1. Chapter 8 Table of Contents Section 1 The Origins of Genetics
Mendel and Heredity
Table of Contents
Section 1 The Origins of Genetics
Section 2 Mendel’s Theory
Section 3 Studying Heredity
Section 4 Complex Patterns of Heredity

2. Chapter 8 Objectives Identify between atoms and elements.
Section 1 The Origins of Genetics
Objectives
Identify between atoms and elements.
List how compounds are formed.
Summarize between covalent bonds, hydrogen bonds, and ionic bonds.
Relate how compounds are formed.

3. Mendel’s Studies of Traits
Chapter 8
Section 1 The Origins of Genetics
Mendel’s Studies of Traits
Many of your traits, including the color and shape of your eyes, the texture of your hair, and even your height and weight, resemble those of your parents.
The passing of traits from parents to offspring is called heredity.

4. Mendel’s Studies of Traits, continued
Chapter 8
Section 1 The Origins of Genetics
Mendel’s Studies of Traits, continued
Mendel’s Breeding Experiments
The scientific study of heredity began more than a century ago with the work of an Austrian monk named Gregor Johann Mendel.
Mendel was the first to develop rules that accurately predict patterns of heredity.
The patterns that Mendel discovered form the basis of genetics, the branch of biology that focuses on heredity.

5. Mendel’s Studies of Traits, continued
Chapter 8
Section 1 The Origins of Genetics
Mendel’s Studies of Traits, continued
Mendel’s Breeding Experiments
Mendel experimented with garden pea heredity by cross-pollinating plants with different characteristics.

6. Mendel’s Studies of Traits, continued
Chapter 8
Section 1 The Origins of Genetics
Mendel’s Studies of Traits, continued
Useful Features in Peas
The garden pea is a good subject for studying heredity for several reasons:
1. Several traits of the garden pea exist in two clearly different forms. (either-or)
2. The male and female reproductive parts of garden peas are enclosed within the same flower. This allows you to easily control mating.
3. The garden pea is small, grows easily, matures quickly, and produces many offspring.

7. Traits Expressed as Simple Ratios
Chapter 8
Section 1 The Origins of Genetics
Traits Expressed as Simple Ratios
Mendel’s initial experiments were monohybrid crosses.
A monohybrid cross is a cross that involves one pair of contrasting traits.
For example, crossing a plant with purple flowers and a plant with white flowers is a monohybrid cross.

8. Traits Expressed as Simple Ratios, continued
Chapter 8
Section 1 The Origins of Genetics
Traits Expressed as Simple Ratios, continued
Mendel carried out his experiments in three steps:
Step 1 Mendel allowed each variety of garden pea to self-pollinate for several generations to ensure that each variety was true-breeding for a particular trait; that is, all the offspring would display only one form of the trait. These true-breeding plants served as the parental generation in Mendel’s experiments. The parental generation, or P generation, are the first two individuals that are crossed in a breeding experiment.

9. Chapter 8
Section 1 The Origins of Genetics
Parental Generation

10. Traits Expressed as Simple Ratios, continued
Chapter 8
Section 1 The Origins of Genetics
Traits Expressed as Simple Ratios, continued
Step 2 Mendel then cross-pollinated two P generation plants that had contrasting forms of a trait, such as purple flowers and white flowers. Mendel called the offspring of the P generation the first filial generation, or F1 generation.
Step 3 Mendel allowed the F1 generation to self-pollinate. He called the offspring of the F1 generation plants the second filial generation, or F2 generation.

11. First Filial Generation
Chapter 8
Section 1 The Origins of Genetics
First Filial Generation

12. Second Filial Generation
Chapter 8
Section 1 The Origins of Genetics
Second Filial Generation

13. Three Steps of Mendel’s Experiments
Chapter 8
Section 1 The Origins of Genetics
Three Steps of Mendel’s Experiments

14. Chapter 8 Mendel’s Experiments Section 1 The Origins of Genetics
Mendel controlled the
fertilization of his pea plants
by removing the male parts,
or stamens.
He then fertilized the female
part, or pistil, with pollen from
a different pea plant.

