Transcript Chapter 14 Power Point File

Mendel & Genes
Chapter 14
Mendel’s contributions
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Used scientific methodology when
conducting experiments.
Used a quantitative approach
Discovered fundamental principals of
heredity
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The Law of Segregation
The Law of Independent Assortment
Mendel’s hypothesis
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2.
3.
4.
Alternative versions of genes (alleles)
account for variations in inherited
characters.
For each character, an organism inherits two
alleles, one from each parent.
If alleles differ, the dominant one will be
expressed and the recessive allele will not
affect the organisms appearance.
The 2 alleles for each character separate
(segregate) during gamete production.
The law of Segregation
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The 2 alleles for a particular character are
packaged into separate gametes.
Thus, an ovum and a sperm each get only
one of the two alleles that are present in the
somatic cells of the organism.
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Allele = a gene that has more than one variation or
different versions of a gene.
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i.e. purple flowers or white flowers.
Yy
Y
y
The law of Independent Assortment
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Pairs of alleles separate independently during
gamete formation. Inheritance of one trait does
not influence the inheritance of another trait.
Each character is INDEPENDENTLY
INHERITED alleles separate as if it were a
monohybrid cross.
Parents
Gametes SB
Example: Color
and Height for Peas
SsBb
Sb
sB
sb
Terminology
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Character: a heritable feature
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(a gene codes for a particular character)
Trait: variations of a character
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(a trait is coded by an allele-defined later)
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Hair color, spot patterns on cats…
P generation: parental generation
F1 generation: first filial generation
(offspring of the P generation)
F2 generation: second filial generation
(self pollinating or F1 cross offspring)
Terminology continued
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Homozygous: 2 identical alleles
Heterozygous: 2 different alleles
Phenotype: organisms visible trait
Genotype: organisms genetic make up
Testcross: this is important and you should
know this!
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Used to reveal the genotype of an organism that
shows a dominant trait.
Completed by crossing the organism with the
unknown genotype with an individual expressing
the recessive trait (a homozygous recessive
individual)
Extending Mendel
Genotype & Phenotype
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Simplest Arrangement: each gene has
only two alleles, one of which is
completely dominant to the other.
Incomplete dominance: not
blended;3phenotypes: both parent &
heterozygote
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Example: snapdragon flowers - Red &
CW
White =Pink
CR
CR CR CR CR CW
CR
CW CR CW CWCW
Phenotype & Genotype
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Codominance: Both Alleles reflected
2 alleles, distinguishable affects
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M and N blood groups
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MM, NN, MN
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Molecules on RBC’s
MN is not INTERMEDIATE
Tay-Sachs disease: lack enzyme to metabolize
lipids in brain cells
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2 Tay-Sachs alleles= disease
Heterozygotes & homozygotes with 2 working alleles=
normal but heterozygotes have less enzyme
Tay-Sachs alleles and normal alleles produce enzyme
Dominance
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Frequency and dominance not related
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Polydactyly dominant
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Achondroplasia, a form of dwarfism, has an
incidence of 1 in 10,000 people.
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Only in 1:400 people
Heterozygous individuals have the dwarf phenotype.
99.99% of the population are homozygous recessive for
this trait & not achondroplastic dwarfs
Range from complete dominance to
codominant
One allele does not subdue the other; they
reflect the pathways of expression
Multiple alleles
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The ABO blood groups in humans are
determined by three alleles, IA, IB, and i.
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Both the IA and IB alleles are dominant to
the i allele
The IA and IB alleles are codominant to each
other.
Because each individual carries two
alleles, there are six possible genotypes
and four possible blood types.
 IA =
oligosaccharside on surface of RBC’s
ABO Blood groups
A quick problem…
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A man with A blood marries a woman
with group B blood. Their child has
group O blood.
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What are the genotypes of these
individuals?
What genotypes and frequencies would you
expect in offspring from this marriage?
Pleiotropic genes
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Gene affects more than one phenotypic
character
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Sickle-cell anemia
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Multiple health problems can result from this
one genetic
Epistasis
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a gene at one locus alters the phenotypic
expression of a gene at a second locus.
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For example, in mice and many other mammals,
coat color depends on two genes.
One, the epistatic gene, determines whether
pigment will be deposited in hair or not.
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The second determines whether the pigment to be
deposited is black (B) or brown (b).
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Presence (C) is dominant to absence (c).
The black allele is dominant to the brown allele.
An individual that is cc has a white (albino) coat
regardless of the genotype of the second gene.
Epistasis
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A cross between two black mice that are heterozygous
(BbCc) will follow the law of independent assortment.
However, unlike the
9:3:3:1 offspring ratio
of an normal Mendelian
experiment, the ratio is
nine black, three brown,
and four white.
Polygenic inheritance
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the additive effects of two or more genes
on a single phenotypic character.
