Mendel conducted extensive experiments with garden pea plants to study patterns of inheritance for different traits. Through his work with monohybrid and dihybrid crosses, he discovered two fundamental principles of genetics: the law of segregation and the law of independent assortment. The law of segregation states that alleles segregate from each other during gamete formation such that each gamete contains one allele for each trait. The law of independent assortment states that genes assort independently of one another during gamete formation. These principles helped establish the particulate nature of inheritance and provided the foundation for modern genetics.

1. Observing Patterns
in Inherited Traits
Chapter 11

2. Impacts, Issues:
The Color of Skin
 Like most human traits, skin color has a genetic
basis; more than 100 gene products affect the
synthesis and deposition of melanins

3. 11.1 Mendel, Pea Plants,
and Inheritance Patterns
 Recurring inheritance patterns are observable
outcomes of sexual reproduction
 Before the discovery of genes, it was thought
that inherited traits resulted from a blend of
parental characters

4. Mendel’s Experimental Approach
 Mendel was a monk with training in plant
breeding and mathematics
 He studied the garden pea (Pisum sativum),
which breeds true for a number of traits

5. Garden Pea Plant:
Self Fertilization and Cross-Fertilization

6. carpel anther
A Garden pea flower, cut in half. Sperm form in
pollen grains, which originate in male floral parts
(anthers). Eggs develop, fertilization takes place,
and seeds mature in female floral parts (carpels).
B Pollen from a plant that breeds true for purple flowers is
brushed onto a floral bud of a plant that breeds true for white
flowers. The white flower had its anthers snipped off. Artificial
pollination is one way to ensure that a plant will not self-fertilize.
C Later, seeds develop inside pods of the cross-fertilized
plant. An embryo in each seed develops into a mature pea plant.
D Each new plant’s flower color is indirect but
observable evidence that hereditary material
has been transmitted from the parent plants.
Fig. 11-3, p. 170

7. Animation: Crossing garden pea plants

8. Terms Used in Modern Genetics
 Genes
• Heritable units of information about traits
• Parents transmit genes to offspring
• Each gene has a specific locus on a
chromosome
 Diploid cells (chromosome number 2n) have
pairs of genes on homologous chromosomes

9. Terms Used in Modern Genetics
 A mutation is a permanent change in a gene
• May cause a trait to change
• Alleles are different molecular forms of a gene
 A hybrid has nonidentical alleles for a trait
• Offspring of a cross between two individuals that
breed true for different forms of a trait are hybrids

10. Terms Used in Modern Genetics
 An individual with nonidentical alleles of a gene
is heterozygous for that gene
 An individual with identical alleles of a gene is
homozygous for that gene

11. Terms Used in Modern Genetics
 An allele is dominant if its effect masks the
effect of a recessive allele paired with it
• Capital letters (A) signify dominant alleles;
lowercase letters (a) signify recessive alleles
• Homozygous dominant (AA)
• Homozygous recessive (aa)
• Heterozygous (Aa)

12. Terms Used in Modern Genetics
 Gene expression
• The process by which information in a gene is
converted to a structural or functional part of a
cell or body
• Expressed genes determine traits

13. Terms Used in Modern Genetics
 Genotype
• The particular alleles an individual carries
 Phenotype
• An individual’s observable traits

14. Terms Used in Modern Genetics
 P stands for parents, F for filial (offspring)
 F1: First generation offspring of parents
 F2: Second generation offspring of parents

15. 11.1 Key Concepts
Where Modern Genetics Started
 Gregor Mendel gathered the first experimental
evidence of the genetic basis of inheritance
 His meticulous work gave him clues that
heritable traits are specified in units
 The units, which are distributed into gametes in
predictable patterns, were later identified as
genes

16. 11.2 Mendel’s Law of Segregation
 Garden pea plants inherit two “units” of
information for a trait, one from each parent

17. Testcrosses
 Testcross
• A method of determining if an individual is
heterozygous or homozygous dominant
• An individual with unknown genotype is crossed
with one that is homozygous recessive (AA x aa)
or (Aa x aa)

18. Monohybrid Experiments
 Monohybrid experiments
• Testcrosses that check for a dominance
relationship between two alleles at a single locus
• May be crosses between true breeding
(homozygous) individuals (AA x aa), or between
identical heterozygotes (Aa x Aa)