15. Traits Expressed as Simple Ratios, continued
Chapter 8
Section 1 The Origins of Genetics
Traits Expressed as Simple Ratios, continued
Mendel’s Results
Each of Mendel’s F1 plants showed only one form of the trait.
But when the F1 generation was allowed to self-pollinate, the missing trait reappeared in some of the plants in the F2 generation.
For each of the seven traits Mendel studied, he found a 3:1 ratio of plants expressing the contrasting traits in the F2 generation.

16. Mendel’s Crosses and Results
Chapter 8
Section 1 The Origins of Genetics
Mendel’s Crosses and Results

17. Mendel drew three important conclusions.
Chapter 8
Section 1 The Origins of Genetics
Mendel drew three important conclusions.
Traits are inherited as discrete units.
Organisms inherit two copies of each gene, one from each parent.
The two copies segregate during gamete formation.
The last two conclusions are called the law of segregation.

18. Chapter 8
Section 2 Mendel’s Theory
Objectives
Describe the four major hypotheses Mendel developed.
Define the terms homozygous, heterozygous, genotype, and phenotype.
Compare Mendel’s two laws of heredity.

19. Chapter 8 A Theory of Heredity
Section 2 Mendel’s Theory
A Theory of Heredity
Mendel correctly concluded from his experiments that each pea has two separate “heritable factors” for each trait—one from each parent.
When gametes (sperm and egg cells) form, each receives only one of the organism’s two factors for each trait.
When gametes fuse during fertilization, the offspring has two factors for each trait, one from each parent. Today these factors are called genes.

20. Chapter 8
Section 2 Mendel’s Theory
Mendel’s Factors

21. A Theory of Heredity, continued
Chapter 8
Section 2 Mendel’s Theory
A Theory of Heredity, continued
Mendel’s Hypotheses
The four hypotheses Mendel developed as a result of his experiments now make up the Mendelian theory of heredity—the foundation of genetics.
1. For each inherited trait, an individual has two copies of the gene—one from each parent.
2. There are alternative versions of genes. Today the different versions of a gene are called its alleles.

22. Chapter 8
Section 2 Mendel’s Theory
Allele

23. A Theory of Heredity, continued
Chapter 8
Section 2 Mendel’s Theory
A Theory of Heredity, continued
Mendel’s Hypotheses
3. When two different alleles occur together, one of them may be completely expressed, while the other may have no observable effect on the organism’s appearance. Mendel described the expressed form of the trait as dominant. The trait that was not expressed when the dominant form of the trait was present was described as recessive.

24. A Theory of Heredity, continued
Chapter 8
Section 2 Mendel’s Theory
A Theory of Heredity, continued
Mendel’s Hypotheses
4. When gametes are formed, the alleles for each gene in an individual separate independently of one another. Thus, gametes carry only one allele for each inherited trait. When gametes unite during fertilization, each gamete contributes one allele.

25. A Theory of Heredity, continued
Chapter 8
Section 2 Mendel’s Theory
A Theory of Heredity, continued
Mendel’s Findings in Modern Terms
Dominant alleles are indicated by writing the first letter of the trait as a capital letter.
Recessive alleles are also indicated by writing the first letter of the dominant trait, but the letter is lowercase.

26. A Theory of Heredity, continued
Chapter 8
Section 2 Mendel’s Theory
A Theory of Heredity, continued
Mendel’s Findings in Modern Terms
If the two alleles of a particular gene present in an individual are the same, the individual is said to be homozygous.
If the alleles of a particular gene present in an individual are different, the individual is heterozygous.
In heterozygous individuals, only the dominant allele is expressed; the recessive allele is present but unexpressed.

27. A Theory of Heredity, continued
Chapter 8
Section 2 Mendel’s Theory
A Theory of Heredity, continued
Mendel’s Findings in Modern Terms
The set of alleles that an individual has is called its genotype.
The physical appearance of a trait is called a phenotype.
Phenotype is determined by which alleles are present.

28. Comparing Genotype and Phenotype
Chapter 8
Section 2 Mendel’s Theory
Comparing Genotype and Phenotype

29. Chapter 8 The Laws of Heredity The Law of Segregation
Section 2 Mendel’s Theory
The Laws of Heredity
The Law of Segregation
The first law of heredity describes the behavior of chromosomes during meiosis.
At this time, homologous chromosomes and then chromatids are separated.
The first law, the law of segregation, states that the two alleles for a trait segregate (separate) when gametes are formed.

