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Chapter 14: Mendel and the Gene Idea
What genetic principles account for the
transmission of traits from parents to offspring?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Early Theories of heredity
• One possible explanation of heredity is a
“blending” hypothesis
– You get a mixing of traits
– The idea that genetic material contributed by
two parents mixes in a manner analogous to
the way blue and yellow paints blend to make
green
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Theories of Inheritance
• An alternative to the blending model is the
“particulate” hypothesis of inheritance: the
gene idea
– Parents pass on discrete heritable units, genes
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Mendel's Ideas
• Gregor Mendel
– Documented a particulate mechanism of
inheritance through his experiments with
garden peas
Figure 14.1
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Mendel
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
• Why did Mendel chose to work with peas?
– Because they are available in many varieties
– Because he could strictly control which plants
mated with which
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Genetic Vocabulary
• Character: a heritable feature, such as flower
color
• Trait: a variant of a character, such as purple or
white flowers
– Often times, traits are used synonymous with
character
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Mendel’s Experiments
• Mendel chose to track
– Only those characters that varied in an “eitheror” manner
• Mendel also made sure that
– He started his experiments with varieties that
were “true-breeding”. Which means selffertilizing. Parents offspring look identical to
the parent.
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The Experiment
• In a typical breeding experiment
– Mendel mated two contrasting, true-breeding
varieties, a process called hybridization
• The true-breeding parents
– Are called the P generation
Mendel’s Parents were purple and white flowers
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Generations
• 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 Cross
• Crossing pea plants
1
APPLICATION By crossing (mating) two true-breeding
varieties of an organism, scientists can study patterns of
inheritance. In this example, Mendel crossed pea plants
that varied in flower color.
TECHNIQUE
Removed stamens
from purple flower
2 Transferred sperm-
bearing pollen from
stamens of white
flower to eggbearing carpel of
purple flower
Parental
generation
(P)
3 Pollinated carpel
Stamens
Carpel (male)
(female)
matured into pod
4 Planted seeds
from pod
When pollen from a white flower fertilizes
TECHNIQUE
RESULTS
eggs of a purple flower, the first-generation hybrids all have purple
flowers. The result is the same for the reciprocal cross, the transfer
of pollen from purple flowers to white flowers.
Figure 14.2
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5 Examined
First
generation
offspring
(F1)
offspring:
all purple
flowers
Results of Cross
• When Mendel crossed contrasting, truebreeding white and purple flowered pea plants
– All of the offspring were purple
• When Mendel crossed the F1 plants
– Many of the plants had purple flowers, but
some had white flowers
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Results of Cross
• Mendel discovered
– A ratio of about three to one, purple to white flowers,
in the F2 generation
EXPERIMENT True-breeding purple-flowered pea plants and
white-flowered pea plants were crossed (symbolized by ). The
resulting F1 hybrids were allowed to self-pollinate or were crosspollinated with other F1 hybrids. Flower color was then observed
in the F2 generation.
P Generation
(true-breeding
parents)

Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had
purple flowers
RESULTS Both purple-flowered plants and whiteflowered plants appeared in the F2 generation. In Mendel’s
experiment, 705 plants had purple flowers, and 224 had white
flowers, a ratio of about 3 purple : 1 white.
Figure 14.3
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F2 Generation
Conclusions
• Mendel reasoned that
– In the F1 plants, only the purple flower factor
was affecting flower color in these hybrids
– Purple flower color was dominant, and white
flower color was recessive
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Other Observations
• Mendel observed the same pattern
– In many other pea plant characters
Table 14.1
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Mendel’s Model: Law of Segregation
• Mendel developed a hypothesis
– To explain the 3:1 inheritance pattern that he
observed among the F2 offspring
• Four related concepts make up this model
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First:
• Alternate forms of genes, now called alleles,
account for variations in characters
Allele for purple flowers
Locus for flower-color gene
Figure 14.4
Allele for white flowers
Alleles are located on homologous
chromosome pairs at the same locus
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Homologous
pair of
chromosomes
Second:
• An organism has two alleles for each inherited
trait, one from each parent.
