chapter 9 ppt
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Patterns of InheritanceTRANSCRIPT
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Purebreds and Mutts–A Difference of Heredity
• Purebred dogs
– Variation?
– Selective breeding?
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• Mutts, or mixed breed dogs on the other hand
– Genetic variation? More? …less? Why?
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• Modern Experimental Genetics
– Gregor Mendel’s quantitative experiments with pea plants
Petal
CarpelStamen
Figure 9.2 BFigure 9.2 A
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• Mendel crossed? ..bred? …pea plants that differed in certain characteristics
• WHY?
– And traced traits from generation to generation
WHY?
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• Mendel hypothesized that there are alternative forms of genes
– The units that determine heritable traits
Flower color
Flower position
Seed color
Seed shape
Pod color
Pod shape
Stem length
Purple White
Axial Terminal
Round Wrinkled
Inflated Constricted
Tall Dwarf
GreenYellow
Green Yellow
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Mendel’s Law’s:
1)Dominance
2)Segregation From his experimental data
– Mendel deduced that an organism has two genes (alleles) for each inherited characteristic
P generation(true-breedingparents)
F1 generation
F2 generation
Purple flowers White flowers
All plants havepurple flowers
Fertilizationamong F1 plants(F1 F1)
of plantshave purple flowers
34 of plants
have white flowers
14
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• For each characteristic
– An organism inherits two alleles, one from each parent
– Hmmm…does this remind you of anything we studied? …what? Be Specific!
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• Mendel’s law of segregation
– Predicts that allele pairs separate from each other during the production of gametes
Figure 9.3 B
P plants
Gametes
Genetic makeup (alleles)
Gametes
F1 plants(hybrids)
F2 plants
PP pp
All P All p
All Pp
Sperm
12 P
P
P
p
p
PP Pp
Pp pp
EggsGenotypic ratio1 PP : 2 Pp: 1 pp
Phenotypic ratio3 purple : 1 white
12 p
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Recal….Homologous chromosomes bear the two alleles for each characteristic
• Alternative forms of a gene
– Reside at the same locus on homologous chromosomes
Genotype: PP aa BbHeterozygous
P a b
P a B
Gene loci
Recessiveallele
Dominantallele
Homozygousfor thedominant allele
Homozygousfor therecessive allele
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• Mendel’s law of independent assortment
– States that alleles of a pair segregate independently of other allele pairs during gamete formation
Hypothesis: Dependent assortment Hypothesis: Independent assortment
RRYY rryy
Gametes Gametes
RRYY rryy
RrYy RrYy
RY ry ryRY
Sperm Sperm
RY ry
ry
RY
ry
Ry
ry
RY
RRYY
RrYY
RRYy
RrYy
RrYY
rrYY
RrYy
rrYy
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
RY ry ryRY
Actual resultscontradict hypothesis
Actual resultssupport hypothesis
Yellowround
Greenround
Yellowwrinkled
Greenwrinkled
Eggs
P generation
F1 generation
F2 generation
Eggs
12
12
12
12
14
14
14
14
14
14
14
14
916
316
316
116
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• An example of independent assortment
• Punnett Squares, Probablility & Predicting F1 & F2
Black coat, normal visionB_N_
Black coat, blind (PRA)B_nn
Chocolate coat, normal visionbbN_
Chocolate coat, blind (PRA)bbnn
Blind Blind
9 black coat, normal vision
3 black coat,blind (PRA)
3 chocolate coat, normal vision
1 chocolate coat, blind (PRA)
BbNn BbNn
PhenotypesGenotypes
Mating of heterozygotes(black, normal vision)
Phenotypic ratioof offspring
Figure 9.5 BPRA: Progressive Retinal Atrophy
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Geneticists a testcross to determine unknown genotypes
• The offspring of a testcross, a mating between an individual of unknown genotype and a homozygous recessive individual
Testcross:
Genotypes
Gametes
Offspring
B_ bb
Two possibilities for the black dog:
BB or Bb
B B b
b Bb b Bb bb
All black 1 black : 1 chocolate
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Mendel’s laws reflect the…..
RULES OF PROBABILITY
• Inheritance follows the rules of probability
Could you use a test cross to determine if an organism was true breeding or pure breeding? How?
