Mendel and the Gene Idea

Сентябрь 9, 2022

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Mendel and the Gene Idea

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2. Overview: Drawing from the Deck of Genes What genetic principles account for the passing of traits
3. The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes) Mendel documented
4. Fig. 14-1
5. Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance Mendel discovered the
6. Mendel’s Experimental, Quantitative Approach Advantages of pea plants for genetic study: There are many varieties with
7. Fig. 14-2 TECHNIQUE RESULTS Parental generation (P) Stamens Carpel 1 2 3 4 First filial gener-
8. Fig. 14-2a Stamens Carpel Parental generation (P) TECHNIQUE 1 2 3 4
9. Fig. 14-2b First filial gener- ation offspring (F1) RESULTS 5
10. Mendel chose to track only those characters that varied in an either-or manner He also used
11. In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding
12. The Law of Segregation When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all
13. Fig. 14-3-1 EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers ×
14. Fig. 14-3-2 EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers × F1 Generation (hybrids) All
15. Fig. 14-3-3 EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers × F1 Generation (hybrids) All
16. Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids
17. Table 14-1
18. Mendel’s Model Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2
19. The first concept is that alternative versions of genes account for variations in inherited characters For
20. Fig. 14-4 Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for
21. The second concept is that for each character an organism inherits two alleles, one from each
22. The third concept is that if the two alleles at a locus differ, then one (the
23. The fourth concept, now known as the law of segregation, states that the two alleles for
24. Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his
25. Fig. 14-5-1 P Generation Appearance: Genetic makeup: Gametes: Purple flowers White flowers PP P pp p
26. Fig. 14-5-2 P Generation Appearance: Genetic makeup: Gametes: Purple flowers White flowers PP P pp p
27. Fig. 14-5-3 P Generation Appearance: Genetic makeup: Gametes: Purple flowers White flowers PP P pp p
28. Useful Genetic Vocabulary An organism with two identical alleles for a character is said to be
29. Because of the different effects of dominant and recessive alleles, an organism’s traits do not always
30. Fig. 14-6 Phenotype Purple Purple 3 Purple Genotype 1 White Ratio 3:1 (homozygous) (homozygous) (heterozygous) (heterozygous)
31. The Testcross How can we tell the genotype of an individual with the dominant phenotype? Such
32. Fig. 14-7 TECHNIQUE RESULTS Dominant phenotype, unknown genotype: PP or Pp? Predictions Recessive phenotype, known genotype:
33. Fig. 14-7a Dominant phenotype, unknown genotype: PP or Pp? Predictions Recessive phenotype, known genotype: pp ×
34. Fig. 14-7b RESULTS All offspring purple or 1/2 offspring purple and 1/2 offspring white
35. The Law of Independent Assortment Mendel derived the law of segregation by following a single character
36. Mendel identified his second law of inheritance by following two characters at the same time Crossing
37. Fig. 14-8 EXPERIMENT RESULTS P Generation F1 Generation Predictions Gametes Hypothesis of dependent assortment YYRR yyrr
38. Fig. 14-8a EXPERIMENT P Generation F1 Generation Predictions Gametes Hypothesis of dependent assortment YYRR yyrr YR
39. Fig. 14-8b RESULTS Phenotypic ratio approximately 9:3:3:1 315 108 101 32
40. Using a dihybrid cross, Mendel developed the law of independent assortment The law of independent assortment
41. Concept 14.2: The laws of probability govern Mendelian inheritance Mendel’s laws of segregation and independent assortment
42. The multiplication rule states that the probability that two or more independent events will occur together
43. Fig. 14-9 Rr Rr × Segregation of alleles into eggs Sperm R R R R R
44. The rule of addition states that the probability that any one of two or more exclusive
45. Solving Complex Genetics Problems with the Rules of Probability We can apply the multiplication and addition
46. Fig. 14-UN1
47. Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics The relationship
48. Extending Mendelian Genetics for a Single Gene Inheritance of characters by a single gene may deviate
49. Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
50. Fig. 14-10-1 Red P Generation Gametes White CRCR CWCW CR CW
51. Fig. 14-10-2 Red P Generation Gametes White CRCR CWCW CR CW F1 Generation Pink CRCW CR
52. Fig. 14-10-3 Red P Generation Gametes White CRCR CWCW CR CW F1 Generation Pink CRCW CR
53. A dominant allele does not subdue a recessive allele; alleles don’t interact Alleles are simply variations
54. Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain At
55. Frequency of Dominant Alleles Dominant alleles are not necessarily more common in populations than recessive alleles
56. The allele for this unusual trait is dominant to the allele for the more common trait
57. Multiple Alleles Most genes exist in populations in more than two allelic forms For example, the
58. Fig. 14-11 IA IB i A B none (a) The three alleles for the ABO blood
59. Pleiotropy Most genes have multiple phenotypic effects, a property called pleiotropy For example, pleiotropic alleles are
60. Extending Mendelian Genetics for Two or More Genes Some traits may be determined by two or
61. Epistasis In epistasis, a gene at one locus alters the phenotypic expression of a gene at
62. Fig. 14-12 BbCc BbCc Sperm Eggs BC bC Bc bc BC bC Bc bc BBCC 1/4
63. Polygenic Inheritance Quantitative characters are those that vary in the population along a continuum Quantitative variation
64. Fig. 14-13 Eggs Sperm Phenotypes: Number of dark-skin alleles: 0 1 2 3 4 5 6
65. Nature and Nurture: The Environmental Impact on Phenotype Another departure from Mendelian genetics arises when the
66. Fig. 14-14
67. Norms of reaction are generally broadest for polygenic characters Such characters are called multifactorial because genetic
68. Integrating a Mendelian View of Heredity and Variation An organism’s phenotype includes its physical appearance, internal
69. Concept 14.4: Many human traits follow Mendelian patterns of inheritance Humans are not good subjects for
70. Pedigree Analysis A pedigree is a family tree that describes the interrelationships of parents and children
71. Fig. 14-15 Key Male Female Affected male Affected female Mating Offspring, in birth order (first-born on
72. Fig. 14-15a Key Male Female Affected male Affected female Mating Offspring, in birth order (first-born on
73. Fig. 14-15b 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) 3rd generation (two sisters) Widow’s
74. Fig. 14-15c Attached earlobe 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) 3rd generation (two
75. Pedigrees can also be used to make predictions about future offspring We can use the multiplication
76. Recessively Inherited Disorders Many genetic disorders are inherited in a recessive manner Copyright © 2008 Pearson
77. The Behavior of Recessive Alleles Recessively inherited disorders show up only in individuals homozygous for the
78. Fig. 14-16 Parents Normal Normal Sperm Eggs Normal Normal (carrier) Normal (carrier) Albino Aa Aa A
79. If a recessive allele that causes a disease is rare, then the chance of two carriers
80. Cystic Fibrosis Cystic fibrosis is the most common lethal genetic disease in the United States,striking one
81. Sickle-Cell Disease Sickle-cell disease affects one out of 400 African-Americans The disease is caused by the
82. Dominantly Inherited Disorders Some human disorders are caused by dominant alleles Dominant alleles that cause a
83. Fig. 14-17 Eggs Parents Dwarf Normal Normal Normal Dwarf Dwarf Sperm Dd × dd d D
84. Huntington’s disease is a degenerative disease of the nervous system The disease has no obvious phenotypic
85. Multifactorial Disorders Many diseases, such as heart disease and cancer, have both genetic and environmental components
86. Genetic Testing and Counseling Genetic counselors can provide information to prospective parents concerned about a family
87. Counseling Based on Mendelian Genetics and Probability Rules Using family histories, genetic counselors help couples determine
88. Tests for Identifying Carriers For a growing number of diseases, tests are available that identify carriers
89. Fetal Testing In amniocentesis, the liquid that bathes the fetus is removed and tested In chorionic
90. Fig. 14-18 Amniotic fluid withdrawn Fetus Placenta Uterus Cervix Centrifugation Fluid Fetal cells Several hours Several
91. Fig. 14-18a Fetus Amniotic fluid withdrawn Placenta Uterus Cervix Centrifugation Fluid Fetal cells Several hours Several
92. Fig. 14-18b (b) Chorionic villus sampling (CVS) Bio- chemical tests Placenta Chorionic villi Fetus Suction tube
93. Newborn Screening Some genetic disorders can be detected at birth by simple tests that are now
94. Fig. 14-UN2 Degree of dominance Complete dominance of one allele Incomplete dominance of either allele Codominance
95. Fig. 14-UN3 Description Relationship among genes Epistasis One gene affects the expression of another Example Polygenic
96. Fig. 14-UN4
97. Fig. 14-UN5 George Sandra Tom Sam Arlene Wilma Ann Michael Carla Daniel Alan Tina Christopher
98. Fig. 14-UN6
99. Fig. 14-UN7
100. Fig. 14-UN8
101. Fig. 14-UN9
102. Fig. 14-UN10
103. Fig. 14-UN11
104. You should now be able to: Define the following terms: true breeding, hybridization, monohybrid cross, P
106. Скачать презентацию

