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CAMPBELL BIOLOGY IN FOCUSUrry Cain Wasserman Minorsky Jackson Reece
11Mendel and
the Gene Idea
(Ayo)
Ayo website:
http://myweb.nutn.edu.tw/~hycheng/
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Overview: Drawing from the Deck of Genes
What genetic principles account for the passing oftraits from parents to offspring?
blendinghypothesis
the way blue and yellow paint blend to make green
particulatehypothesis is the idea that parents
pass on discrete heritable units (genes).
Mendeldocumented a particulatemechanismthrough his experiments with garden peas.
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Chap.11 Mendel and the Gene Idea
11.1: Mendel used the scientific approachtoidentify two laws of inheritance
11.2: The laws of probability()govern
Mendelian inheritance 11.3: Inheritance patterns are often more complex
than predicted by simple Mendelian genetics
11.4: Many human traits follow Mendelian patternsof inheritance
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Gregor Mendel
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5
11.1: Mendel used the scientific approach toidentify two laws of inheritance
Mendel discovered the basic principles of heredity bybreeding garden peas in carefully planned
experiments
Mendel probably chose to work with peasbecause1. 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.
2. He could control mating between plants
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Figure 11.2
Parentalgeneration(P)
Stamens
First filialgenerationoffspring(F1)
Carpel
Technique
Results
1
2
3
4
5
Mendels Experimental,
Quantitative Approach
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In a typical experiment, Mendel mated twocontrasting varieties, a process called
hybridization.
The parents are the P generation.
The hybrid offspring of the P generation are called
the F1generation.
When F1individuals self-pollinate or cross- pollinate
with other F1hybrids, the F2generation is produced.
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The Law of Segregation
When Mendel crossed contrasting white- and purple-flowered pea plants, all of the F1hybrids were purple
When Mendel crossed the F1hybrids, 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 F2generation
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Figure 11.3-1
P Generation
Experiment
Purple flowers White flowers
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Figure 11.3-2
P Generation
Experiment
F1Generation
(hybrids)
Purple flowers White flowers
All plants had purple flowers
Self- or cross-pollination
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Figure 11.3-3
P Generation
Experiment
F1Generation
F2Generation
(hybrids)
Purple flowers White flowers
All plants had purple flowers
Self- or cross-pollination
705 purple-flowered
plants
224 white-flowered
plants 11
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Mendel called the purple flower color a dominanttrait and the white flower color a recessive trait.
The factor for white flowers was not diluted or
destroyed because it reappeared in the F2
generation.
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 ( ).
T bl 11 1
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Table 11.1a
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T bl 11 1b
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Table 11.1b
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MendelsModel
Four related concepts make up this model First,alternative versions of genesaccount for
variations in inherited characters.
These alternative versions of a gene are now calledalleles ( ).
Each generesides at a specific locus()on a
specific chromosome
Fig re 11 4
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Figure 11.4
Allele for purple flowers
Pair of
homologouschromosomes
Allele for white flowers
Locus for flower-color gene
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Second, for each character, an organism inheritstwo alleles, one from each parent.
Mendel made this deduction without knowing about
the existence of chromosomes
Two alleles at a particular locus may be identical, as
in the true-breeding plants of MendelsP generation
Alternatively, the two alleles at a locus may differ, as
in the F1hybrids
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Third, if the two alleles at a locus differ, then one(the dominant allele) determines the organisms
appearance, and the other (the recessive allele)
has no noticeable effect on appearance
In the flower-color example, the F1plants had purple
flowers because the allele for that trait is dominant
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Fourth(now known as the law of segregation),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 organism
This segregation of alleles corresponds to the
distribution of homologous chromosomes to differentgametes in meiosis
Figure 11 5-1
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Figure 11.5-1
P Generation
Gametes:
Appearance:Genetic makeup:
Purple flowersPP
White flowerspp
P p
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Figure 11 5-3
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Figure 11.5 3
P Generation
Gametes:
Appearance:Genetic makeup:
F1Generation
F2Generation
Purple flowersPP
White flowerspp
Gametes:
Appearance:Genetic makeup:
Eggs fromF1(Pp) plant
Sperm fromF1(Pp) plant
Purple flowersPp
P
P
p
p
P p
P
p
PP
pp
Pp
Pp
3 : 1 22
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Mendelssegregation model accounts for the 3:1ratio he observed in the F2generation of hisnumerous crosses
The possible combinations of sperm and egg can be
shown using a Punnett square, a diagram forpredicting the results of a genetic cross betweenindividuals of known genetic makeup
A capital letter represents a dominant allele, and alowercase letter represents a recessive allele
For example, Pis the purple-flower allele andpis thewhite-flower allele
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Useful Genetic Vocabulary
An organism with two identical alleles for acharacter 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, heterozygotesare not true-
breeding.