30. The Laws of Heredity, continued
Chapter 8
Section 2 Mendel’s Theory
The Laws of Heredity, continued
The Law of Independent Assortment
Mendel found that for the traits he studied, the inheritance of one trait did not influence the inheritance of any other trait.
The law of independent assortment states that the alleles of different genes separate independently of one another during gamete formation.

31. Chapter 8
Section 3 Studying Heredity
Objectives
Predict the results of monohybrid genetic crosses by using Punnett squares.
Apply a test cross to determine the genotype of an organism with a dominant phenotype.
Predict the results of monohybrid genetic crosses by using probabilities.
Analyze a simple pedigree.

32. Chapter 8 Punnett Squares
Section 3 Studying Heredity
Punnett Squares
A Punnett square is a diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in the cross.
The possible gametes that one parent can produce are written along the top of the square. The possible gametes that the other parent can produce are written along the left side of the square.
Each box inside the square is filled in with two letters obtained by combining the allele along the top of the box with the allele along the side of the box.

33. Punnett Squares, continued
Chapter 8
Section 3 Studying Heredity
Punnett Squares, continued
One Pair of Contrasting Traits
Punnett squares can be used to predict the outcome of a monohybrid cross (a cross that considers one pair of contrasting traits between two individuals).
Punnett squares allow direct and simple predictions to be made about the outcomes of genetic crosses.

34. Monohybrid Cross: Homozygous Plants
Chapter 8
Section 3 Studying Heredity
Monohybrid Cross: Homozygous Plants

35. Punnett Square with Homozygous Cross
Chapter 8
Section 3 Studying Heredity
Punnett Square with Homozygous Cross

36. Monohybrid Cross: Heterozygous Plants
Chapter 8
Section 3 Studying Heredity
Monohybrid Cross: Heterozygous Plants

37. Punnett Square with Heterozygous Cross
Chapter 8
Section 3 Studying Heredity
Punnett Square with Heterozygous Cross

38. Punnett Squares, continued
Chapter 8
Section 3 Studying Heredity
Punnett Squares, continued
Determining Unknown Genotypes
Animal breeders, horticulturists, and others involved in breeding organisms often need to know whether an organism with a dominant phenotype is heterozygous or homozygous for a trait.
In a test cross, an individual whose phenotype is dominant, but whose genotype is not known, is crossed with a homozygous recessive individual.

39. Chapter 8
Section 3 Studying Heredity
Test Cross

40. Chapter 8 Outcomes of Crosses
Section 3 Studying Heredity
Outcomes of Crosses
Like Punnett squares, probability calculations can be used to predict the results of genetic crosses.
Probability is the likelihood that a specific event will occur.

41. Calculating Probability
Chapter 8
Section 3 Studying Heredity
Calculating Probability

42. Outcomes of Crosses, continued
Chapter 8
Section 3 Studying Heredity
Outcomes of Crosses, continued
Probability of Specific Allele in a Gamete
Consider the possibility that a coin tossed into the air will land on heads (one possible outcome). The total number of all possible outcomes is two—heads or tails. Thus, the probability that a coin will land on heads is ½.
The same formula can be used to predict the probability of an allele being present in a gamete.

43. Outcomes of Crosses, continued
Chapter 8
Section 3 Studying Heredity
Outcomes of Crosses, continued
Probability of the Outcome of a Cross
Because two parents are involved in a genetic cross, both parents must be considered when calculating the probability of the outcome of a genetic cross.
To find the probability that a combination of two independent events will occur, multiply the separate probabilities of the two events.

44. Probability with Two Coins
Chapter 8
Section 3 Studying Heredity
Probability with Two Coins

45. Chapter 8 Inheritance of Traits
Section 3 Studying Heredity
Inheritance of Traits
Geneticists often prepare a pedigree, a family history that shows how a trait is inherited over several generations.
Pedigrees are particularly helpful if the trait is a genetic disorder and the family members want to know if they are carriers or if their children might get the disorder.

46. Chapter 8
Section 3 Studying Heredity
Pedigree

47. Inheritance of Traits, continued
Chapter 8
Section 3 Studying Heredity
Inheritance of Traits, continued
Scientists can determine several pieces of genetic information from a pedigree:
Autosomal or Sex-Linked? If a trait is autosomal, it will appear in both sexes equally. If a trait is sex-linked, it is usually seen only in males. A sex-linked trait is a trait whose allele is located on the X chromosome.
Dominant or Recessive? If the trait is autosomal dominant, every individual with the trait will have a parent with the trait. If the trait is recessive, an individual with the trait can have one, two, or neither parent exhibit the trait.