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Overview: Drawing from the Deck of Genes
What genetic principles account for

the passing of traits from parents to offspring?
The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green)

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The “particulate” hypothesis is the idea that parents pass on discrete

heritable units (genes)
Mendel documented a particulate mechanism through his experiments with garden peas

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Fig. 14-1

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Concept 14.1: Mendel used the scientific approach to identify two laws

of inheritance

Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments

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Mendel’s Experimental, Quantitative Approach
Advantages of pea plants for genetic study:
There are

many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits
Mating of plants can be controlled
Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels)
Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another

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Fig. 14-2
TECHNIQUE
RESULTS
Parental
generation
(P)
Stamens
Carpel
1
2
3
4
First
filial
gener-
ation
offspring
(F1)
5

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Fig. 14-2a
Stamens
Carpel
Parental
generation
(P)
TECHNIQUE
1
2
3
4

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Fig. 14-2b
First
filial
gener-
ation
offspring
(F1)
RESULTS
5

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Mendel chose to track only those characters that varied in an

either-or manner
He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate)

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In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a

process called hybridization
The true-breeding parents are the P generation
The hybrid offspring of the P generation are called the F1 generation
When F1 individuals self-pollinate, the F2 generation is produced

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The Law of Segregation
When Mendel crossed contrasting, true-breeding white and purple

flowered pea plants, all of the F1 hybrids were purple
When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white
Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation

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Fig. 14-3-1
EXPERIMENT
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
×

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Fig. 14-3-2
EXPERIMENT
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
×
F1 Generation
(hybrids)
All plants had
purple flowers

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Fig. 14-3-3
EXPERIMENT
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
×
F1 Generation
(hybrids)
All plants had
purple flowers
F2 Generation
705

purple-flowered
plants

224 white-flowered
plants

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Mendel reasoned that only the purple flower factor was affecting flower

color in the F1 hybrids
Mendel called the purple flower color a dominant trait and the white flower color a recessive trait
Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits
What Mendel called a “heritable factor” is what we now call a gene

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Table 14-1

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Mendel’s Model
Mendel developed a hypothesis to explain the 3:1 inheritance pattern

he observed in F2 offspring
Four related concepts make up this model
These concepts can be related to what we now know about genes and chromosomes

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The first concept is that alternative versions of genes account for

variations in inherited characters
For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers
These alternative versions of a gene are now called alleles
Each gene resides at a specific locus on a specific chromosome

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Fig. 14-4
Allele for purple flowers
Homologous
pair of
chromosomes
Locus for flower-color gene
Allele for white

flowers

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The second concept is that for each character an organism inherits

two alleles, one from each parent
Mendel made this deduction without knowing about the role of chromosomes
The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation
Alternatively, the two alleles at a locus may differ, as in the F1 hybrids

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The third concept is that if the two alleles at a

locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance
In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant

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The fourth concept, now known as the law of segregation, states

that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism
This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

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Mendel’s segregation model accounts for the 3:1 ratio he observed in

the F2 generation of his numerous crosses
The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup
A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele

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Fig. 14-5-1
P Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
White flowers
PP
P
pp
p