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Because of the effects of dominant and recessivealleles, an organismstraits do not always reveal its
genetic composition
Therefore, we distinguish between an organisms
phenotype(
), or physical appearance, and its
genotype( ), or genetic makeup
In the example of flower color in pea plants, PP and
Ppplants have the same phenotype (purple) butdifferent genotypes
Figure 11.6
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g
Phenotype
1
Genotype
Purple
Purple
Purple
White
Ratio 3:1
PP(homozygous)
Pp(heterozygous)
Pp(heterozygous)
pp(homozygous)
Ratio 1:2:1
2
3
1
1
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The Testcross
How can we tell the genotype of an individual withthe dominant phenotype?
Such an 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, themystery parent must be heterozygous
Figure 11.7T h i
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gTechnique
Predictions
Dominant phenotype,unknown genotype:
PPor Pp?
Eggs
Sperm
offspring purple and
offspring white
Recessive phenotype,known genotype:
pp
If purple-floweredparent isPP
If purple-floweredparent isPp
Eggs
Sperm
All offspring purple
Results
or
or
p p
P
p
Pp
pp
Pp
pp
p p
P
P
Pp
Pp
Pp
Pp
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The Law of Independent Assortment
Mendel derived the law of segregation by followinga single character
The F1offspring 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 byfollowing two characters at the same time
Crossing two true-breeding parents differing in
two characters produces dihybrids in the F1
generation, heterozygousfor both characters
A dihybrid cross, a cross between F1dihybrids,
can determine whether two characters are
transmitted to offspring as a package orindependently
Figure 11.8a
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Experiment
P Generation
F1Generation
Gametes
YYRR yyrr
YyRr
YR y r
31
Figure 11.8b
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YR yr
YR
y r
YYRR
yyr r
YyRr
YyRr
Predicted
offspring inF2generation
Results
Eggs
Eggs
Sperm
Sperm
Hypothesis of
dependent assortment
Phenotypic ratio 3:1
Hypothesis of
independent assortment
Phenotypic ratio approximately 9:3:3:1
Phenotypic ratio 9:3:3:1
YR yr
YR
yr
YYRR
yyr r
YYRr YyRr
Yr yR
Yr
yR
YyRR
YYRr YYrr YyrrYyRr
YyRR YyRr yyRryyRR
YyRr Yyrr yyRr
315 108 101 32
916 316316 116
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The results of Mendels dihybrid experiments arethe basis for the law of independent
assortment
It states that each pair of alleles segregates
independentlyof each other pair of allelesduring gamete formation
11 2 Th l f b bili M d li
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11.2: The laws of probability govern Mendelianinheritance
Mendels laws of segregation and independentassortment 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 independentlyof another
genes alleles
Th M l i li i d Addi i R l A li d
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The multiplication rulestates that the probabilitythat two or more independent events will occur
together is the product of their individual
probabilities
This can be applied to an F1monohybrid cross
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 therecessive allele
The Multiplication and Addition Rules Appliedto Monohybrid Crosses
12
12
Figure 11.9
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R r
R
r
R
r
Segregation ofalleles into eggs
Eggs
Sperm
Segregation ofalleles into sperm
Rr Rr
R
R
r
r
R
r
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The addition rulestates that the probability that anyone of two or more mutually exclusive events will
occur is calculated by adding together their
individual probabilities
It can be used to figure out the probability that an F2plant from a monohybrid cross will be heterozygous
rather than homozygous
S l i C l G ti P bl ith th
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Solving Complex Genetics Problems with theRules of Probability
We can apply the rules of probability to predict theoutcome 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
Figure 11.UN01
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39
For example, if we cross F1heterozygotes of
genotype YyRr, we can calculate the
probability of different genotypes among the
F2generation
Figure 11.UN02
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40
For example, for the cross PpYyRr Ppyyrr,
we can calculate the probability of offspringshowing at least two recessive traits
11 3: Inheritance patterns are often more complex
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11.3: Inheritance patterns are often more complexthan predicted by simple Mendelian genetics
Not all heritable characters are determined assimply as the traits Mendel studied
However, the basic principles of segregation and
independent assortment apply even to more
complex patternsof inheritance
E t di M d li G ti f Si l G
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Extending Mendelian Genetics for a Single Gene
Inheritance of characters by a single gene maydeviatefrom simple Mendelian patterns in the
