Mendel observed patterns of inheritance in pea plants through experimentation with traits such as flower color, seed shape, and pod color. His work provided evidence that heritable traits are specified by discrete units (later identified as genes) that are transmitted from parents to offspring in predictable patterns. Through experiments involving one trait (monohybrid crosses) and two traits (dihybrid crosses), Mendel deduced that genes assort and transmit independently during gamete formation and fertilization. Later work showed that traits are influenced not only by genes but also environmental factors and that variations exist in patterns of gene expression and dominance.

1. Observing Patterns in
Inherited Traits

2. 10.1 Mendel,
Pea Plants, and Inheritance Patterns
• By experimenting with pea plants, Mendel was
the first to gather evidence of patterns by
which parents transmit genes to offspring

3. Mendel’s Experiments

4. a Garden pea flower, cut in half. Sperm form in
pollen grains, which originate in male floral parts
(stamens). 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
stamens snipped off. This is one way to assure
cross-fertilization of plants.
c Later, seeds develop inside pods of the cross-
fertilized plant. An embryo within 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. 10.3, p.154
carpel stamen

5. Animation: Crossing garden pea
plants
CLICK HERE TO PLAY

6. Producing Hybrids
• Hybrids
• Offspring of a cross between two individuals that
breed true for different forms of a trait
• Each inherits nonidentical alleles for a trait
being studied

7. Producing Hybrid Offspring

8. fertilization
produces
heterozygous
offspring
meiosis II
meiosis
I
(chromosomes
duplicated
before meiosis)
Homozygous
dominant parent
Homozygous
recessive parent
(gametes) (gametes)
Fig. 10.5, p.156

9. Heritable Units of Information
• Genes
• Heritable units of information about traits
• Each has its own locus on the chromosome
• Alleles
• Different molecular forms of the same gene
• Mutation
• Permanent change in a gene’s information

10. Heritable Units of Information

11. b A gene locus (plural, loci), the location
for a specific gene on a chromosome.
Alleles are at corresponding loci on a
pair of homologous chromosomes
d Three pairs of genes (at three
loci on this pair of homologous
chromosomes); same thing as
three pairs of alleles.
Fig. 10.4, p.155
a A pair of homologous chromosomes,
both unduplicated. In most species, one
is inherited from a female parent and its
partner from a male parent.
c A pair of alleles may be identical
or not. Alleles are represented in
the text by letters such as D or d.

12. Modern Genetic Terms
• Homozygous dominant
• Has two dominant alleles for a trait (AA)
• Homozygous recessive
• Has two recessive alleles (aa)
• Heterozygote
• Has two nonidentical alleles (Aa)

13. Modern Genetic Terms
• Dominant allele may mask effect of recessive
allele on the homologous chromosome
• Genotype
• An individual’s alleles at any or all gene loci
• Phenotype
• An individual’s observable traits

14. Animation: Genetic terms
CLICK HERE TO PLAY

15. Key Concepts:
MODERN GENETICS
• Gregor Mendel gathered the first indirect,
experimental evidence of the genetic basis of
inheritance
• His meticulous work tracking traits in many
generations of pea plants gave him clues that
heritable traits are specified in units
• The units, distributed into gametes in predictable
patterns, were later identified as genes

16. 10.2 Mendel’s Theory of
Segregation
• Mendel’s Theory of Segregation:
• Diploid organisms have pairs of genes, on pairs of
homologous chromosomes
• Based on monohybrid experiments
• During meiosis
• Genes of each pair separate
• Each gamete gets one or the other gene

17. Producing Hybrid Offspring
• Crossing two true-breeding parents of
different genotypes yields hybrid offspring
• All F1 offspring are heterozygous for a gene,
and can be used in monohybrid experiments
• All F1 offspring of parental cross AA x aa are Aa

18. A Monohybrid Cross
• Crosses between F1 monohybrids resulted in
these allelic combinations among F2 offspring
• Phenotype ratio 3:1
• Evidence of dominant and recessive traits

19. F2 Offspring:
Dominant and Recessive Traits

20. Trait
Studied
Dominant
Form
Recessive
Form
F2 Dominant-
to-Recessive
Ratio
Seed
shape
Seed
color
Pod
shape
Pod
color
Flower
color
Flower
position
Stem
length
2.98:1
3.01:1
2.95:1
2.82:1
3.15:1
3.14:1
2.84:1
787 tall 277 dwarf
651 long stem 207 at tip
705 purple 224 white
152 yellow
428 green
299 wrinkled
882 inflated
6,022 yellow 2,001 green
5,474 round 1,850 wrinkled
Fig. 10.6, p.156