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Fig. 14-5-2
P Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
White flowers
PP
P
pp
p
F1 Generation
Gametes:
Genetic makeup:
Appearance:
Purple flowers
Pp
P
p
1/2
1/2

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Fig. 14-5-3
P Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
White flowers
PP
P
pp
p
F1 Generation
Gametes:
Genetic makeup:
Appearance:
Purple flowers
Pp
P
p
1/2
1/2
F2 Generation
Sperm
Eggs
P
P
PP
Pp
p
p
Pp
pp
3
1

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Useful Genetic Vocabulary
An organism with two identical alleles for a character

is said to be homozygous for the gene controlling that character
An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character
Unlike homozygotes, heterozygotes are not true-breeding

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Because of the different effects of dominant and recessive alleles, an

organism’s traits do not always reveal its genetic composition
Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup
In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes

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Fig. 14-6
Phenotype
Purple
Purple
3
Purple
Genotype
1
White
Ratio 3:1
(homozygous)
(homozygous)
(heterozygous)
(heterozygous)
PP
Pp
Pp
pp
Ratio 1:2:1
1
1
2

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The Testcross
How can we tell the genotype of an individual with

the dominant phenotype?
Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous
The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual
If any offspring display the recessive phenotype, the mystery parent must be heterozygous

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Fig. 14-7
TECHNIQUE
RESULTS
Dominant phenotype,
unknown genotype:
PP or Pp?
Predictions
Recessive phenotype,
known genotype:
pp
×
If

If Pp

Sperm

Eggs

All offspring purple

1/2 offspring purple and
1/2 offspring white

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Fig. 14-7a
Dominant phenotype,
unknown genotype:
PP or Pp?
Predictions
Recessive phenotype,
known genotype:
pp
×
If

If Pp

Sperm

Eggs

TECHNIQUE

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Fig. 14-7b
RESULTS
All offspring purple
or
1/2 offspring purple and
1/2 offspring white

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The Law of Independent Assortment
Mendel derived the law of segregation by

following a single character
The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one character
A cross between such heterozygotes is called a monohybrid cross

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Mendel identified his second law of inheritance by following two characters

at the same time
Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters
A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

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Fig. 14-8
EXPERIMENT
RESULTS
P Generation
F1 Generation
Predictions
Gametes
Hypothesis of
dependent
assortment
YYRR
yyrr
YR
yr
YyRr
×
Hypothesis of
independent
assortment
or
Predicted
offspring of
F2 generation
Sperm
Sperm
YR
YR
yr
yr
Yr
YR
yR
Yr
yR
yr
YR
YYRR
YYRR
YyRr
YyRr
YyRr
YyRr
YyRr
YyRr
YYRr
YYRr
YyRR
YyRR
YYrr
Yyrr
Yyrr
yyRR
yyRr
yyRr
yyrr
yyrr
Phenotypic ratio 3:1
Eggs
Eggs
Phenotypic ratio

9:3:3:1

1/2

1/4

3/4

9/16

3/16

1/16

Phenotypic ratio approximately 9:3:3:1

315

108

101

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Fig. 14-8a
EXPERIMENT
P Generation
F1 Generation
Predictions
Gametes
Hypothesis of
dependent
assortment
YYRR
yyrr
YR
yr
YyRr
×
Hypothesis of
independent
assortment
or
Predicted
offspring of
F2 generation
Sperm
Sperm
YR
YR
yr
yr
Yr
YR
yR
Yr
yR
yr
YR
YYRR
YYRR
YyRr
YyRr
YyRr
YyRr
YyRr
YyRr
YYRr
YYRr
YyRR
YyRR
YYrr
Yyrr
Yyrr
yyRR
yyRr
yyRr
yyrr
yyrr
Phenotypic ratio 3:1
Eggs
Eggs
Phenotypic ratio

9:3:3:1

1/2

1/4

3/4

9/16

3/16

1/16

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Fig. 14-8b
RESULTS
Phenotypic ratio approximately 9:3:3:1
315
108
101
32

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Using a dihybrid cross, Mendel developed the law of independent assortment
The

law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation
Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes
Genes located near each other on the same chromosome tend to be inherited together

