1. Mendel and Genes Chapter 14

2. Fig. 14-1
The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green)
The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes)
Figure 14.1 What principles of inheritance did Gregor Mendel discover by breeding garden pea plants?

3. How can we tell the genotype of an individual with the dominant phenotype?
The answer is to carry out a testcross:

4. Fig. 14-7 TECHNIQUE RESULTS Dominant phenotype, unknown genotype:
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype: pp
Predictions
If PP
If Pp or
Sperm
Sperm p p p p P P Pp Pp Pp Pp
Eggs
Eggs
Figure 14.7 The testcross P p Pp Pp pp pp
RESULTS or
All offspring purple
1/2 offspring purple and
1/2 offspring white

5. 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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6. 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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7. 1/2 1/2 1/2 1/4 1/4 1/2 1/4 1/4 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

8. 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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9. 1/2 1/2 1/2 1/4 1/4 1/2 1/4 1/4 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

10. Summary of Mendel’s Principles
The inheritance of biological characteristics is determined by individual units known as genes. In organisms that reproduce sexually, genes are passed from parents to their offspring.
In cases in which two or more forms of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.
In most sexually reproducing organisms, each adult has two copies of each gene—one from each parent. These genes are segregated from each other when gametes are formed.
The alleles for different genes usually segregate independently of one another.

11. 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
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12. Beyond dominant/recessive
Fig
Beyond dominant/recessive
P Generation
Red
White
CRCR
CWCW
incomplete dominance - situation in which one allele is not completely dominant over another
There is no white no red: new phenotype pink
Gametes CR CW
Pink
F1 Generation
CRCW
Gametes
1/2 CR
1/2 CW
Sperm
1/2 CR
1/2 CW
F2 Generation
1/2 CR
CRCR
CRCW
Eggs
1/2 CW
CRCW
CWCW

13. Beyond dominant and recessive alleles
Codominance - situation in which both alleles of a gene contribute to the phenotype of the organism

14. Beyond dominant and recessive alleles
multiple alleles - three or more alleles of the same gene

15. Fig. 14-11 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

16. 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
Height and Skin color in humans is an example of polygenic inheritance

17. 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

18. 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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19. 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
PKU
Sickle-cell disease
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20. 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

21. 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 Fig. 14-12  BbCc BbCc Sperm Eggs BBCC
Figure An example of epistasis
BBCc
BbCc
BBcc
Bbcc
1/4 bc
BbCc
bbCc
Bbcc
bbcc 9
: 3
: 4

22. Nature and Nurture: The Environmental Impact on Phenotype
Fig
Nature and Nurture: The Environmental Impact on Phenotype
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
multifactorial
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23. 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

24. (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?

25. (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?

26. (Start Next)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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

27. Parents Normal Normal Aa Sperm A a Eggs Aa AA A Normal (carrier)
Fig
Parents
Normal
Normal Aa  Aa
Sperm A a
Eggs Aa AA A
Normal
(carrier)
Normal
Figure Albinism: a recessive trait Aa aa a
Normal
(carrier)
Albino

28. Autosomal Disorders Genes for these disorders are located on autosomes
Recessive disorder
Dominant disorders
Codominant disorders

29. Cystic Fibrosis

30. Parents Dwarf Normal Sperm Eggs Dwarf Normal Dwarf Normal Dd dd D d Dd
Fig
Parents
Dwarf
Normal Dd  dd
Sperm D d
Eggs Dd dd d
Figure Achondroplasia: a dominant trait
Dwarf
Normal Dd dd d
Dwarf
Normal

32. 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

33. Genetic Testing and Counseling
Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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

35. Karyotype Making a Karyotype Photograph chromosomes during mitosis
Cut chromosomes out of photograph
Group them in order, in pairs
Male 46XY
Female 46XX

37. Sex-Linked Genes
Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive.

38. Some disorders caused by recessive alleles on the X chromosome in humans:
Color blindness
Duchenne muscular dystrophy
Hemophilia
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

39. Heterozygous phenotype same as that of homo- zygous dominant PP Pp
Fig. 14-UN2
Degree of dominance
Description
Example
Complete dominance
of one allele
Heterozygous phenotype
same as that of homo-
zygous dominant PP Pp
Incomplete dominance
of either allele
Heterozygous phenotype
intermediate between
the two homozygous
phenotypes
CRCR
CRCW
CWCW
Codominance
Heterozygotes: Both
phenotypes expressed
IAIB
Multiple alleles
In the whole population,
some genes have more
than two alleles
ABO blood group alleles
IA , IB , i
Pleiotropy
One gene is able to
affect multiple
phenotypic characters
Sickle-cell disease

40. Relationship among genes Description Example Epistasis
Fig. 14-UN3
Relationship among
genes
Description
Example
Epistasis
One gene affects
the expression of
another
BbCc
BbCc BC bC Bc bc BC bC Bc bc 9
: 3
: 4
Polygenic
inheritance
A single phenotypic
character is
affected by
two or more genes
AaBbCc
AaBbCc