following situations
1. When alleles are not completely dominant or
recessive
2. When a gene has more than two alleles
3. When a single gene influences multiple
phenotypes
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1. Degrees of Dominance
Complete dominanceoccurs when phenotypes ofthe heterozygote and dominant homozygote are
identical
In incomplete dominance, the phenotype of F1
hybrids is somewhere between the phenotypes ofthe two parental varieties
In codominance, two dominant alleles affect the
phenotype in separate, distinguishable ways
Figure 11.10-1
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P Generation
Gametes
WhiteCWCW
RedCRCR
CWCR
44
Figure 11.10-2
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P Generation
F1Generation
Gametes
Gametes
WhiteCWCW
PinkCRCW
RedCRCR
CWCR
CWCR
45
Figure 11.10-3
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Eggs
P Generation
F1Generation
Gametes
F2Generation
Gametes
Sperm
WhiteCWCW
PinkCRCW
RedCRCR
CWCW
CRCWCRCR
CWCR
CWCR
CW
CR
CW
CR
CRCW
46
Th R l ti hi B t D i d
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The Relationship Between Dominance and
Phenotype
Alleles are simply variations in a genesnucleotide
sequence
When a dominant allele coexists with a recessive
allele in a heterozygote, they do not actually interact
at all
For any character, dominant/recessive relationships
of alleles depend on the level at which we examine
the phenotype
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Tay-Sachsdisease (
) is fatal; adysfunctional enzyme causes an accumulation of
lipids in the brain
At the organismallevel, the allele is recessive
At the biochemicallevel, the phenotype (i.e., the
enzyme activity level) is incompletely dominant
At the molecularlevel, 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, a dominant
trait called polydactyly
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Mul tiple 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 of
the gene: IA
, IB
, and i. The enzyme (I) adds specific carbohydrates to the
surface of blood cells
The enzyme encoded by I
A
adds the A carbohydrate,and the enzyme encoded by IBadds the B
carbohydrate; the enzyme encoded by the iallele
adds neither
Figure 11.11
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Carbohydrate
(b) Blood group genotypes and phenotypes
Allele
Red blood cellappearance
Genotype
noneBA
IB
Phenotype(blood group)
iIA
IA IB i iIA IA orIA i IBIB orIBi
BA OAB
(a) The three alleles for the ABO blood groups and theircarbohydrates
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Pleiotropy
Most genes have multiple phenotypic effects, aproperty 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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Epistasis (
)
In epistasis, a gene at one locus alters thephenotypic expression of a gene at a second locus
For example, in Labrador retrievers and many other
mammals, coat color depends on two genes.
One gene determines the pigment color (with alleles B
for black and bfor brown).
The other gene (with alleles Cfor color and cfor no
color) determines whether the pigment will bedeposited in the hair.
Figure 11.12
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BE Be
BE
be
BBEE
bbee
BbEE BbEe
bE be
bE
Be
BBEe
BbEE bbEE bbEeBbEe
BBEe BbEe BbeeBBee
BbEe bbEe Bbee
9 : 4: 3
Eggs
Sperm
BbEe BbEe
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Polygenic Inheritance
Quantitative charactersare those that vary in thepopulation 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
Figure 11.13
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Eggs
Sperm
AaBbCc AaBbCc
Phenotypes:
0
Number of
dark-skin alleles: 1 2 3 4 5 6
164
164
664
664
164
1564
1564
2064
18
1 8
18
18
18
18
18
18
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
56
Nature and Nurture: The Environmental Impact
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Nature and Nurture: The Environmental Impacton Phenotype
Another departure from Mendelian genetics ariseswhen the phenotype for a character depends on
environmentas well as genotype
The norm of reaction is the phenotypic range of a
genotype influenced by the environment.
The phenotypic range is generally broadest for
polygenic characters.
Such characters are called multifactorialbecause
genetic and environmental factors collectively
influence phenotype.
11 4: Many human traits follow Mendelian
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11.4: Many human traits follow Mendelianpatterns of inheritance
Humans are notgood subjects for genetic research1. Generation time is too long
2. Parents produce relatively few offspring
3. Breeding experiments are unacceptable
However, basic Mendelian genetics endures as the
foundation of human genetics
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Pedigree Analysis
A pedigreeis a family tree that describes theinterrelationships of parents and children across
generations.
Inheritance patterns of particular traits can be traced
and described using pedigrees.
Pedigreescan also be used to make predictions
about future offspring.
