hidden” (unseen) unless the dominant allele is absent
Now called Mendel’s Law of Dominance

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

22. Law of Segregation

23. Genetics 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. Vocabulary Homozygous - When the two alleles are the same (TT/tt)
Heterozygous- When the two alleles are different (Tt)
Notice (for single-gene traits:
Three choices for genotypes
Homo Dom (TT), Homo Rec (tt), Hetero (Tt)

26. 6 Mendelian Crosses are Possible
Notice the 3:1 ratio!!!
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
Goal: to determine genotype of unknown
Ex: T? X tt
*If TT - all Dominant
*If Tt - 1 Dominant: 1 Recessive

29. Dihybrid Cross Cross with two genetic traits
Di = two
Need 4 letters (two for each trait) to code for the cross
Ex: TtRr (Mono = Tt OR Rr)
Each Gamete - Must get 1 letter for each trait
Ex. TR, Tr, etc. (when combine = 4 letters)

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

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

32. TtRr X TtRr Dihybrid Cross
Each parent can produce 4 types of gametes. (n=2; 22=4)
TR, Tr, tR, tr
Cross is a 4 X 4 = 16 possible offspring

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

34. Law of Independent Assortment
The inheritance of 1st genetic trait is NOT dependent on the inheritance of the 2nd trait
Ex: Inheritance of height is independent of the inheritance of flower color
This relates to dihybrid crosses – one character’s inheritance is NOT connected to the inheritance of another!

35. 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) = 9:3:3:1
*Use FOIL method to attain ratio

36. 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

38. 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

39. Rule of Multiplication
The probability that two alleles will come together at fertilization, is equal to the product of their separate probabilities
Steps to determining probability:
1) Determine ratios for each character/trait
How? Do “little” Punnett squares for EACH trait
2) Multiply ratios together

40. Example: TtRr X TtRr The probability of getting a tall offspring is ¾.
The probability of getting a red offspring is ¾. (use same Punnett square as above – only with R/r)
The probability of getting a tall red offspring is ¾ x ¾ = 9/16

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

42. TtrrGG X TtRrgg 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

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

44. Incomplete Dominance
When the F1 hybrids show a phenotype somewhere between the phenotypes of the two parents
Ex. Red X White snapdragons
F1 = all pink
F2 = 1 red: 2 pink: 1 white
NOT BLENDING!!!!!

45. Not enough red pigment made

46. Result from Inc Dominance
No hidden recessive
3 phenotypes and 3 genotypes (Hint! – often a “dose” effect)
Red = CR CR
Pink = CRCW
White = CWCW

47. Another example

48. Codominance Both alleles are expressed equally in the phenotype
NOT an intermediate (like incomplete dominance
Ex. MN blood group
MM, MN, NN
Ex: Rooster/chicken feathers
Ex: flower petal color

49. Result from Codominance
No hidden recessive
3 phenotypes and 3 genotypes (but not a “dose” effect)

50. Multiple Alleles When there are more than 2 alleles for a trait
*Remember: only 2 alleles exist for Mendel’s pea plants
Ex. ABO blood group
IA - A type antigen
IB - B type antigen
i - no antigen

51. Result from Multiple Alleles
Multiple genotypes and phenotypes
Very common event in many traits

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

53. Blood types IA and IB are dominant A and B are CODOMINANT
A and B are the names for two different carbohydrates found on the surface of RBCs
Blood types are actually ways of differentiating the type of antigens on a person's red blood cells

56. Rh factor
Rh blood factor is a separate factor from the ABO blood group
Rh+ = dominant
Rh- = recessive

57. Blood Type Problem Wife is type A Husband is type AB Child is type O
Question - Is this possible?
Comment - Wife’s boss is type O…There’s some explaining to be done!

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

59. 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

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

62. 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

63. Pedigree Chart Symbols
Male Female Person with trait

64. Sample Pedigree

65. Recessive Trait
Dominant Trait

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

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

69. Tay-Sachs Only affects Eastern European Jews
Brain cells unable to metabolize type of lipid; accumulation of the lipid causes brain damage
Death in infancy or early childhood

70. 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

71. Recessive Pattern Usually rare Skips generations
Occurrence increases with consaguineous matings (people descended from the same ancestor)
Often an enzyme defect
Affects males and females equally

72. Human Dominant Disorders
Less common then recessives
Affects males and females equally
Ex:
Huntington’s disease
Achondroplasia
Familial Hypercholesterolemia

73. 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.

74. Newborn Screening
Blood tests for recessive conditions that can have the phenotypes treated to avoid damage
Genotypes are NOT changed
Ex: PKU
Required by law in all states
Tests 1- 6 conditions
Required of “home” births too

75. Multifactorial Diseases
Where Genetic and Environment Factors interact to cause the disease
Ex: Heart Disease factors
Genetics
Diet
Exercise
Bacterial infections

76. Summary
Recognize Mendel's experiments and their role in the scientific discovery of genetic principles.
Identify Mendel's Laws of Genetics.
Recognize the use and application of probability in genetics.
Recognize the basic Mendelian crosses and genetic terminology.
Recognize various extensions of Mendelian genetics and their effect on inheritance patterns.
Identify human traits that exhibit Mendelian inheritance patterns.
Recognize methods used in genetic screening and counseling.