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• The Mule of Multiplication, OR …the Product Rule
– Calculates the probability of two independent events
• The Rule of Addition
– Calculates the probability of an event that can occur in alternate ways
Figure 9.7
F1 genotypes
Bb female
Formation of eggs
F2 genotypes
Bb male
Formation of sperm
B b
BB B B b
b b B b b
12
12
12
12
14
14
14
14
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• Family pedigrees
– Can be used to determine individual genotypes
DdJoshuaLambert
DdAbigailLinnell
D ?JohnEddy
D ?HepzibahDaggett
D ?Abigail
Lambert
ddJonathanLambert
DdElizabeth
Eddy
Dd Dd dd Dd Dd Dd dd
Female MaleDeafHearing
Figure 9.8 BComet Hale Bopp seen from path to Lambert’s Cove Beach…Martha’s Vineyard
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CONNECTION
9.9 Many inherited disorders in humans are controlled by a single gene
• Some autosomal disorders in humans
Table 9.9
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Parents
Offspring
Sperm
NormalDd
NormalDd
D d
Eggs
D
d
DDNormal
DdNormal(carrier)
DdNormal(carrier)
ddDeaf
Figure 9.9 A
Recessive Disorders• Most human genetic disorders are recessive
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Dominant Disorders• Some human genetic disorders are dominant• http://www.youtube.com/watch?v=zS7vCd8KQIA• http://en.wikipedia.org/wiki/Human_genetics#Autosomal_dominant_inheritance
Figure 9.9 B
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CONNECTION
New technologies can provide insight into one’s genetic legacy
• New technologies
– Can provide insight for reproductive decisions
http://www.gaucherdisease.com/
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Amniocentesis Chorionic villus sampling (CVS)
Ultrasoundmonitor
Fetus
Uterus
Amnioticfluid
Fetalcells
Severalweeks
Biochemicaltests
Severalhours
Fetalcells
Uterus
Cervix
Suction tube insertedthrough cervix to extracttissue from chorionic villi
Needle insertedthrough abdomen toextract amniotic fluid
Centrifugation
Ultrasoundmonitor
Fetus
Placenta
Chorionicvilli
Karyotyping
Placenta
Cervix
Fetal Testing• Amniocentesis and chorionic villus sampling (CVS)
– Allow doctors to remove fetal cells that can be tested for genetic abnormalities
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Fetal Imaging• Ultrasound imaging
– Uses sound waves to produce a picture of the fetus
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NON-MENDELIAN INHERITANCE
Genotype = Phenotype?
1)What does this mean?
2)Mendel’s principles are valid for all sexually reproducing species
3)D’OH…. genotype often does not dictate phenotype in the simple way his laws describe
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Incomplete Dominance?
When an offspring’s phenotype is in between the phenotypes of its parents, it exhibits incomplete dominance.
P generation
F1 generation
F2 generation
RedRR
Gametes
Whiterr
Gametes
Sperm
Eggs
PinkRr
R
R
R
r
rR
r
r
RedRR
PinkrR
PinkRr
Whiterr
12
12
12
12
12
12
Genotypes:
HHHomozygous
for ability to makeLDL receptors
HhHeterozygous
hhHomozygous
for inability to makeLDL receptors
Phenotypes:
LDL
LDLreceptor
Cell
Normal Mild disease Severe disease
Figure 9.12 B
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Multiple Alleles!!
• In a population
– Multiple alleles often exist for a characteristic
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• The ABO blood type in humans
– Involves three alleles of a single gene
• The alleles for A and B blood types are codominant
– And both are expressed in the phenotype
Figure 9.13
BloodGroup(Phenotype) Genotypes
AntibodiesPresent inBlood
Reaction When Blood from Groups Below Is Mixed withAntibodies from Groups at Left
O A B AB
O
A
B
AB
ii
IAIA
orIAi
IBIB
orIBi
IAIB
Anti-AAnti-B
Anti-B
Anti-A
—
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Pleiotropy: A single gene may affect many phenotypic characteristics•Pleiotropy describes the genetic effect of a single gene on multiple phenotypic traits. The underlying mechanism is that the gene codes for a product that is, for example, used by various cells, or has a signaling function on various targets.
PKU (phenylketonuria) Symptoms:mental retardation reduced hair skin pigmentation,
…caused by any of a large number of mutations in a single gene that codes for the enzyme (phenylalanine hydroxylase), which converts the amino acid phenylalanine to tyrosine, another amino acid.