Figure 11.14
K
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WW
or
Ww
ww
wwww
ww ww
WwWw
Ww Ww
No widows peakWidows peak
wwWw
1st generation(grandparents)
3rd generation(two sisters)
2nd generation(parents,
aunts, anduncles)
Affectedmale
Affectedfemale
Male Female
Key
Mating
Attachedearlobe
Freeearlobe
Offspring, inbirth order(first-born on left)
FF
or
Ff
ff
ff ff
Ff ff
FfFF or
Ff
Ff Ff
ffFf
(a) Is a widows peak a dominant or recessive trait? (b) Is an attached earlobe a dominant
or recessive trait?
60
Figure 11.14a
Key
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WW
or
Ww
ww
ww ww
ww ww
WwWw
Ww Ww
Widows peak
wwWw
1st generation(grandparents)
3rd generation(two sisters)
2nd generation(parents, aunts,and uncles)
Affectedmale
Affectedfemale
Male
Female
Mating
Offspring, inbirth order(first-born on left)
(a) Is a widows peak a dominant or recessive trait?
No widows peak
61
Figure 11.14b
Aff t dM l
Key
M ti
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Affectedmale
Affectedfemale
Male
Female
Mating
Offspring, inbirth order(first-born on left)
1st generation(grandparents)
3rd generation(two sisters)
2nd generation(parents, aunts,and uncles)
Attached earlobe Free earlobe
FF
or
Ff
ff
ff ff
Ff ff
FfFF or
Ff
Ff Ff
ffFf
(b) Is an attached earlobe a dominant or recessive trait?
62
Recessively Inherited Disorders
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Recessively Inherited Disorders
Many genetic disorders are inherited in a recessive
manner
These range from relatively mild to life-threatening
The Behavior of Recessive Alleles
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The Behavior of Recessive Alleles
Recessively inherited disorders show up only in
individuals homozygous for the allele
Carriersare heterozygous individuals who carry the
recessive allele but are phenotypically normal
Most people who have recessive disorders are born
to parents who are carriers of the disorder
Figure 11.15
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Parents
Sperm
NormalAa
NormalAa
EggsAA
Normal
AaNormal(carrier)
AaNormal
(carrier)
aaAlbino
A
a
A a
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If a recessive allele that causes a disease is rare,
then the chance of two carriers meeting and mating
is low
Consanguineous (between close relatives) matings
increase the chance of mating between two carriersof the same rare allele
Most societies and cultures have laws or taboos
against marriages between close relatives
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Cystic F ibrosis
Cystic fibrosisis 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 plasmamembranes leading to a buildup of chloride ions
outside the cell
Symptoms include mucus buildup in some internalorgans and abnormal absorption of nutrients in the
small intestine
Sickle-Cell Disease: A Genetic Disorder with
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Evolutionary Implications
Sickle-cell diseaseaffects 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
In homozygous individuals, all hemoglobin is
abnormal (sickle-cell)
Symptoms include physical weakness, pain, organ
damage, and even paralysis
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Heterozygotes (said to have sickle-cell trait) are
usually healthy but may suffer some symptoms
About one out of ten African-Americans has sickle-cell
trait, an unusually high frequency of an allele with
detrimental effects in homozygotes
Heterozygotes are less susceptible to the malaria
parasite, so there is an advantage to being
heterozygous
D i tl I h it d Di d
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Dominantly Inherited Disorders
Some human disordersare caused by dominant
alleles
Dominant alleles that cause a lethal disease are
rare and arise by mutation
Achondroplasiais a form of dwarfismcaused by a
rare dominant allele
Figure 11.16
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Parents
Sperm
DwarfDd
Normaldd
Eggs
DdDwarf
ddNormal
DdDwarf
ddNormal
d
d
D d
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The timing of onset of a disease significantly affects
its inheritance.
Huntingtonsdiseaseis a degenerative disease of
the nervous system.
The disease has no obvious phenotypic effects until
the individual is about 35 to 45 years of age.
Once the deterioration of the nervous system begins
the condition is irreversible and fatal.
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Multifactorial Disorders
Many diseases, such as heart disease, diabetes,
alcoholism, mental illnesses, and cancer, have both
geneticand environmentalcomponents.
Lifestyle has a tremendous effect on phenotype for
cardiovascular health and other multifactorialcharacters.
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Genetic Counseling Based on Mendelian Genetics
Genetic counselors can provide information to
prospective parents concerned about a family history
for a specific disease.
Avoiding simple Mendelian disorders is possible
when the risk of a particular genetic disorder can beassessed before a child is conceived or during the
early stages of the pregnancy.
Many hospitals have genetic counselors who canprovide information to prospective parents concerned
about a family history for a specific disease.
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Ayo NUTN website: http://myweb.nutn.edu.tw/~hycheng/
http://myweb.nutn.edu.tw/~hycheng/http://myweb.nutn.edu.tw/~hycheng/