21. Predicting Probability: Punnett
Squares

22. female gametes
male
gametes
Fig. 10.7a, p.157
a From left to right, step-by-step construction of a Punnett square. Circles
signify gametes. A stands for a dominant allele and a for a recessive allele at
the same gene locus. Offspring genotypes are indicated inside the squares.
A
A A A A
AA
A
A
A
Aa
Aa
Aa
Aa
Aa
aa aa aa aa
a
a
a
a
a
a
a
a

23. a
A
aa
Aa
A
a
a
A
aa
Aa
Aa
A
a
a
A
aa
Aa
Aa
AA
A
a
a
A
a aa
A
female gametes
male
gametes
Stepped Art
Fig. 10-7a, p.157

24. Predicting F1 Offspring

25. Fig. 10.7b, p.157
A
AA
Aa
a Aa
Aa
Aa
A
a
aa
Aa Aa
Aa
Aa
True-breeding homozygous
recessive parent plant
F1 offspring
b Cross between two plants that breed true for different forms of a trait.
True-breeding homozygous
dominant parent plant

26. Predicting F2 Offspring

27. Fig. 10.7c, p.157
A
Aa
A
Aa
Aa
a
a
Aa
AA Aa
aa
Aa
F2 offspring
Heterozygous
F1 offspring
Heterozygous
F1 offspring
c Cross between heterozygous F1 offspring.
aa
AA

28. Key Concepts:
MONOHYBRID EXPERIMENTS
• Some experiments yielded evidence of gene
segregation
• When one chromosome separates from its
homologous partner during meiosis, the pairs
of alleles on those chromosomes also separate
and end up in different gametes

29. Animation: Monohybrid cross
CLICK HERE TO PLAY

30. Animation: F2 ratios interaction
CLICK HERE TO PLAY

31. Animation: Testcross
CLICK HERE TO PLAY

32. 10.3 Mendel’s Theory of
Independent Assortment
• Mendel’s Theory of Independent Assortment:
• Meiosis assorts gene pairs of homologous
chromosomes independently of gene pairs on all
other chromosomes
• Based on dihybrid experiments
• Pairs of homologous chromosomes align
randomly at metaphase I

33. Independent Assortment in
Meiosis I

34. Fig. 10.8, p.158
One of two possible alignments The only other possible alignment
c Possible
combinations
of alleles in
gametes:
b The resulting
alignments at
metaphase II:
a Chromosome
alignments at
metaphase I:
A
a
AB Ab
ab aB
a
a
a
a a
b
b
b b
b
b
A A
A
A
b b
A A B B
B
B
B B
a
a
a
a
a
a
b
b
b
b
B
B
B
B B
B
A
A
A
A

35. Dihybrid Experiments
• Start with a cross between true-breeding
heterozygous parents that differ for alleles of
two genes (AABB x aabb)
• All F1 offspring are heterozygous for both
genes (AaBb)

36. Mendel’s Dihybrid Experiments
• AaBb x AaBb
• Phenotypes of the F2 offspring of F1 hybrids
were close to a 9:3:3:1 ratio
• 9 dominant for both traits
• 3 dominant for A, recessive for b
• 3 dominant for B, recessive for a
• 1 recessive for both traits

37. Results of
Mendel’s Dihybrid Experiments

38. Fig. 10.9, p.159
parent homozygous recessive
for white flowers, short stems
Gametes at fertilization
parent homozygous dominant
for purple flowers, tall stems
Meiosis, gamete formation
in true-breeding parent
plants
Possible genotypes resulting from a cross between two F1 plants:
meiosis,
gamete
formation
All F1 plants are AaBb
heterozygotes with purple
flowers and tall stems.
meiosis,
gamete
formation

39. Key Concepts:
DIHYBRID EXPERIMENTS
• Some experiments yielded evidence of
independent assortment
• During meiosis, the members of a pair of
homologous chromosomes are distributed into
gametes independently of all other pairs

40. Animation: Dihybrid cross
CLICK HERE TO PLAY

41. 10.4 Beyond Simple Dominance
• Other types of gene expression
• Codominant alleles
• Incomplete dominance
• Epistasis
• Pleiotropy

42. Codominant Alleles
• Both expressed at the same time in heterozygotes
• Example: Multiple alleles in ABO blood typing