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Concept 14.2: The laws of probability govern Mendelian inheritance
Mendel’s laws of

segregation and independent assortment reflect the rules of probability
When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss
In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles

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The multiplication rule states that the probability that two or more

independent events will occur together is the product of their individual probabilities
Probability in an F1 monohybrid cross can be determined using the multiplication rule
Segregation in a heterozygous plant is like flipping a coin: Each gamete has a chance of carrying the dominant allele and a chance of carrying the recessive allele

The Multiplication and Addition Rules Applied to Monohybrid Crosses

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Fig. 14-9
Rr
Rr
×
Segregation of
alleles into eggs
Sperm
R
R
R
R
R
R
r
r
r
r
r
r
1/2
1/2
1/2
1/2
Segregation of
alleles into sperm
Eggs
1/4
1/4
1/4
1/4

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The rule of addition states that the probability that any one

of two or more exclusive events will occur is calculated by adding together their individual probabilities
The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous

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Solving Complex Genetics Problems with the Rules of Probability
We can apply

the multiplication and addition rules to predict the outcome of crosses involving multiple characters
A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously
In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together

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Fig. 14-UN1

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Concept 14.3: Inheritance patterns are often more complex than predicted by

simple Mendelian genetics

The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied
Many heritable characters are not determined by only one gene with two alleles
However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

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Extending Mendelian Genetics for a Single Gene
Inheritance of characters by a

single gene may deviate from simple Mendelian patterns in the following situations:
When alleles are not completely dominant or recessive
When a gene has more than two alleles
When a gene produces multiple phenotypes

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Degrees of Dominance
Complete dominance occurs when phenotypes of the heterozygote

and dominant homozygote are identical
In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

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Fig. 14-10-1
Red
P Generation
Gametes
White
CRCR
CWCW
CR
CW

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Fig. 14-10-2
Red
P Generation
Gametes
White
CRCR
CWCW
CR
CW
F1 Generation
Pink
CRCW
CR
CW
Gametes
1/2
1/2

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Fig. 14-10-3
Red
P Generation
Gametes
White
CRCR
CWCW
CR
CW
F1 Generation
Pink
CRCW
CR
CW
Gametes
1/2
1/2
F2 Generation
Sperm
Eggs
CR
CR
CW
CW
CRCR
CRCW
CRCW
CWCW
1/2
1/2
1/2
1/2

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A dominant allele does not subdue a recessive allele; alleles don’t

interact
Alleles are simply variations in a gene’s nucleotide sequence
For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype

The Relation Between Dominance and
Phenotype

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Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of

lipids in the brain
At the organismal level, the allele is recessive
At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant
At the molecular level, the alleles are codominant

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Frequency of Dominant Alleles
Dominant alleles are not necessarily more common in

populations than recessive alleles
For example, one baby out of 400 in the United States is born with extra fingers or toes

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The allele for this unusual trait is dominant to the allele

for the more common trait of five digits per appendage
In this example, the recessive allele is far more prevalent than the population’s dominant allele

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Multiple Alleles
Most genes exist in populations in more than two allelic

forms
For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.
The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither

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Fig. 14-11
IA
IB
i
A
B
none
(a) The three alleles for the ABO blood groups
and

their associated carbohydrates

IAIA or IA i

IBIB or IB i

IAIB

(b) Blood group genotypes and phenotypes

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Pleiotropy
Most genes have multiple phenotypic effects, a property called pleiotropy
For

example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

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Extending Mendelian Genetics for Two or More Genes
Some traits may be

determined by two or more genes

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Epistasis
In epistasis, a gene at one locus alters the phenotypic expression

of a gene at a second locus
For example, in mice and many other mammals, coat color depends on two genes
One gene determines the pigment color (with alleles B for black and b for brown)
The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair

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Fig. 14-12
BbCc
BbCc
Sperm
Eggs
BC
bC
Bc
bc
BC
bC
Bc
bc
BBCC
1/4
1/4
1/4
1/4
1/4
1/4
1/4
1/4
BbCC
BBCc
BbCc
BbCC
bbCC
BbCc
bbCc
BBCc
BbCc
BbCc
bbCc
BBcc
Bbcc
Bbcc
bbcc
9
: 3
: 4
×