Depending on the mutation involved, this results in reduced or zero conversion of phenylalanine to tyrosine, and phenylalanine concentrations increase to toxic levels, causing damage at several locations in the body. PKU is totally benign if a diet free from phenylalanine is maintained
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A single characteristic may be influenced by many genes
• Polygenic inheritance: Creates a continuum of phenotypes
http://www.athro.com/evo/inherit.html
In humans three genes involved in eye color are known. They explain typical patterns of inheritance of brown, green, and blue eye colors. However, they don't explain everything. Grey eye color, Hazel eye color, and multiple shades of blue, brown, green, and grey are not explained. The molecular basis of these genes is not known. What proteins they produce and how these proteins produce eye color is not known. Eye color at birth is often blue, and later turns to a darker color. Why eye color can change over time is not known. An additional gene for green is also postulated, and there are reports of blue eyed parents producing brown eyed children (which the three known genes can't easily explain [mutations, modifier genes that supress brown, and additional brown genes are all potential explanations]).
The known Human Eye color genes are: EYCL1 (also called gey), the Green/blue eye color gene, located on chromosome 19 (though there is also evidence that another gene with similar activity exists but is not on chromosome 19). EYCL2 (also called bey1), the central brown eye color gene, possibly located on chromosome 15. EYCL3 (also called bey2), the Brown/blue eye color gene located on chromosome 15. EYCL3 probably involves mutations in the regulatory region just before the OCA2 gene (which produces a protein that is expressed in melanocytes). A second gene for green has also been postulated. Other eye colors including grey and hazel are not yet explained. We do not yet know what these genes make, or how they produce eye colors. The two gene model (EYCL1 and EYCL3) used above explains only a portion of human eye color inheritance. Both additional eye color genes and modifier genes are almost certainly involved
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The environmental affects many characteristics
• Many traits are affected, in varying degrees
– By both genetic and environmental factors
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• Genetic testing can detect disease-causing alleles
• Predictive genetic testing – May inform people of their risk for developing genetic
diseases– http://www.wired.com/wiredscience/2009/03/
designerdebate/– http://www.geneticsandsociety.org/article.php?id=4561
Designer Babies?
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THE CHROMOSOMAL BASIS OF INHERITANCE
Chromosome behavior accounts for Mendel’s laws
• The structure and assembly of a eukaryotic chromosome: http://www.youtube.com/watch?v=gbSIBhFwQ4s
• Genes are located on chromosomes
– Whose behavior during meiosis and fertilization accounts for inheritance patterns
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• The chromosomal basis of Mendel’s laws
All round yellow seeds(RrYy)
Metaphase Iof meiosis
(alternative arrangements)
Anaphase Iof meiosis
Metaphase IIof meiosis
Gametes
F1 generation
F2 generation
Fertilization among the F1 plants
(See Figure 9.5A)
1
4 RY
1
4ry
R
R
R
R RR
R
y
Y
Y
Y
Y Yy Y Y
r
r
y
R
Y
r
y
R r r r r
rr
y
r
Y
R
y
r
Y
R
y
1
4rY
1
4 Ry
9 : 3 : 3 : 1
y y y
yY
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Genes on the same chromosome tend to be inherited together
• Certain genes are linked
– They tend to be inheritedtogether because they reside close together onthe same chromosome
Experiment
Explanation: linked genes
PpLI PpLILong pollen
Observed PredictionPhenotypes offspring (9:3:3:1)
Purple longPurple roundRed longRed round
Parentaldiploid cellPpLI
Most gametes
Mostoffspring Eggs
3 purple long : 1 red roundNot accounted for: purple round and red long
Meiosis
Fertilization
Sperm
284212155
215717124
P I
P L
P L
P L
P L
P L
P I
P L P I
P I
P L
P I
P I
P I
P I
P L
Purple flower
Figure 9.19
Trait A Trait BDominant or Recessive
References
Blond hair Blue eyes both recessive [5]
Flexibility Anxiety disorderA is recessive B is dominant
[6]
Large ears broad nose both dominant [7]
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Crossing over produces new combinations of alleles??? HOW?
• Crossing over can separate linked alleles
– Producing gametes with recombinant chromosomes
A B
a b
Tetrad Crossing over
A B
A b
a b
a B
Gametes
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• Thomas Hunt Morgan
– Performed some of the early studies of crossing over using the fruit fly Drosophila melanogaster
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Thomas Hunt Morgan• Morgan began working seriously with Drosophila in 1907.