– A genetic locus is actually represented twice
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Third:
• When two different alleles occur together, one
of them, called the dominant allele,
determines the organisms appearance while
the other, the recessive allele, has no
observable effect on the organisms
appearance.
– Dominant alleles “show”
– Recessive alleles are “masked”
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Fourth:
• Allele pairs separate (segregate) during the
formation of gametes.
• This is Mendel’s Law of Segregation
• An egg or sperm carries only one allele for
each inherited character
– This is consistent with chromosome behavior
during meiosis
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Punnett Squares
• Does Mendel’s segregation model account for
the 3:1 ratio he observed in the F2 generation
of his numerous crosses?
– We can answer this question using a Punnett
square
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An Example
• Mendel’s law of segregation, probability and
the Punnett square
Each true-breeding plant of the
parental generation has identical
alleles, PP or pp.
Gametes (circles) each contain only
one allele for the flower-color gene.
In this case, every gamete produced
by one parent has the same allele.
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
PP
pp
Gametes:
p
P
Union of the parental gametes
produces F1 hybrids having a Pp
combination. Because the purpleflower allele is dominant, all
these hybrids have purple flowers.
F1 Generation
When the hybrid plants produce
gametes, the two alleles segregate,
half the gametes receiving the P
allele and the other half the p allele.
Gametes:
This box, a Punnett square, shows
all possible combinations of alleles
in offspring that result from an
F1  F1 (Pp  Pp) cross. Each square
represents an equally probable product
of fertilization. For example, the bottom
left box shows the genetic combination
resulting from a p egg fertilized by
a P sperm.

Appearance:
Genetic makeup:
Purple flowers
Pp
1/
1/
2 P
F1 sperm
P
p
PP
Pp
F2 Generation
P
F1 eggs
p
pp
Pp
Figure 14.5
Random combination of the gametes
results in the 3:1 ratio that Mendel
observed in the F2 generation.
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2 p
3
:1
Useful Genetic Vocabulary
• An organism that is homozygous for a
particular gene
– Has a pair of identical alleles for that gene
– Exhibits true-breeding
• An organism that is heterozygous for a
particular gene
– Has a pair of alleles that are different for that
gene
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Genetic Vocabulary
• An organism’s phenotype
– Is its expressed traits
• physical appearance
• An organism’s genotype
– Is its genetic makeup
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Examples of Phenotype and genotype
• Phenotype versus genotype
Phenotype
Purple
3
Purple
Genotype
PP
(homozygous)
1
Pp
(heterozygous)
2
Pp
(heterozygous)
Purple
1
Figure 14.6
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
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1
The Testcross
• In pea plants with purple flowers
– The genotype is not immediately obvious
• A testcross
– Allows us to determine the genotype of an
organism with the dominant phenotype, but
unknown genotype
– Crosses an individual with the dominant
phenotype with an individual that is
homozygous recessive for a trait
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Testcross
APPLICATION An organism that exhibits a dominant trait,
such as purple flowers in pea plants, can be either homozygous for
the dominant allele or heterozygous. To determine the organism’s
genotype, geneticists can perform a testcross.

TECHNIQUE In a testcross, the individual with the
unknown genotype is crossed with a homozygous individual
expressing the recessive trait (white flowers in this example).
By observing the phenotypes of the offspring resulting from this
cross, we can deduce the genotype of the purple-flowered
parent.
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype:
pp
If PP,
then all offspring
purple:
If Pp,
then 2 offspring purple
and 1⁄2 offspring white:
p
1⁄
p
p
p
Pp
Pp
pp
pp
RESULTS
P
P
Pp
Pp
P
p
Pp
Figure 14.7
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Pp
Testcross example
Deafness is caused by a recessive gene.