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Polygenic Inheritance
Quantitative characters are those that vary in the population along

a continuum
Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype
Skin color in humans is an example of polygenic inheritance

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Fig. 14-13
Eggs
Sperm
Phenotypes:
Number of
dark-skin alleles:
0
1
2
3
4
5
6
1/64
6/64
15/64
20/64
15/64
6/64
1/64
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
AaBbCc
AaBbCc
×

Слайд 65

Nature and Nurture: The Environmental Impact on Phenotype
Another departure from Mendelian

genetics arises when the phenotype for a character depends on environment as well as genotype
The norm of reaction is the phenotypic range of a genotype influenced by the environment
For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity

Слайд 66

Fig. 14-14

Слайд 67

Norms of reaction are generally broadest for polygenic characters
Such characters are

called multifactorial because genetic and environmental factors collectively influence phenotype

Слайд 68

Integrating a Mendelian View of Heredity and Variation
An organism’s phenotype includes

its physical appearance, internal anatomy, physiology, and behavior
An organism’s phenotype reflects its overall genotype and unique environmental history

Слайд 69

Concept 14.4: Many human traits follow Mendelian patterns of inheritance
Humans are

not good subjects for genetic research
– Generation time is too long
– Parents produce relatively few offspring
– Breeding experiments are unacceptable
However, basic Mendelian genetics endures as the foundation of human genetics

Слайд 70

Pedigree Analysis
A pedigree is a family tree that describes the interrelationships

of parents and children across generations
Inheritance patterns of particular traits can be traced and described using pedigrees

Слайд 71

Fig. 14-15
Key
Male
Female
Affected
male
Affected
female
Mating
Offspring, in
birth order
(first-born on left)
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
3rd generation
(two

sisters)

Widow’s peak

No widow’s peak

(a) Is a widow’s peak a dominant or recessive trait?

1st generation
(grandparents)

2nd generation
(parents, aunts,
and uncles)

3rd generation
(two sisters)

Attached earlobe

Free earlobe

(b) Is an attached earlobe a dominant or recessive trait?

Слайд 72

Fig. 14-15a
Key
Male
Female
Affected
male
Affected
female
Mating
Offspring, in
birth order
(first-born on left)

Слайд 73

Fig. 14-15b
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
3rd generation
(two sisters)
Widow’s peak
No widow’s peak
(a)

Is a widow’s peak a dominant or recessive trait?

Слайд 74

Fig. 14-15c
Attached earlobe
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
3rd generation
(two sisters)
Free earlobe
(b) Is

an attached earlobe a dominant or recessive trait?

Слайд 75

Pedigrees can also be used to make predictions about future offspring
We

can use the multiplication and addition rules to predict the probability of specific phenotypes

Слайд 76

Recessively Inherited Disorders
Many genetic disorders are inherited in a recessive manner
Copyright

Слайд 77

The Behavior of Recessive Alleles
Recessively inherited disorders show up only in

individuals homozygous for the allele
Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e., pigmented)
Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair

Слайд 78

Fig. 14-16
Parents
Normal
Normal
Sperm
Eggs
Normal
Normal
(carrier)
Normal
(carrier)
Albino
Aa
Aa
A
A
AA
Aa
a
Aa
aa
a
×

Слайд 79

If a recessive allele that causes a disease is rare, then

the chance of two carriers meeting and mating is low
Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele
Most societies and cultures have laws or taboos against marriages between close relatives

Слайд 80

Cystic Fibrosis
Cystic fibrosis is the most common lethal genetic disease in

the United States,striking one out of every 2,500 people of European descent
The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes
Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

Слайд 81

Sickle-Cell Disease
Sickle-cell disease affects one out of 400 African-Americans
The disease is

caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
Symptoms include physical weakness, pain, organ damage, and even paralysis

Слайд 82

Dominantly Inherited Disorders
Some human disorders are caused by dominant alleles
Dominant alleles

that cause a lethal disease are rare and arise by mutation
Achondroplasia is a form of dwarfism caused by a rare dominant allele