• But despite much effort and the breeding of successive generations, Morgan initially failed to detect a single mutation. "Two years work wasted," he lamented to one visitor to his laboratory. "I have been breeding those flies for all that time and I've got nothing out of it."(Harrison, R.G., "Embryology and Its Relations")
• April 1910 he suddenly had a breakthrough…one male fly with white : How did this white eye color originate? What determines eye color?
• Morgan bred this white-eyed (mutant) male to a red-eyed (wild-type) virgin sister and found that white-colored eyes are inherited in a special way. In the first generation of brother-sister mating, labeled F1, there were only red-eyed offspring, suggesting that red eye color is dominant and that white eye color is recessive. To prove this idea Morgan carried out brother-sister matings with the next generation (F2) and found that the offspring followed the expected Mendelian ratio for a recessive trait: three red-eyed flies to every one white-eyed fly. With these experiments Morgan started a tradition, which continues to this day, whereby he named the gene "white" by the result of its mutation. But then came a surprise. He had expected there would be an equal number of males and females with white eyes, but it turned out that all the female flies had red eyes; only males had white eyes, and, even more, only some of them displayed the trait. Morgan realized that white eye color is not only recessive but is also linked in some way to sex.
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Morgan…continued• By 1910, it was already known that chromosomes occur in pairs and that Drosophila had four pairs of chromosomes. Several decades earlier, these thread-shaped structures had been seen under a microscope to be located in the nucleus, but nobody knew their function. Morgan later was to describe them in the following terms:
•"The egg of every species of animal or plant carries a definite number of bodies called chromosomes. The sperm carries the same number. Consequently, when the sperm unites with the egg, the fertilized egg will contain the double number of chromosomes. For each chromosome contributed by the sperm there is a corresponding chromosome contributed by the egg, i.e., there are two chromosomes of each kind, which together constitute a pair." (Morgan, T.H. et al., The Mechanism of Mendelian Heredity) When Morgan turned to examining the fruit fly's chromosomes under the microscope, he immediately appreciated that not all four pairs of chromosomes were always identical. In particular, whereas female flies had two identical-looking X chromosomes, in the male the X chromosome was paired with a Y chromosome, which looks different and is never present in the female.
• Morgan deduced that a male must inherit the X chromosome from his mother and Y from his father, and he immediately spotted a correlation between these sex-linked chromosomes and the segregation of the factors determining eye color. When the mother was homozygous and had two copies of the gene for red eyes, the male offspring invariably had red eyes, even if the father had white eyes. But when the mother had white eyes, the male offspring did too, even if the father's eyes were red. In contrast, a female fly gets one X chromosome from each parent, and if one passed along an X chromosome with a gene for red eyes, the offspring had red eyes because the color is dominant over white. Only when both parents gave her an X chromosome with a gene for white eyes did she display the recessive trait. From these observations, Morgan concluded that the allele-producing eye color must lie on the X chromosome that governs sex. This provided the first correlation between a specific trait and a specific chromosome.
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• Morgan’s experiments
– Demonstrated the roleof crossing over in inheritance
Figure 9.20 C
Experiment
Gray body,long wings(wild type)
GgLI
Female
Black body,vestigial wings
ggll
Male
Offspring
Gray long
965 944 206 185
Black vestigial Gray vestigial Black long
Parentalphenotypes
Recombinantphenotypes
Recombination frequency = = 0.17 or 17%391 recombinants
2,300 total offspring
Explanation
GgLI(female)
ggll(male)
G L
g l
g l
g l
G L g l G l g L g l
Eggs Sperm
G L g l
g l g l g l g l
LglG
Offspring
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Geneticists use crossover data to map genes
• Morgan and his students
– Used crossover data to map genes in Drosophila
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• Recombination frequencies
– Can be used to map the relative positions of genes on chromosomes.