A breeder has a dog that isn’t deaf. She would
like to breed this dog and wants to make sure it
is “pure”. Develop a test cross and explain the
outcomes (use a punnett square)….
a) if the dog is “pure”
b) if the dog isn’t “pure”
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The Law of Independent Assortment
• Mendel derived the law of segregation by
following a single trait
• The F1 offspring produced in this cross were
monohybrids, heterozygous for one character
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The Law of Independent Assortment
• 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
• How are two characters transmitted from parents to
offspring?
– As a package?
– Independently?
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A dihybrid cross
• A dihybrid cross
– Illustrates the inheritance of two characters
• Produces four phenotypes in the F2 generation
EXPERIMENT Two true-breeding pea plants—
one with yellow-round seeds and the other with
green-wrinkled seeds—were crossed, producing
dihybrid F1 plants. Self-pollination of the F1 dihybrids,
which are heterozygous for both characters,
produced the F2 generation. The two hypotheses
predict different phenotypic ratios. Note that yellow
color (Y) and round shape (R) are dominant.
P Generation
YYRR
yyrr
Gametes
F1 Generation
YR

Hypothesis of
dependent
assortment
yr
YyRr
Hypothesis of
independent
assortment
Sperm
1⁄ YR
2
RESULTS
CONCLUSION The results support the hypothesis of
independent assortment. The alleles for seed color and seed
shape sort into gametes independently of each other.
Sperm
yr
1⁄
2
Eggs
1
F2 Generation ⁄2 YR YYRR YyRr
(predicted
offspring)
1 ⁄ yr
2
YyRr yyrr
3⁄
4
1⁄
4
1⁄
4
Yr
1⁄
4
yR
1⁄
4
yr
Eggs
1 ⁄ YR
4
1⁄
4
Yr
1⁄
4
yR
1⁄
4
yr
1⁄
4
Phenotypic ratio 3:1
YR
9⁄
16
YYRR YYRr YyRR YyRr
YYrr
YYrr YyRr
Yyrr
YyRR YyRr yyRR yyRr
YyRr
3⁄
16
Yyrr
yyRr
3⁄
16
yyrr
1⁄
16
Phenotypic ratio 9:3:3:1
Figure 14.8
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315
108
101
32
Phenotypic ratio approximately 9:3:3:1
Law of Independent Assortment
• Using the information from a dihybrid cross,
Mendel developed the law of independent
assortment
– Each pair of alleles segregates independently
during gamete formation
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Mendel’s Laws…A Review
• Law of Segregation
– The two alleles for a character, separate
during gamete formation and end up in
different gametes.
• Law of Independent Assortment
– Pairs of alleles will also segregate
independently during gamete formation
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Laws of Probability
• Concept 14.2: The laws of probability govern
Mendelian inheritance
• Mendel’s laws of segregation and independent
assortment
– Reflect the rules of probability
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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
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• Probability in a monohybrid cross
– Can be determined using this rule
Rr
Rr

Segregation of
alleles into eggs
Segregation of
alleles into sperm
Sperm
1⁄
R
2
1⁄
Eggs
r
1⁄
2
r
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1⁄
4
R
1⁄
Figure 14.9
r
R
R
2
r
2
R
R
1⁄
1⁄
4
r
4
r
1⁄
4
Complex inheritance patterns
• More complex inheritance patterns are
dependent upon…
alleles are not completely dominant or
recessive
when a gene has more than 2 alleles
when a gene produces multiple phenotypes
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The Spectrum of Dominance
• Complete dominance
– Occurs when the phenotypes of the
heterozygote and dominant homozygote are
identical
• Ex. The F1 generation in Mendel’s cross
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Codominance
• In codominance
–
both alleles are expressed in the
heterozygote’s phenotype.
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Codominance
• Example – Blood type AB
–
people who have the alleles A and B express
both in their blood group.