43. Fig. 10.10, p.160
Phenotypes
(Blood type):
Genotypes:
A AB B O
or
AB
AO BO OO
BB
AA
or

44. Animation: Codominance: ABO
blood types
CLICK HERE TO PLAY

45. Incomplete Dominance
• An allele is not fully dominant over its partner
on a homologous chromosome
• Both are expressed
• Produces a phenotype between the two homozygous
conditions

46. Incomplete Dominance

47. Fig. 10.11, p.160
Cross two of the
F1 plants, and
the F2 offspring
will show three
phenotypes in
a 1:2:1 ratio:
homozygous
parent (RR)
homozygous
parent (rr)
heterozygous
F1 offspring (Rr)
x
RR Rr Rr rr

48. Animation: Incomplete
dominance
CLICK HERE TO PLAY

49. Epistasis
• Interacting products of one or more genes
affect the same trait

50. Fig. 10.12, p.161
9/16 walnut 3/16 rose 3/16 pea 1/16 single comb
F2 offspring:
RRpp (rose comb) X rrPP (pea comb)
F1 of spring: RrPp (all walnut comb)
RrPp RrPp
RRPP, RRPp,
RrPP, or RrPp
RRpp or Rrpp rrPP or rrPp rrpp
X

51. Animation: Comb shape in
chickens
CLICK HERE TO PLAY

52. More Epistasis

53. Fig. 10.13, p.161
EB Eb eB eb
EB
Eb
eB
eb
EeBb
black
EeBB
black
EEBb
black
EEBB
black
EEBb
black
EeBB
black
EeBb
black
Eebb
chocolate
EeBb
black
EEbb
chocolate
EeBb
black
Eebb
chocolate
eeBB
yellow
eeBb
yellow
eebb
yellow
eeBb
yellow

54. Animation: Coat color in
Labrador retrievers
CLICK HERE TO PLAY

55. Pleiotropy
• A single gene may affects two or more traits
• Example: Marfan syndrome

56. Animation: Pleiotropic effects
of Marfan syndrome
CLICK HERE TO PLAY

57. 10.5 Linkage Groups
• All genes on the same chromosome are part of
one linkage group
• Crossing over between homologous
chromosomes disrupts gene linkages

58. Linkage Groups and Meiosis
• During meiosis, genes relatively close together
on a chromosome tend to stay together
• Few crossover events occur between them
• Genes that are relatively far apart tend to
assort independently into gametes
• Greater frequency of crossing over between them

59. Linkage and Crossing Over

60. Fig. 10.15, p.162
Parental
generation
F1 offspring
Gametes
Most gametes have
parental genotypes
A smaller number have
recombinant genotypes
meiosis, gamete formation
All AaCc
ac
AC
×
A
A
A A a
a
a
a
c
c
c c C
C
C
C

61. Animation: Crossover review
CLICK HERE TO PLAY

62. 10.6 Genes and Environment
• Environmental factors may affect gene
expression in individuals
• Example: Temperature and fur color

63. Animation: Coat color in the
Himalayan rabbit
CLICK HERE TO PLAY

64. Elevation and Plant Height

65. Fig. 10.17, p.163
c Mature cutting
at low elevation
(30 meters above
sea level)
b Mature cutting
at mid-elevation
(1,400 meters
above sea level)
a Mature cutting
at high elevation
(3,060 meters
above sea level)
Height
(centimeters)
Height
(centimeters)
Height
(centimeters)
60
0
60
0
60
0

66. Predation and Body Form

67. 10.7 Complex Variations in
Traits
• Polygenic Inheritance
• When products of many
genes influence a trait,
individuals of a population
show a range of continuous
variation for the trait

68. Continuous Variation

69. Fig. 10.19a, p.164
Range of values for the trait
This red graph line of the
range of variation for a trait
in a population plots out as
a bell-shaped curve. Such
curves indicate continuous
variation in a population.
Number
of
individuals
with
a
measurable
value
for
the
trait

70. Animation: Continuous variation
in height
CLICK HERE TO PLAY

71. Variations in Gene Expression
• Gene interactions and environmental factors
affect most phenotypes
• Gene products control metabolic pathways
• Mutations may alter or block pathways

72. Key Concepts:
VARIATIONS ON MENDEL’S THEME
• Not all traits have clearly dominant or
recessive forms
• One allele of a pair may be fully or partially
dominant over its nonidentical partner, or
codominant with it

73. VARIATIONS ON MENDEL’S THEME
(cont.)
• Two or more gene pairs often influence the
same trait, and some single genes influence
many traits
• The environment also influences variation in
gene expression