1. Biology, 9th ed,Sylvia Mader
Chapter 11
Chapter 11
Mendelian Inheritance
Mendelian Inheritance

2. Human Genetic Disorders
Outline
Blending Inheritance
Monohybrid Cross
Law of Segregation
Modern Genetics
Genotype vs. Phenotype
Punnett Square
Dihybrid Cross
Law of Independent Assortment
Human Genetic Disorders

3. Formulated fundamental laws of heredity in early 1860s
Gregor Mendel
Austrian monk
Studied science and mathematics at University of Vienna
Conducted breeding experiments with the garden pea Pisum sativum
Carefully gathered and documented mathematical data from his experiments
Formulated fundamental laws of heredity in early 1860s
Had no knowledge of cells or chromosomes
Did not have a microscope

4. Gregor Mendel

5. Fruit and Flower of the Garden Pea

6. Garden Pea Traits Studied by Mendel

7. Theories of inheritance in Mendel’s time:
Blending Inheritance
Theories of inheritance in Mendel’s time:
Based on blending
Parents of contrasting appearance produce offspring of intermediate appearance
Mendel’s findings were in contrast with this
He formulated the particulate theory of inheritance
Inheritance involves reshuffling of genes from generation to generation

8. Mendel’s Monohybrid Crosses: An Example

9. One-Trait Inheritance
Mendel performed cross-breeding experiments
Used “true-breeding” (homozygous) plants
Chose varieties that differed in only one trait (monohybrid cross)
Performed reciprocal crosses
Parental generation = P
First filial generation offspring = F1
Second filial generation offspring = F2
Formulated the Law of Segregation

10. Law of Segregation
Each individual has a pair of factors (alleles) for each trait
The factors (alleles) segregate (separate) during gamete (sperm & egg) formation
Each gamete contains only one factor (allele) from each pair
Fertilization gives the offspring two factors for each trait

11. Modern Genetics View
Each trait in a pea plant is controlled by two alleles (alternate forms of a gene)
Dominant allele (capital letter) masks the expression of the recessive allele (lower- case)
Alleles occur on a homologous pair of chromosomes at a particular gene locus
Homozygous = identical alleles
Heterozygous = different alleles

12. Homologous Chromosomes

13. Genotype Versus Phenotype
Refers to the two alleles an individual has for a specific trait
If identical, genotype is homozygous
If different, genotype is heterozygous
Phenotype
Refers to the physical appearance of the individual

14. Table listing all possible genotypes resulting from a cross
Punnett Square
Table listing all possible genotypes resulting from a cross
All possible sperm genotypes are lined up on one side
All possible egg genotypes are lined up on the other side
Every possible zygote genotypes are placed within the squares

15. Punnett Square Showing Earlobe Inheritance Patterns

16. Try this one
MONOHYBRID CROSS
Cross a heterozygous tall plant with a heterozygous tall plant (use T = tall and t = short) Determine expected genotype and phenotype ratios.

17. Test cross determines genotype of individual having dominant phenotype
Monohybrid Testcross
Individuals with recessive phenotype always have the homozygous recessive genotype
However, Individuals with dominant phenotype have indeterminate genotype
May be homozygous dominant, or
Heterozygous
Test cross determines genotype of individual having dominant phenotype

18. IS MY DOG A PURE BREED?
Do a test cross: G = golden retriever
g = other

19. Not so much!

20. One-Trait Test Cross Unknown is Heterozygous

21. One-Trait Test Cross Unknown is Homozygous Dominant

22. Two-Trait Inheritance
Dihybrid cross uses true-breeding plants differing in two traits
Observed phenotypes among F2 plants
Formulated Law of Independent Assortment
The pair of factors for one trait segregate independently of the factors for other traits
All possible combinations of factors can occur in the gametes

23. Try Mendel’s Classic Dihybrid Cross
Cross two heterozygous tall, heterozygous green pod producing plants. Use a punnett square to show expected offspring and complete a phenotype ratio.
Key:
T = tall G = green pods
t = short g = yellow pods

24. Two-Trait (Dihybrid) Cross

25. Hartford’s example

26. Rat Answers

27. Two-Trait Test Cross

28. Human Genetic Disorders
Autosome - Any chromosome other than a sex chromosome
Genetic disorders caused by genes on autosomes are called autosomal disorders
Some genetic disorders are autosomal dominant
An individual with AA has the disorder
An individual with Aa has the disorder
An individual with aa does NOT have disorder
Other genetic disorders are autosomal recessive
An individual with AA does NOT have disorder
An individual with Aa does NOT have disorder, but is a carrier
An individual with aa DOES have the disorder

29. Autosomal Recessive Pedigree Chart

30. Example: albinism

31. Autosomal Dominant Pedigree Chart

32. Autosomal Recessive Disorders
Tay-Sachs Disease
Progressive deterioration of psychomotor functions
Cystic Fibrosis
Mucus in bronchial tubes and pancreatic ducts is particularly thick and viscous
Phenylketonuria (PKU)
Lack enzyme for normal metabolism of phenylalanine

33. Cystic Fibrosis Therapy

34. Autosomal Dominant Disorders
Neurofibromatosis
Tan or dark spots develop on skin and darken
Small, benign tumors may arise from fibrous nerve coverings
Huntington Disease
Neurological disorder
Progressive degeneration of brain cells
Severe muscle spasms
Personality disorders

35. A Victim of Huntington Disease

36. Huntington Disease: Normal and Diseased Brain

37. Sex-linked – disorders carried on the X chromosome
Colorblindness
Hemophilia
Baldness (?)

39. Phenotype reveals genotype without test cross
Incomplete Dominance
Heterozygote has phenotype intermediate between that of either homozygote
Homozygous red has red phenotype
Homozygous white has white phenotype
Heterozygote has pink (intermediate) phenotype
Phenotype reveals genotype without test cross