More on blood groups later
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Another example
• roan coat in some animals.
•
A cross between homozygous red shorthorn cattle and
homozygous white shorthorn cattle results in heterozygous
offspring with a roan coat. The roan coat consists of a mixture of
all red hairs and all white hairs. Because each hair is either all red
or all white, the condition is codominance.
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Incomplete Dominance
• In incomplete dominance
– The phenotype of F1 hybrids is somewhere between
the phenotypes of the two parental varieties
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Incomplete Dominance
• F2 generation has a 1:2:1 ratio
P Generation
Red
CRCR
Gametes CR
“Blended”
CW
Pink
CRCW
F1 Generation
Gametes
When F1’s are crossed
the red color comes
back.
White
CW CW

Eggs
F2 Generation
Figure 14.10
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1⁄
2
CR
1⁄
2
Cw
1⁄
2
1⁄
2
CR
1⁄
2
CR
CR 1⁄2 CR
CR CR CR CW
CR CW CW CW
Sperm
Incomplete Dominance
• Incomplete dominance in humans
A good example of incomplete
dominance is sickle cell disease.
Homozygous recessive individuals
have sickle cell disease; heterozygous
individuals exhibit a mild version of the
disease; homozygous dominant
individuals are normal
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Frequency of Alleles
• Frequency of Dominant Alleles
• Dominant alleles
– A dominant trait does not make it more
common
– Whether or not a trait is common has to do
with how many copies of that gene version (or
allele) are in the population. It has little or
nothing to do with whether the trait is dominant
or recessive.
http://science.kqed.org/quest/2011/06/06/dominant-isn%E2%80%99t-always-common/
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Multiple Alleles
• Most genes exist in populations
– In more than two allelic forms
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Multiple Alleles
• The ABO blood group in humans
– Is determined by multiple alleles
Alleles code for an
enzyme that attaches a
carbohydrate to the
surface of red blood cells
The carbohydrates are
specific to the group
O blood types have no
carbohydrates
Table 14.2
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• Getting the wrong blood type can cause
clumping of the blood.
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Pleiotropy
• In pleiotropy
– A gene has multiple phenotypic effects
– The gene influences the phenotype of more
than one part of the body
The underlying mechanism is that the gene
codes for a product that is for example used
by various cells
http://www.nature.com/scitable/topicpage/pleiotropy-one-gene-can-affect-multiple-traits-569
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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
• Refer to website for examples
http://www.nature.com/scitable/topicpage/epistasis-gene-interaction-and-phenotype-effects-460
http://www.ndsu.edu/pubweb/~mcclean/plsc431/mendel/mendel6.htm
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Polygenic Inheritance
• Many human characters
– Vary in the population along a continuum and
are called quantitative characters
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Polygenic Inheritance
• Quantitative variation usually indicates
polygenic inheritance
– An additive effect of two or more genes on a
single phenotype

AaBbCc
Examples are height and
skin color
aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCcAABBCC
20⁄
Each dominant allele
contributes one “unit” to the
phenotype
The more dominant alleles,
the darker the skin, or taller
the person
Figure 14.12
AaBbCc
15⁄
6⁄
64
64
64
1⁄
64
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Nature and Nurture: The Environmental Impact
on Phenotype
• Another departure from simple Mendelian
genetics arises
– When the phenotype for a character depends
on environment as well as on genotype
– Polygenic characters are often multifactorial
• A combination of genetic and environmental
factors influences phenotype.
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Nature and Nurture
• The norm of reaction
– Is the phenotypic range of a particular
genotype that is influenced by the environment
– Blood group doesn’t change
– Blood count (white to red blood cells will)
– Based on physical activity, presence of
infectious agents.