Слайд 83

Fig. 14-17
Eggs
Parents
Dwarf
Normal
Normal
Normal
Dwarf
Dwarf
Sperm
Dd
×
dd
d
D
Dd
dd
dd
Dd
d
d

Слайд 84

Huntington’s disease is a degenerative disease of the nervous system
The disease

has no obvious phenotypic effects until the individual is about 35 to 40 years of age

Huntington’s Disease

Слайд 85

Multifactorial Disorders
Many diseases, such as heart disease and cancer, have both

genetic and environmental components
Little is understood about the genetic contribution to most multifactorial diseases

Слайд 86

Genetic Testing and Counseling
Genetic counselors can provide information to prospective parents

concerned about a family history for a specific disease

Слайд 87

Counseling Based on Mendelian Genetics and Probability Rules
Using family histories, genetic

counselors help couples determine the odds that their children will have genetic disorders

Слайд 88

Tests for Identifying Carriers
For a growing number of diseases, tests are

available that identify carriers and help define the odds more accurately

Слайд 89

Fetal Testing
In amniocentesis, the liquid that bathes the fetus is removed

and tested
In chorionic villus sampling (CVS), a sample of the placenta is removed and tested
Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero

Video: Ultrasound of Human Fetus I

Слайд 90

Fig. 14-18
Amniotic fluid
withdrawn
Fetus
Placenta
Uterus
Cervix
Centrifugation
Fluid
Fetal
cells
Several
hours
Several
weeks
Several
weeks
(a) Amniocentesis
(b) Chorionic villus sampling (CVS)
Several
hours
Several
hours
Fetal
cells
Bio-
chemical
tests
Karyotyping
Placenta
Chorionic
villi
Fetus
Suction tube
inserted
through
cervix

Слайд 91

Fig. 14-18a
Fetus
Amniotic fluid
withdrawn
Placenta
Uterus
Cervix
Centrifugation
Fluid
Fetal
cells
Several
hours
Several
weeks
Several
weeks
Bio-
chemical
tests
Karyotyping
(a) Amniocentesis

Слайд 92

Fig. 14-18b
(b) Chorionic villus sampling (CVS)
Bio-
chemical
tests
Placenta
Chorionic
villi
Fetus
Suction tube
inserted
through
cervix
Fetal
cells
Several
hours
Several
hours
Karyotyping

Слайд 93

Newborn Screening
Some genetic disorders can be detected at birth by simple

tests that are now routinely performed in most hospitals in the United States

Слайд 94

Fig. 14-UN2
Degree of dominance
Complete dominance
of one allele
Incomplete dominance
of either allele
Codominance
Description
Heterozygous phenotype
same

as that of homo-
zygous dominant

Heterozygous phenotype
intermediate between
the two homozygous
phenotypes

Heterozygotes: Both
phenotypes expressed

Multiple alleles

Pleiotropy

In the whole population,
some genes have more
than two alleles

One gene is able to
affect multiple
phenotypic characters

CRCR

CRCW

CWCW

IAIB

IA , IB , i

ABO blood group alleles

Sickle-cell disease

Example

Слайд 95

Fig. 14-UN3
Description
Relationship among
genes
Epistasis
One gene affects
the expression of
another
Example
Polygenic
inheritance
A single phenotypic
character is
affected by
two

or more genes

BbCc

: 3

: 4

AaBbCc

Слайд 96

Fig. 14-UN4

Слайд 97

Fig. 14-UN5
George
Sandra
Tom
Sam
Arlene
Wilma
Ann
Michael
Carla
Daniel
Alan
Tina
Christopher

Слайд 98

Fig. 14-UN6

Слайд 99

Fig. 14-UN7

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Fig. 14-UN8

Слайд 101

Fig. 14-UN9

Слайд 102

Fig. 14-UN10

Слайд 103

Fig. 14-UN11

Слайд 104

You should now be able to:
Define the following terms: true breeding,

hybridization, monohybrid cross, P generation, F1 generation, F2 generation
Distinguish between the following pairs of terms: dominant and recessive; heterozygous and homozygous; genotype and phenotype
Use a Punnett square to predict the results of a cross and to state the phenotypic and genotypic ratios of the F2 generation