40. Incomplete Dominance

41. Multiple Allelic Traits
Some traits controlled by multiple alleles
The gene exists in several allelic forms (but each individual only has two)
ABO blood types
The alleles:
IA = A antigen on red cells, anti-B antibody in plasma
IB = B antigen on red cells, anti-AB antibody in plasma
I = Neither A nor B antigens, both antibodies
Phenotype (Blood Type)
Genotype
A (actually AA or AO)
IAIA or IAi
B (actually BB or BO)
IBIB or IBi AB
IAIB
O (actually OO) ii

42. Inheritance of Blood Type

43. Answers to blood type problems
Blood Type Mystery
Answers to blood type problems

44. Polygenic Inheritance
Occurs when a trait is governed by two or more genes having different alleles
Each dominant allele has a quantitative effect on the phenotype
These effects are additive
Result in continuous variation of phenotypes

45. Height in Human Beings

46. Frequency Distributions in Polygenic Inheritance

47. Terminology Pleiotropy Codominance Epistasis
A gene that affects more than one characteristic of an individual
Sickle-cell (incomplete dominance)
Codominance
More than one allele is fully expressed
ABO blood type (multiple allelic traits)
Epistasis
A gene at one locus interferes with the expression of a gene at a different locus
Human skin color (polygenic inheritance)

48. Pleiotropy

49. Epistasis – disrupts the expected

50. Environment and Phenotype: Himalayan Rabbits

51. SOLVING GENETICS PROBLEMS
Rules of Probability
*segregation and independent assortment of alleles during meiosis and fertilization are random events
*if we know genotype of parents we can predict the EXPECTED genotypes of offspring using rules of probability

52. The probability of all possible outcomes for an event must add up to 1
Probability Scale
Scale range: 0-1
The probability of all possible outcomes for an event must add up to 1

53. EVENT PROBABILITY *tossing heads w/normal coin………………1/2 *tossing coins
Common Examples
EVENT PROBABILITY
*tossing heads
w/normal coin………………1/2
*tossing coins 1

54. Probability of rolling 3 on a six-sided die? 1/6
Try a common one
Probability of rolling 3 on a six-sided die?
1/6
Probability of rolling a number other than 3?
5/6

55. Random events are independent of one another!
If a couple has 6 sons and tries for the little girl: there is still a ½ chance of getting their wish. Regardless of the past events!

56. Rule of Multiplication
The probability that independent events will occur simultaneously is the product of their individual probs.

57. A: egg w/ t = ½ sperm w/ t = ½
Ex 1 - monohybrid
Q: In a Mendelian cross between two heterozygous pea plants (Tt), what is the probability that the offspring will be homozygous recessive?
A: egg w/ t = ½ sperm w/ t = ½
overall probability that two will unite is ½ x ½ = ¼

58. A: prob. of egg w/ YR = ½ x ½ = ¼ prob. of sperm w/ YR = ½ x ½ = ¼
Ex 2. Dihybrid cross
Q: For YyRr x YyRr, what is the probability of an F2 plant having genotype YYRR?
A: prob. of egg w/ YR = ½ x ½ = ¼
prob. of sperm w/ YR = ½ x ½ = ¼
Overall prob. for offspring:
¼ x ¼ = 1/16

59. Rule of Addition
The probability of an een that can occur in two or more independent ways is the sum of the separate probabilities of the different ways.
Think heterozygotes!

60. Ex. 1: monohybrids
Q: In a Mendelian cross between pea plants that are heterozygous (Pp), what is the probability of the offspring being heterozygous?
A: there are two different ways for this to occur- 1) sperm gives P and egg gives p OR 2) egg gives P and sperm gives p

61. So…
sperm gives P and egg gives p
½ X ½ = ¼
2) egg gives P and sperm gives p
OVERALL PROB. of way 1 or way 2
¼ + ¼ = ½

62. A: Look at each allele pair separately Aa x Aa: prob. of aa = ¼
Ex 2 Trihybrid
Q: What is the probability that a trihybrid cross between two organisms with genotypes AaBbCc and AaBbCc produce an aabbcc offspring?
A: Look at each allele pair separately
Aa x Aa: prob. of aa = ¼
Bb x Bb: prob. of bb = ¼
Cc x Cc: prob. of cc = ¼
Overall that this will occur simultaneously:
¼ x ¼ x ¼ = 1/64

63. ppyyRr, ppYyrr, Ppyyrr, PPyyrr, and ppyyrr
Ex. 3 Trihybrid
Q: PpYyRr x Ppyyrr
What is the chance that the offspring will show at least two recessive phenotypes out of the three traits?
A: break down the possibilities of all the genotypes that are homozygous recessive for at least two traits:
ppyyRr, ppYyrr, Ppyyrr, PPyyrr, and ppyyrr

64. PpYyRr x Ppyyrr
ppyyRr ¼ x ½ x ½ = 1/16
ppYyrr ¼ x ½ x ½ = 1/16
Ppyyrr ½ x ½ x ½ = 2/16
PPyyrr ¼ x ½ x ½ = 1/16
ppyyrr ¼ x ½ x ½ = 1/16
6/16 or 3/8 chance of an offspring showing at least 2 of the recessive traits

65. Human Genetic Disorders
Review
Blending Inheritance
Monohybrid Cross
Law of Segregation
Modern Genetics
Genotype vs. Phenotype
Punnett Square
Dihybrid Cross
Law of Independent Assortment
Human Genetic Disorders

66. Biology, 9th ed,Sylvia Mader
Chapter 11
Ending Slide Chapter 11
Mendelian Inheritance
Mendelian Inheritance