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Integrating a Mendelian View of Heredity and Variation
• An organism’s phenotype
– Includes its physical appearance, internal
anatomy, physiology, and behavior
– Reflects its overall genotype and unique
environmental history
• Even in more complex inheritance patterns
– Mendel’s fundamental laws of segregation and
independent assortment still apply
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Human Traits
• Many human traits follow Mendelian patterns
of inheritance
• Humans are not convenient subjects for
genetic research
– However, the study of human genetics
continues to advance
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Pedigree Analysis
• A pedigree
– Is a family tree that describes the
interrelationships of parents and children
across generations
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Pedigree Charts
• A chart which shows the inheritance of
genetic disorders. Based on phenotypes.
Squares represent Males
Circles represent Females
Horizontal lines represent a
marriage
Vertical lines represent parents
and their children
A shaded square or circle
indicates that a person expresses
the trait.
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Pedigree
• Inheritance patterns of particular traits
– Can be traced and described using pedigrees
– Can be used to make predictions about future
offspring
Ww
ww
Ww ww ww Ww
WW
or
Ww
Widow’s peak
ww
Ww
Ww
ww
First generation
(grandparents)
Second generation
(parents plus aunts
and uncles)
Ff
FF or Ff
Ff
Ff
Third
generation
(two sisters)
ww
No Widow’s peak
(a) Dominant trait (widow’s peak)
Figure 14.14 A, B
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Attached earlobe
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
Free earlobe
(b) Recessive trait (attached earlobe)
Inheritance Patterns
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Autosomal Disorders
• Affect chromosome #’s 1-22
• Caused by abnormal or nonfunctioning alleles
• Are inherited 3 ways
– By a dominant allele
– By recessive alleles
– By codominant alleles
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Autosomal recessive disorders
• Remember, alternative versions of genes are
called alleles, and a person needs 2 alleles to
express a trait
• In order for a person to express an autosomal
recessive disorder, they must have 2 recessive
alleles.
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Autosomal recessive
Phenylketonuria – PKU
• A mutation that does not allow the person to
produce the enzyme required to break down
phenylalanine. If untreated, it causes brain
damage.
• All newborn babies are tested for this disease.
• Can be treated with diet restrictions.
– Check out your gum packets
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Autosomal recessive
• Tay-Sachs disease
–
Tay-Sachs disease affects the Jews mostly of
eastern or central European descent. Death
occurs one in 360,000 newborns worldwide.
The symptoms for Tay Sachs disease will not appear until the child is at 3 to 6
months. In acute phases, this includes:
Seizures
Decreased eye contact
Listlessness
Increased irritability
Delayed mental and social skills
Slow body growth but the head size is increased.
Changing behaviors such as the infant doesn’t smile, crawl, or doesn’t roll over
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Autosomal recessive
Cystic Fibrosis
• Cystic fibrosis (CF) is caused by a defective gene which
causes the body to produce abnormally thick and sticky
fluid, called mucus. This mucus builds up in the breathing
passages of the lungs and in the pancreas, the organ that
helps to break down and absorb food.
• This collection of sticky mucus results in life-threatening
lung infections and serious digestion problems. The
disease may also affect the sweat glands and a man's
reproductive system.
In 2008, the median predicted age of
survival rose to 37.4 years, up from 32 in
2000 (about 30,000people in US are living
with this)
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Cystic Fibrosis
1in
2,500-3,500
(Caucasians)
• The severity of cystic fibrosis symptoms is different from
person to person. The most common symptoms are:
• Very salty-tasting skin
• Persistent coughing at times producing phlegm
• Frequent lung infections, like pneumonia or bronchitis
• Wheezing or shortness of breath
• Poor growth/weight gain in spite of a good appetite
• Frequent greasy, bulky stools or difficulty in bowel
movements
• Small, fleshy growths in the nose called nasal polyps
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Autosomal recessive
Albinism
Albinism occurs when one of several
genetic defects makes the body unable to
produce or distribute melanin, a natural
substance that gives color to your hair,
skin, and iris of the eye.
