1. Observing Patterns in Inherited Traits
Chapter 10

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. carpel
stamen
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

5. Animation: Crossing garden pea plants
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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. Homozygous dominant parent Homozygous recessive parent
(chromosomes duplicated before meiosis)
meiosis I
meiosis II
(gametes)
(gametes)
fertilization produces heterozygous offspring
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. 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.
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
c A pair of alleles may be identical
or not. Alleles are represented in
the text by letters such as D or d.
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

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

21. Predicting Probability: Punnett Squares

22. female gametes A a A a A a A a A A A Aa A AA Aa male gametes a aa a Aa
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.
Fig. 10.7a, p.157

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

24. Predicting F1 Offspring

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

26. Predicting F2 Offspring

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

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
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30. Animation: F2 ratios interaction
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31. Animation: Testcross
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32. 10.3 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. A a a A A a a B B b b b b B B A A a a A A a a B B b b b b B B B A A B
One of two possible alignments
The only other possible alignment
a Chromosome
alignments at
metaphase I: A a a A A a a B B b b b b B B
b The resulting
alignments at
metaphase II: A A a a A A a a B B b b b b B B B A A B b a a b b A A b B a a B
c Possible
combinations
of alleles in
gametes: AB ab Ab aB
Fig. 10.8, p.158

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

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

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

48. Animation: Incomplete dominance
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49. Epistasis
Interacting products of one or more genes affect the same trait

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

51. Animation: Comb shape in chickens
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52. More Epistasis

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

54. Animation: Coat color in Labrador retrievers
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55. Pleiotropy A single gene may affects two or more traits
Example: Marfan syndrome

56. Animation: Pleiotropic effects of Marfan syndrome
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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. × AC ac Parental generation A a C c A a C c F1 offspring All AaCc
meiosis, gamete formation
Gametes A a A a c c C C
Most gametes have
parental genotypes
A smaller number have
recombinant genotypes
Fig , p.162

61. Animation: Crossover review
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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
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64. Elevation and Plant Height

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

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. 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
Range of values for the trait
Fig a, p.164

70. Animation: Continuous variation in height
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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

74. Video: One Bad Transporter and Cystic Fibrosis
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