19. Mendel’s Monohybrid Experiments
 Mendel used monohybrid experiments to find
dominance relationships among pea plant traits
• When he crossed plants that bred true for white
flowers with plants that bred true for purple
flowers, all F1 plants had purple flowers
• When he crossed two F1 plants, ¾ of the F2 plants
had purple flowers, ¼ had white flowers

20. Segregation of Alleles at a Gene Locus

21. homozygous homozygous
dominant parent recessive parent
(chromosomes
duplicated before
meiosis)
meiosis I
meiosis II
(gametes) (gametes)
fertilization
produces
heterozygous
offspring
Fig. 11-5, p. 172

22. homozygous homozygous
dominant parent recessive parent
(chromosomes
duplicated before
meiosis)
meiosis I
meiosis II
(gametes) (gametes)
fertilization
produces
heterozygous
Stepped Art
offspring
Fig. 11-5, p. 172

23. Mendel’s Monohybrid Experiments

24. Trait Dominant Recessive F2 Dominant-to-
Studied Form Form Recessive Ratio
Seed
shape 5,474 round 1,850 wrinkled 2.98 to 1
Seed
color 6,022 yellow 2,001 green 3.01 to 1
Pod
shape 882 inflated 299 wrinkled 2.95 to 1
Pod
color 428 green 152 yellow 2.82 to 1
Flower
color 705 purple 224 white 3.15 to 1
Flower
position 651 along stem 207 at tip 3.14 to 1
Stem
length 787 tall 277 dwarf 2.84 to 1
Fig. 11-6, p. 172

25. Calculating Probabilities
 Probability
• A measure of the chance that a particular
outcome will occur
 Punnett square
• A grid used to calculate the probability of
genotypes and phenotypes in offspring

26. Construction of a Punnett Square

27. Phenotype Ratios
in a Monohybrid Experiment

28. Phenotype Ratios
in a Monohybrid Experiment

29. Fig. 11-7, p. 173

30. female gametes
A a A a A a A a
male gametes
A A A Aa A AA Aa
a aa a Aa aa a Aa aa a Aa aa
A From left to right, step-by-step construction of a Punnett square. Circles
signify gametes, and letters signify alleles: A is dominant; a is recessive.
The genotypes of the resulting offspring are inside the squares.
Fig. 11-7a, p. 173

31. F1 offspring
aa
True-breeding homozygous
recessive parent plant
a a
Aa Aa
A Aa Aa
AA
A Aa Aa
True-breeding homozygous Aa Aa
dominant parent plant
B A cross between two plants that breed true for different forms
of a trait produces F1 offspring that are identically heterozygous.
Fig. 11-7b, p. 173

32. F2 offspring
Aa
Heterozygous
F1 offspring
A a
AA Aa
A AA Aa
Aa
a Aa aa
Heterozygous
F1 offspring Aa aa
C A cross between the F1 offspring is the monohybrid experiment. The
phenotype ratio of F2 offspring in this example is 3:1 (3 purple to 1 white).
Fig. 11-7c, p. 173

33. Mendel’s Law of Segregation
 Mendel observed a phenotype ratio of 3:1 in the
F2 offspring of his monohybrid crosses
• Consistent with the probability of the aa genotype
in the offspring of a heterozygous cross (Aa x Aa)
 This is the basis of Mendel’s law of segregation
• Diploid cells have pairs of genes on pairs of
homologous chromosomes
• The two genes of each pair separate during
meiosis, and end up in different gametes

34. 11.2 Key Concepts
Insights from Monohybrid Experiments
 Some experiments yielded evidence of gene
segregation: When one chromosome separates
from its homologous partner during meiosis, the
alleles on those chromosomes also separate
and end up in different gametes

35. 11.3 Mendel’s Law
of Independent Assortment
 Mendel’s law of independent assortment
• Many genes are sorted into gametes
independently of other genes

36. Dihybrid Experiments
 Dihybrid experiments
• Tests for dominance relationships between
alleles at two loci
• Individuals that breed true for two different traits
are crossed (AABB x aabb)
• F2 phenotype ratio is 9:3:3:1 (four phenotypes)
• Individually, each dominant trait has an F2 ratio of
3:1 – inheritance of one trait does not affect
inheritance of the other