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Autosomal recessive- Albinism
• A person with albinism will have one of the following
symptoms:
• Absence of color in the hair, skin, or iris of the eye
• Lighter than normal skin and hair
• Patchy, missing skin color
• Many forms of albinism are associated with the following
symptoms:
• Crossed eyes (strabismus)
• Light sensitivity (photophobia)
• Rapid eye movements (nystagmus)
• Vision problems, or functional blindness
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• Genotypes of autosomal recessive
disorders
• Example: cystic fibrosis (c)
• CC = normal
• Cc = carrier
• cc = has the disorder
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How would two people who are normal have a
child with CF?
• Show the cross
– Cc x Cc
– (both parents would have to be carriers of the
disorder and display no symptoms)
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Autosomal Dominant Disorders
• If you have one dominant allele for a genetic
disorder, it will be expressed
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Examples of autosomal dominant disorders
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Huntington’s Disease
A result of a single DOMINANT allele.
Disease symptoms appear when a person is in
their 30’s or 40’s (after they have already
passed on the allele to their offspring).
The disease is fatal.
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Huntington’s Disease (p. 346)
•
The defect causes a part of DNA, called a CAG repeat, to occur
many more times than it is supposed to. Normally, this section
of DNA is repeated 10 to 35 times. But in persons with
Huntington's disease, it is repeated 36 to 120 times.
Behavior changes may occur before movement problems, and can include:
Antisocial behaviors
Hallucinations
Irritability
Abnormal movement may include
Quick, sudden, sometimes wild jerking movements of
the arms, legs, face, and other body parts
Dementia will then set in.
Persons with this disease usually die within 15
to 20 years. The cause of death is often
infection, although suicide is also common
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Achondroplasia
• One form of dwarfism.
• Caused by a dominant allele.
affects 1 in 15,000 to
40,000 births
Newborns who inherit both genes are considered to have a severe
form of achondroplasia, where survival is usually less than 12
months after birth.
– The genotype would be Aa
– Inheritance of 2 dominant alleles does not
produce a viable offspring
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Achondroplasia
•
The term "achondroplasia" is a Greek word that means "without
cartilage formation." Characteristics of a person with achondroplasia
dwarfism include:
•
A short stature with proportionately short arms and legs
•
A large head
•
A prominent forehead,
•
A flattened bridge of the nose.
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ACHONDROPLASIA
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Inheriting an autosomal dominant disorder
• Genotype
• Example Huntington’s disease
– A person will be Hh
– What are the chances a child will have
Huntington’s disease if one parent has the
disease and the other doesn’t?
• The cross
• Hh x hh
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Multifactorial Disorders
• Many human diseases
– Have both genetic and environment
components
• Examples include
– Heart disease and cancer
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Genetic Testing and Counseling
• Genetic counselors
– Can provide information to prospective parents
concerned about a family history for a specific
disease
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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
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Tests for Identifying Carriers
• For a growing number of diseases
– Tests are available that identify carriers and
help define the odds more accurately
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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
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• Fetal testing
(b) Chorionic villus sampling (CVS)
(a) Amniocentesis
Amniotic
fluid
withdrawn
A sample of chorionic villus
tissue can be taken as early
as the 8th to 10th week of
pregnancy.
A sample of
amniotic fluid can
be taken starting at
the 14th to 16th
week of pregnancy.
Fetus
Fetus
Suction tube
Inserted through
cervix
Centrifugation
Placenta
Placenta
Uterus
Chorionic viIIi
Cervix
Fluid
Fetal
cells
Fetal
cells
Biochemical tests can be
Performed immediately on
the amniotic fluid or later
on the cultured cells.
Fetal cells must be cultured
for several weeks to obtain
sufficient numbers for
karyotyping.
Biochemical
tests
Several
weeks
Several
hours
Karyotyping
Figure 14.17 A, B
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Karyotyping and biochemical
tests can be performed on
the fetal cells immediately,
providing results within a day
or so.
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
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