48. Chapter 8
Section 3 Studying Heredity
Sex Linkage

49. Inheritance of Traits, continued
Chapter 8
Section 3 Studying Heredity
Inheritance of Traits, continued
Scientists can determine several pieces of genetic information from a pedigree:
Heterozygous or Homozygous? If individuals with autosomal traits are homozygous dominant or heterozygous, their phenotype will show the dominant characteristic. If individuals are homozygous recessive, their phenotype will show the recessive characteristic.

50. Section 4 Complex Patterns of Heredity
Chapter 8
Objectives
Identify five factors that influence patterns of heredity.
Describe how mutations can cause genetic disorders.
List two genetic disorders, and describe their causes and symptoms.
Evaluate the benefits of genetic counseling.

51. Complex Control of Characters
Section 4 Complex Patterns of Heredity
Chapter 8
Complex Control of Characters
Characters Influenced by Several Genes
When several genes influence a trait, the trait is said to be a polygenic trait.
The genes for a polygenic trait may be scattered along the same chromosome or located on different chromosomes.
Familiar examples of polygenic traits in humans include eye color, height, weight, and hair and skin color.

52. Complex Control of Characters, continued
Section 4 Complex Patterns of Heredity
Chapter 8
Complex Control of Characters, continued
Intermediate Characters
In some organisms, however, an individual displays a trait that is intermediate between the two parents, a condition known as incomplete dominance.
For example, when a snapdragon with red flowers is crossed with a snapdragon with white flowers, a snapdragon with pink flowers is produced.

53. Section 4 Complex Patterns of Heredity
Chapter 8
Incomplete Dominance

54. Complex Control of Characters, continued
Section 4 Complex Patterns of Heredity
Chapter 8
Complex Control of Characters, continued
Characters Controlled by Genes with Three or More
Alleles
Genes with three or more alleles are said to have multiple alleles.
Even for traits controlled by genes with multiple alleles, an individual can have only two of the possible alleles for that gene.

55. Comparing Single Allele, Multiple Allele, and Polygenic Traits
Section 4 Complex Patterns of Heredity
Chapter 8
Comparing Single Allele, Multiple Allele, and Polygenic Traits

56. Complex Control of Characters, continued
Section 4 Complex Patterns of Heredity
Chapter 8
Complex Control of Characters, continued
Characters with Two Forms Displayed at the Same
Time
For some traits, two dominant alleles are expressed at the same time.
In this case, both forms of the trait are displayed, a phenomenon called codominance.
Codominance is different from incomplete dominance because both traits are displayed.

57. Comparing Complete, Incomplete, and Codominance
Section 4 Complex Patterns of Heredity
Chapter 8
Comparing Complete, Incomplete, and Codominance

58. Complex Control of Characters, continued
Section 4 Complex Patterns of Heredity
Chapter 8
Complex Control of Characters, continued
Characters Influenced by the Environment
An individual’s phenotype often depends on conditions in the environment.
Because identical twins have identical genes, they are often used to study environmental influences.
Because identical twins are genetically identical, any differences between them are attributed to environmental influences.

59. Section 4 Complex Patterns of Heredity
Chapter 8
Genetic Disorder

60. Chapter 8 Genetic Disorders Sickle Cell Anemia
Section 4 Complex Patterns of Heredity
Chapter 8
Genetic Disorders
Sickle Cell Anemia
An example of a recessive genetic disorder is sickle cell anemia, a condition caused by a mutated allele that produces a defective form of the protein hemoglobin.
In sickle cell anemia, the defective form of hemoglobin causes many red blood cells to bend into a sickle shape.
The recessive allele that causes sickle-shaped red blood cells helps protect the cells of heterozygous individuals from the effects of malaria.

61. Section 4 Complex Patterns of Heredity
Chapter 8
Sickle Cell Anemia

62. Genetic Disorders, continued
Section 4 Complex Patterns of Heredity
Chapter 8
Genetic Disorders, continued
Cystic Fibrosis (CF)
Cystic fibrosis, a fatal recessive trait, is the most common fatal hereditary disorder among Caucasians.
One in 25 Caucasian individuals has at least one copy of a defective gene that makes a protein necessary to pump chloride into and out of cells.
The airways of the lungs become clogged with thick mucus, and the ducts of the liver and pancreas become blocked.

