1. 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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2. The Multiplication and Addition Rules Applied to Monohybrid Crosses
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
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3. 1/2 1/2 1/2 1/4 1/4 1/2 1/4 1/4 Rr Rr  Segregation of Segregation of
Fig. 14-9 Rr Rr 
Segregation of
alleles into eggs
Segregation of
alleles into sperm
Sperm
1/2 R
1/2 r R R
1/2 R R r
Figure 14.9 Segregation of alleles and fertilization as chance events
1/4
1/4
Eggs r r
1/2 R r r
1/4
1/4

4. 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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5. 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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6. Fig. 14-UN1

7. 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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8. 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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9. 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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10. P Generation Red White CRCR CWCW Gametes CR CW Pink F1 Generation CRCW
Fig
P Generation
Red
White
CRCR
CWCW
Gametes CR CW
Pink
F1 Generation
CRCW
Gametes
1/2 CR
1/2 CW
Figure Incomplete dominance in snapdragon color
Sperm
1/2 CR
1/2 CW
F2 Generation
1/2 CR
CRCR
CRCW
Eggs
1/2 CW
CRCW
CWCW

11. The Relation Between Dominance and Phenotype
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
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12. 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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13. 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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14. 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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15. Most genes exist in populations in more than two allelic forms
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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16. Red blood cell appearance Phenotype (blood group)
Fig
Allele
Carbohydrate IA A IB B i
none
(a) The three alleles for the ABO blood groups
and their associated carbohydrates
Red blood cell
appearance
Phenotype
(blood group)
Genotype
IAIA or IA i A
IBIB or IB i B
Figure Multiple alleles for the ABO blood groups
IAIB AB ii O
(b) Blood group genotypes and phenotypes

17. 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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18. Extending Mendelian Genetics for Two or More Genes
Some traits may be determined by two or more genes
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19. 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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20. 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4  BbCc BbCc Sperm Eggs BBCC BbCC BBCc
Fig 
BbCc
BbCc
Sperm
1/4
1/4
1/4
1/4 BC bC Bc bc
Eggs
1/4 BC
BBCC
BbCC
BBCc
BbCc
1/4 bC
BbCC
bbCC
BbCc
bbCc
1/4 Bc
Figure An example of epistasis
BBCc
BbCc
BBcc
Bbcc
1/4 bc
BbCc
bbCc
Bbcc
bbcc 9
: 3
: 4

21. 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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22. Fig 
AaBbCc
AaBbCc
Sperm
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
Eggs
1/8
1/8
Figure A simplified model for polygenic inheritance of skin color
1/8
1/8
Phenotypes:
1/64
6/64
15/64
20/64
15/64
6/64
1/64
Number of
dark-skin alleles: 1 2 3 4 5 6

23. 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
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24. Fig
Figure The effect of environment on phenotype

25. Norms of reaction are generally broadest for polygenic characters
Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype
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26. 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
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27. Humans are not good subjects for genetic research
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
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28. 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
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29. Figure 14.15 Pedigree analysis
Key
Male
Affected
male
Mating
Offspring, in
birth order
(first-born on left)
Female
Affected
female
1st generation
(grandparents) Ww ww ww Ww
2nd generation
(parents, aunts,
and uncles) Ww ww ww Ww Ww ww
3rd generation
(two sisters) WW ww or Ww
Widow’s peak
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
1st generation
(grandparents) Ff Ff ff Ff
Figure Pedigree analysis
2nd generation
(parents, aunts,
and uncles) FF or Ff ff ff Ff Ff ff
3rd generation
(two sisters) ff FF or Ff
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?

30. Key Male Affected male Mating Offspring, in birth order
Fig a
Key
Male
Affected
male
Mating
Offspring, in
birth order
(first-born on left)
Female
Affected
female
Figure Pedigree analysis

31. (a) Is a widow’s peak a dominant or recessive trait?
Fig b
1st generation
(grandparents) Ww ww ww Ww
2nd generation
(parents, aunts,
and uncles) Ww ww ww Ww Ww ww
3rd generation
(two sisters) WW ww
Figure 14.15a Pedigree analysis or Ww
Widow’s peak
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?

32. (b) Is an attached earlobe a dominant or recessive trait?
Fig c
1st generation
(grandparents) Ff Ff ff Ff
2nd generation
(parents, aunts,
and uncles) FF or Ff ff ff Ff Ff ff
3rd generation
(two sisters) ff FF or Ff
Figure 14.15b Pedigree analysis
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?

33. 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
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