Mutant phenotypes
Shortaristae
Blackbody(g)
Cinnabareyes(c)
Vestigialwings(l)
Browneyes
Long aristae(appendageson head)
Gray body(G)
Redeyes(C)
Normalwings(L)
Redeyes
Wild-type phenotypes
Chromosomeg c l
9% 9.5%
17%
Recombinationfrequencies
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• The Y chromosome– Has genes for the development of testes
• The absence of a Y chromosome– Allows ovaries to develop
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• Other systems of sex determination exist in other animals and plants
22+
XX
22+X
76+
ZW
76+
ZZ
32 16
Figure 9.22 D
Figure 9.22 C
Figure 9.22 B
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Sex-linked genes exhibit a unique pattern of inheritance
• All genes on the sex chromosomes
– Are said to be sex-linked
• In many organisms
– The X chromosome carries many genes unrelated to sex
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• In Drosophila
– White eye color is a sex-linked trait
Figure 9.23 A
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• The inheritance pattern of sex-linked genes
– Is reflected in females and males
Female Male
Sperm
Xr Y
XR
Xr Y XR XR
XR Xr XR YEggs
R = red-eye alleler = white-eye allele
Female Male
Sperm
XR Y
XR
XR Y XR Xr
XR XR XR Y
Eggs
Xr Xr XR Xr Y
Female
Sperm
Xr Y
XR
Xr Y XR Xr
XR Xr XR Y
Eggs
Male
Xr Xr Xr Xr Y
Figure 9.23 B Figure 9.23 C Figure 9.23 D
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Genetic Determination of Sex Creates Dosage Problems
In mammals females have 2 X chromosomes, the males only 1
If nothing were done to compensate the females would get a double dose of any gene products from the X chromosome, compared to the dose that males get
Nature solves this problem by shutting down one whole X chromosome in mammalian females
• X chromosome inactivation is called Lyonization after Mary Lyon who discovered it
• Inactive X chromosome appears in condensed state as a Barr body (p. 272, text)
• Inactivation of X chromosomes in different cells is somewhat random
The calico cat is a product of X chromosome inactivation
• Genes for coat color of the cat are on the X chromosome
• One gene produces a black color; its allele produces orange
• To get a calico coat a cat must be heterozygous, with genes for both the orange and the black color
• If the X chromosome with the black gene is inactivated that cell will produce orange
• If the X chromosome with the orange gene is inactivated the cell will produce black
• Inactivation occurs in patches, giving the orange and black coat of the calico
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CONNECTION
Sex-linked disorders affect mostly males
• Most sex-linked human disorders
– Are due to recessive alleles
– Are mostly seen in males
Queenvictoria
Albert
Alice Louis
Alexandra CzarNicholas IIof Russia
AlexisFigure 9.24 A Figure 9.24 B
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There are about 1,098 human X-linked genes.
Most of them code for something other than female anatomical traits. Many of the non-sex determining X-linked genes are responsible for abnormal conditions such as…
Hemophilia Duchenne Muscular Dystrophy Fragile-X SyndromeSome High Blood PressureCongenital Night BlindnessG6PD DeficiencyRed-Green Color Blindness. Male Pattern Baldness
Mechanism of PRO051 in the restoration of Dystrophin Expression through Exon Skipping.
Normal muscle produces dystrophin, a critical protein, in response to signals encoded in a precise lockstep manner into mRNA. The mRNA is then translated into dystrophin protein. In the muscle of patients with Duchenne muscular dystrophy, mutations in the dystrophin gene lead to the loss of one or more exons. The mRNA splices together the remaining exons; however, the missing pieces lead to errors in translation (frame shift) and loss of production of the dystrophin protein. Intramuscular injection of a small modified DNA molecule can enter Duchenne-affected muscle through abnormal muscle membranes; then enters the nucleus and binds to the dystrophin mRNA. The modified DNA molecule allows the mRNA to skip over the affected exons, and restores the reading frame of the mRNA, for new production of dystrophin. The dystrophin that is produced is not normal but probably retains considerable function.
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Mechanism of PRO051 in the Restoration of
Dystrophin Expression through Exon Skipping.
Normal muscle produces dystrophin, a critical protein, in response to signals encoded in a precise lockstep manner into mRNA. The mRNA is then translated into dystrophin protein. In the muscle of patients with Duchenne muscular dystrophy, mutations in the dystrophin gene lead to the loss of one or more exons. The mRNA splices together the remaining exons; however, the missing pieces lead to errors in translation (frame shift) and loss of production of the dystrophin protein. Intramuscular injection of a small modified DNA molecule can enter Duchenne-affected muscle through abnormal muscle membranes; then enters the nucleus and binds to the dystrophin mRNA. The modified DNA molecule allows the mRNA to skip over the affected exons, and restores the reading frame of the mRNA, for new production of dystrophin. The dystrophin that is produced is not normal but probably retains considerable function.
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• A male receiving a single X-linked allele from his mother
– Will have the disorder• A female
– Has to receive the allele from both parents to be affected