37. Independent Assortment at Meiosis

38. One of two possible alignments The only other possible alignment
a Chromosome A Aa a
A Aa a
alignments at
metaphase I: B Bb b b bB B
b The resulting A A a a A A a a
alignments at
metaphase II: B B b b b b B B
c Possible B A A B b a a b b A A b B a a B
combinations
of alleles in
gametes: AB ab Ab aB
Fig. 11-8, p. 174

39. One of two possible alignments The only other possible alignment
a Chromosome
A Aa a A Aa a
alignments at
metaphase I: B Bb b b bB B
b The resulting A A a a A A a a
alignments at
metaphase II: B B b b b b B B
c Possible B A A B b a a b b A A b B a a B
combinations
of alleles in
gametes: AB ab Ab aB
Stepped Art
Fig. 11-8, p. 174

40. Mendel’s Dihybrid Experiments

41. Fig. 11-9a, p. 175

42. parent plant parent plant
homozygous homozygous
P
for purple for white
generation
flowers flowers
A Meiosis in homozygous and long and short
individuals results in one stems stems
kind of gamete. aabb
AABB
B A cross between plants
homozygous for two different traits AB x ab
yields one possible combination of gametes:
Fig. 11-9a, p. 175

43. Fig. 11-9b, p. 175

44. AaBb AaBb AaBb
F1 All F1 offspring are AaBb,
generation with purple flowers and tall stems.
C Meiosis in AaBb dihybrid plants
results in four kinds of gametes:
AB Ab aB ab
F2
These gametes can meet up in one of 16
generation
possible wayswhen the dihybrids are
crossed (AaBb X AaBb):
Fig. 11-9b, p. 175

45. Fig. 11-9c, p. 175

46. AB Ab aB ab
AB AABB AABb AaBB AaBb
Ab AABb AAbb AaBb Aabb
aB AaBB AaBb aaBB aaBb
ab AaBb Aabb aaBb aabb
D Out of 16 possible genetic outcomes of this dihybrid cross, 9 will result in plants that
are purple-flowered and tall; 3, purple-flowered and short; 3, white-flowered and tall;
and 1, white-flowered and short. The ratio of phenotypes of this dihybrid cross is 9:3:3:1.
Fig. 11-9c, p. 175

47. Animation: Dihybrid cross

48. Mendel’s Law of Independent
Assortment
 Mendel’s dihybrid experiments showed that
“units” specifying one trait segregated into
gametes separately from “units” for other traits
 Exception: Genes that have loci very close to
one another on a chromosome tend to stay
together during meiosis

49. 11.3 Key Concepts
Insights from Dihybrid Experiments
 Some experiments yielded evidence of
independent assortment: Genes are typically
distributed into gametes independently of other
genes

50. 11.4 Beyond Simple Dominance
 Mendel focused on traits based on clearly
dominant and recessive alleles; however, the
expression patterns of genes for some traits are
not as straightforward

51. Codominance in ABO Blood Types
 Codominance
• Two nonidentical alleles of a gene are both fully
expressed in heterozygotes, so neither is
dominant or recessive
• May occur in multiple allele systems
 Multiple allele systems
• Genes with three or more alleles in a population
• Example: ABO blood types

52. Codominance in ABO Blood Types

53. AA BB
or or
Genotypes: AO AB BO OO
Phenotypes
(Blood type): A AB B O
Fig. 11-10, p. 176

54. Animation: Codominance: ABO blood
types

55. Incomplete Dominance
 Incomplete dominance
• One allele is not fully dominant over its partner
• The heterozygote’s phenotype is somewhere
between the two homozygotes, resulting in a
1:2:1 phenotype ratio in F2 offspring
 Example: Snapdragon color
• RR is red
• Rr is pink
• rr is white

56. Incomplete Dominance in Snapdragons

57. Fig. 11-11a, p. 176

58. homozygous homozygous heterozygous F1
x parent (rr)
parent (RR) offspring (Rr)
A Cross a red-flowered with a white-flowered plant,
and all of the F1 offspring will be pink.
Fig. 11-11a, p. 176