Mendel and the Gene Idea

Содержание

Overview: Drawing from the Deck of GenesWhat genetic principles account for

The “particulate” hypothesis is the idea that parents pass on discrete

Fig. 14-1

Concept 14.1: Mendel used the scientific approach to identify two laws

Mendel’s Experimental, Quantitative ApproachAdvantages of pea plants for genetic study:There are

Fig. 14-2TECHNIQUERESULTSParentalgeneration(P)StamensCarpel1234Firstfilialgener-ationoffspring(F1)5

Fig. 14-2aStamensCarpelParentalgeneration(P)TECHNIQUE1234

Fig. 14-2bFirstfilialgener-ationoffspring(F1)RESULTS5

Mendel chose to track only those characters that varied in an

In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a

The Law of SegregationWhen Mendel crossed contrasting, true-breeding white and purple

Fig. 14-3-1EXPERIMENTP Generation(true-breeding parents)Purpleflowers Whiteflowers×

Fig. 14-3-2EXPERIMENTP Generation(true-breeding parents)Purpleflowers Whiteflowers×F1 Generation (hybrids)All plants hadpurple flowers

Fig. 14-3-3EXPERIMENTP Generation(true-breeding parents)Purpleflowers Whiteflowers×F1 Generation (hybrids)All plants hadpurple flowersF2 Generation705

Mendel reasoned that only the purple flower factor was affecting flower

Table 14-1

Mendel’s ModelMendel developed a hypothesis to explain the 3:1 inheritance pattern

The first concept is that alternative versions of genes account for

Fig. 14-4Allele for purple flowersHomologouspair ofchromosomesLocus for flower-color geneAllele for white

The second concept is that for each character an organism inherits

The third concept is that if the two alleles at a

The fourth concept, now known as the law of segregation, states

Mendel’s segregation model accounts for the 3:1 ratio he observed in

Fig. 14-5-1P GenerationAppearance:Genetic makeup:Gametes:Purple flowersWhite flowersPPPppp

Fig. 14-5-2P GenerationAppearance:Genetic makeup:Gametes:Purple flowersWhite flowersPPPpppF1 GenerationGametes:Genetic makeup:Appearance:Purple flowersPpPp1/21/2

Fig. 14-5-3P GenerationAppearance:Genetic makeup:Gametes:Purple flowersWhite flowersPPPpppF1 GenerationGametes:Genetic makeup:Appearance:Purple flowersPpPp1/21/2F2 GenerationSpermEggsPPPPPpppPppp31

Useful Genetic VocabularyAn organism with two identical alleles for a character

Because of the different effects of dominant and recessive alleles, an

Fig. 14-6PhenotypePurplePurple3PurpleGenotype1WhiteRatio 3:1(homozygous)(homozygous)(heterozygous)(heterozygous)PPPpPpppRatio 1:2:1112

The TestcrossHow can we tell the genotype of an individual with

Fig. 14-7TECHNIQUERESULTSDominant phenotype, unknown genotype:PP or Pp?PredictionsRecessive phenotype, known genotype: pp×If

Fig. 14-7aDominant phenotype, unknown genotype:PP or Pp?PredictionsRecessive phenotype, known genotype: pp×If

Fig. 14-7bRESULTSAll offspring purpleor1/2 offspring purple and1/2 offspring white

The Law of Independent AssortmentMendel derived the law of segregation by

Mendel identified his second law of inheritance by following two characters

Fig. 14-8bRESULTSPhenotypic ratio approximately 9:3:3:131510810132

Using a dihybrid cross, Mendel developed the law of independent assortmentThe

Concept 14.2: The laws of probability govern Mendelian inheritanceMendel’s laws of

The multiplication rule states that the probability that two or more

Fig. 14-9RrRr×Segregation ofalleles into eggsSpermRRRRRRrrrrrr1/21/21/21/2Segregation ofalleles into spermEggs1/41/41/41/4

The rule of addition states that the probability that any one

Solving Complex Genetics Problems with the Rules of ProbabilityWe can apply