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For example, skin color in humans is
controlled by at least three different genes.
Imagine that each gene has two alleles,
one light and one dark, that demonstrate
incomplete dominance.
An AABBCC individual is dark and aabbcc
is light.
Polygenic
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A cross between two AaBbCc individuals
(intermediate skin shade) would produce offspring
covering a wide range of shades.
 Individuals with
intermediate skin shades
would be the most likely
offspring, but very light
and very dark individuals
are possible as well.
 The range of phenotypes
forms a normal
distribution.
Nature vs Nurture
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Phenotype depends on environment and
genes.
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A single tree has leaves that vary in size, shape,
and greenness, depending on exposure to wind
and sun.
Humans, nutrition influences height, exercise
alters build, sun-tanning darkens the skin, and
experience improves performance on intelligence
tests.
Even identical twins, genetic equals, accumulate
phenotypic differences as a result of their unique
experiences.
Very old and hotly contested debate.
Nature vs nurture
• The product of a genotype is generally not a rigidly defined
phenotype, but a range of phenotypic possibilities, the norm of
reaction, that are determined by the environment.
• In some cases the norm of reaction has no breadth (for
example, blood type).
• Norms of reactions are broadest for polygenic characters.
• For these multifactorial
characters, environment
contributes to their
quantitative nature.
• Genotype can refer not just to a single
genetic locus, but to an organism’s entire
genetic makeup.
• An organism’s phenotype reflects its
overall genotype and unique
environmental history
Pedigree analysis
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3rd generation lacks a widow’s peak, but both her parents have
widow’s peaks, then her parents must be heterozygous for that
gene
If some siblings in the second generation lack a widow’ peak
and one of the grandparents (first generation) also lacks one,
then we know the other grandparent must be heterozygous and
we can determine the genotype of almost all other individuals.
Pedigree analysis
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We can use the normal Mendelian rules,
including multiplication and addition, to predict
the probability of specific phenotypes.
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For example, these rules could be used to predict
the probability that a child with WwFf parents will
have a widow’s peak and attached earlobes.
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The chance of having a widow’s peak is 3/4 (1/2 [WW] +
1/4 [Ww]).
The chance of having attached earlobes is 1/4 [ff].
This combination has a probability of 3/4 x 1/4 = 3/16
Human disorders
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From the relatively mild (albinism) to lifethreatening (cystic fibrosis).
The recessive behavior of the alleles occurs
because the allele codes for either a
malfunctioning protein or no protein at all.
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Heterozygotes have a normal phenotype because
one “normal” allele produces enough of the
required protein
Two carriers have a 1/4 chance of having a child
with the disorder, 1/2 chance of a carrier, and 1/4
free.
Human disorders
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Multifactorial basis.
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These have a genetic component plus a significant
environmental influence.
Multifactorial disorders include heart disease,
diabetes, cancer, alcoholism, and certain mental
illnesses, such a schizophrenia and manicdepressive disorder.
The genetic component is typically polygenic.
Human disorders
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Huntington’s disease, a degenerative
disease of the nervous system.
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The dominant lethal allele has no obvious
phenotypic effect until an individuals is
about 35 to 45 years old.
The deterioration of the nervous system is
irreversible and inevitably fatal.
Tip of chromosome 4
Cystic fibrosis
One of every 2,500 whites of European descent.
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One in 25 whites is a carrier.
The normal allele codes for a membrane protein
that transports Cl- between cells and the
environment.
If these channels are defective or absent, there are
abnormally high extracellular levels of chloride that
causes the mucus coats of certain cells to become
thicker and stickier than normal.
This mucus build-up in the pancreas, lungs,
digestive tract, and elsewhere favors bacterial
infections.
Without treatment, affected children die before five,
but with treatment can live past their late 20’s.
Human Disorders
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Tay-Sachs disease is lethal recessive
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Dysfunctional enzyme that fails to break down
specific brain lipids.
Symptoms: seizures, blindness, and degeneration
of motor and mental performance a few months
after birth.
Inevitably, the child dies after a few years.
Among Ashkenazic Jews (those from central
Europe) this disease occurs in one of 3,600 births,
about 100 times greater than the incidence among
non-Jews or Mediterranean (Sephardic) Jews.
Human Disorders: Sickle Cell
Human Disorders
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Some genetic tests can be detected at birth by simple tests
that are now routinely performed in hospitals.
One test can detect the presence of a recessively inherited
disorder, phenyketonuria (PKU).
 This disorder occurs in one in 10,000 to 15,000 births.
 Individuals with this disorder accumulate the amino acid
phenylalanine and its derivative phenypyruvate in the
blood to toxic levels.
 This leads to mental retardation.
 If the disorder is detected, a special diet low in
phenyalalanine usually promotes normal development