59. Fig. 11-11b, p. 176

60. R r
B Cross two F1 plants, R
and the three phenotypes
RR Rr
of the F2 offspring will
occur in a 1:2 :1 ratio:
r
Rr rr
Fig. 11-11b, p. 176

61. Epistasis
 Epistasis
• Two or more gene products influence a trait
• Typically, one gene product suppresses the effect
of another
 Example: Coat color in dogs
• Alleles B and b designate colors (black or brown)
• Two recessive alleles ee suppress color

62. Epistasis in Coat Colors

63. EB Eb eB eb
EEBB EEBb EeBB EeBb
EB black black black black
EEBb EEbb EeBb Eebb
Eb black chocolate black chocolate
EeBB EeBb eeBB eeBb
eB black yellow yellow
black
EeBb Eebb eeBb eebb
eb black chocolate yellow yellow
Fig. 11-13a, p. 177

64. Epistasis in Chicken Combs

65. Pleiotropy
 Pleiotropy
• One gene product
influences two or
more traits
• Example: Some tall,
thin athletes have
Marfan syndrome, a
potentially fatal
genetic disorder

66. 11.5 Linkage Groups
 The farther apart two genes are on a
chromosome, the more often crossing over
occurs between them
 Linkage group
• All genes on one chromosome
• Linked genes are very close together; crossing
over rarely occurs between them

67. Linkage and Crossing Over

68. Parental AC ac
generation
X
F1 offspring All AaCc
meiosis, gamete formation
Gametes
Most gametes have A smaller number have
parental genotypes recombinant genotypes
Fig. 11-15, p. 178

69. Animation: Crossover review

70. The Distance Between Genes
 The probability that a crossover event will
separate alleles of two genes is proportional to
the distance between those genes

71. 11.6 Genes and the Environment
 Expression of some genes is affected by
environmental factors such as temperature,
altitude, or chemical exposure
 The result may be variation in traits

72. Effects of Temperature
on Gene Expression
 Enzyme tyrosinase, works at low temperatures

73. Animation: Coat color in the Himalayan
rabbit

74. Effects of Altitude
on Gene Expression

75. Height (centimeters) Height (centimeters) Height (centimeters)
60
a Mature
cutting at high
elevation (3,060
meters above sea
level)
0
60
b Mature
cutting at mid-
elevation (1,400
meters above
sea level)
0
60
c Mature
cutting at low
elevation (30
meters above
sea level)
0
Fig. 11-17, p. 179

76. Effects of Predation
on Gene Expression
 Predators of daphnias emit chemicals that
trigger a different phenotype

77. Fig. 11-18a, p. 179

78. Fig. 11-18b, p. 179

79. 11.7 Complex
Variations in Traits
 Individuals of most
species vary in some
of their shared traits
 Many traits (such as
eye color) show a
continuous range of
variation

80. Continuous Variation
 Continuous variation
• Traits with a range of small differences
• The more factors that influence a trait, the more
continuous the distribution of phenotype
 Bell curve
• When continuous phenotypes are divided into
measurable categories and plotted as a bar chart,
they form a bell-shaped curve

81. Continuous Variation and the Bell Curve

82. Fig. 11-19a, p. 180

83. Fig. 11-19b, p. 180

84. Fig. 11-19c, p. 180

85. Animation: Continuous variation in
height

86. Regarding the Unexpected Phenotype
 Phenotype results from complex interactions
among gene products and the environment
• Enzymes and other gene products control steps
of most metabolic pathways
• Mutations, interactions among genes, and
environmental conditions may affect one or more
steps

87. 11.4-11.7 Key Concepts
Variations on Mendel’s Theme
 Not all traits appear in Mendelian inheritance
patterns
• An allele may be partly dominant over a
nonidentical partner, or codominant with it
• Multiple genes may influence a trait; some genes
influence many traits
• The environments also influences gene
expression

88. Animation: Testcross

89. Animation: Coat color in Labrador
retrievers

90. Animation: Comb shape in chickens

91. Animation: F2 ratios interaction

92. Animation: Genetic terms

93. Animation: Incomplete dominance

94. Animation: Monohybrid cross

95. Animation: Pleiotropic effects of Marfan
syndrome

96. Video: Genetics of skin color