63. Genetic Disorders, continued
Section 4 Complex Patterns of Heredity
Chapter 8
Genetic Disorders, continued
Hemophilia
Another recessive genetic disorder is hemophilia, a condition that impairs the blood’s ability to clot.
Hemophilia is a sex-linked trait.
A mutation on one of more than a dozen genes coding for the proteins involved in blood clotting on the X chromosome causes the form of hemophilia called hemophilia A.

64. Comparing X-Linked and Sex-Influenced Traits
Section 4 Complex Patterns of Heredity
Chapter 8
Comparing X-Linked and Sex-Influenced Traits

65. Section 4 Complex Patterns of Heredity
Chapter 8
Hemophilia

66. Genetic Disorders, continued
Section 4 Complex Patterns of Heredity
Chapter 8
Genetic Disorders, continued
Huntington’s Disease (HD)
Huntington’s disease is a genetic disorder caused by a dominant allele located on an autosome.
The first symptoms of HD—mild forgetfulness and irritability—appear in victims in their thirties or forties.
In time, HD causes loss of muscle control, uncontrollable physical spasms, severe mental illness, and eventually death.

67. Section 4 Complex Patterns of Heredity
Chapter 8
Huntington’s Disease

68. Some Human Genetic Disorders
Section 4 Complex Patterns of Heredity
Chapter 8
Some Human Genetic Disorders

69. Treating Genetic Disorders
Section 4 Complex Patterns of Heredity
Chapter 8
Treating Genetic Disorders
Most genetic disorders cannot be cured, although progress is being made.
A person with a family history of genetic disorders may wish to undergo genetic counseling before becoming a parent.
In some cases, a genetic disorder can be treated if it is diagnosed early enough.

70. Treating Genetic Disorders, continued
Section 4 Complex Patterns of Heredity
Chapter 8
Treating Genetic Disorders, continued
Gene Therapy
Gene technology may soon allow scientists to correct certain recessive genetic disorders by replacing defective genes with copies of healthy ones, an approach called gene therapy.
The essential first step in gene therapy is to isolate a copy of the gene.

71. Chapter 8 Multiple Choice
Standardized Test Prep
Multiple Choice
The diagram below shows the expected results of a cross between two pea plants. T and t represent the alleles for the tall and dwarf traits, respectively. Use the figure below to answer questions 1–3.

72. Multiple Choice, continued
Chapter 8
Standardized Test Prep
Multiple Choice, continued
1. What are the genotypes of the plants that were crossed?
A. tt on the top; tt along the side
B. Tt on the top; tt along the side
C. Tt on the top; Tt along the side
D. TT on the top; TT along the side

73. Multiple Choice, continued
Chapter 8
Standardized Test Prep
Multiple Choice, continued
1. What are the genotypes of the plants that were crossed?
A. tt on the top; tt along the side
B. Tt on the top; tt along the side
C. Tt on the top; Tt along the side
D. TT on the top; TT along the side

74. Multiple Choice, continued
Chapter 8
Standardized Test Prep
Multiple Choice, continued
2. What genotypic ratio is expected in the offspring of this cross?
F. 1 Tt : 1 tt
G. 3 Tt : 1 tt
H. 1 Tt : 3 tt
J. 1 TT : 1 tt

75. Multiple Choice, continued
Chapter 8
Standardized Test Prep
Multiple Choice, continued
2. What genotypic ratio is expected in the offspring of this cross?
F. 1 Tt : 1 tt
G. 3 Tt : 1 tt
H. 1 Tt : 3 tt
J. 1 TT : 1 tt

76. Multiple Choice, continued
Chapter 8
Standardized Test Prep
Multiple Choice, continued
3. If this cross produced 240 offspring, how many of the offspring would be expected to have the dwarf trait?
A. 0
B. 60
C. 120
D. 180

77. Multiple Choice, continued
Chapter 8
Standardized Test Prep
Multiple Choice, continued
3. If this cross produced 240 offspring, how many of the offspring would be expected to have the dwarf trait?
A. 0
B. 60
C. 120
D. 180