Transcript Mendel and the Gene Idea

Chapter 14:
Mendel and the Gene
Idea
Genetic Theories
1. Blending Theory traits were like paints and mixed evenly from both
parents.
2. Incubation Theory only one parent controlled the traits of the children.
Ex: Spermists and Ovists
3. Particulate Model parents pass on traits as discrete units that retain
their identities in the offspring.
Gregor Mendel
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Father of Modern Genetics.
Reasons for Mendel's Success
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Used an experimental approach.
Applied mathematics to the study of natural
phenomena.
Kept good records.
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Mendel was a pea picker.
He used peas as his study
organism. Why?
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Short life span.
Bisexual.
Many traits known.
Cross- and self-pollinating.
(You can eat the failures).
Cross-pollination
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Two parents.
Results in hybrid offspring where the
offspring may be different than the parents.
Self-pollination
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One flower as both parents.
Natural event in peas.
Results in pure-bred offspring where the offspring
are identical to the parents.
Mendel's Work
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Used seven characters, each with two
expressions or traits.
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Example:
Character - height
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Traits - tall or short.
Monohybrid or Mendelian Crosses
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Crosses that work with a single character at a
time.
Example - Tall X short
P Generation
The Parental generation or the first two
individuals used in a cross.
Example - Tall X short
 Mendel used reciprocal crosses, where the
parents alternated for the trait.
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Offspring
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F1 - first filial generation.
F2 - second filial generation, bred by
crossing two F1 plants together or allowing a
F1 to self-pollinate.
Another Sample Cross
P Tall X short (TT x tt)
F1 all Tall (Tt)
F2 3 tall to 1 short
(1 TT: 2 Tt: 1 tt)
Results - Summary
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In all crosses, the F1 generation showed
only one of the traits regardless of which
was male or female.
The other trait reappeared in the F2 at
~25% (3:1 ratio).
Mendel's Hypothesis
1. Genes can have alternate versions called
alleles.
2. Each offspring inherits two alleles, one from
each parent.
3. If the two alleles differ, the dominant allele is
expressed. The recessive allele remains
hidden unless the dominant allele is absent.
Does dominant mean it occurs the most frequently?
Mendel's Hypothesis
4. The two alleles for each trait separate during
gamete formation. This now called: Mendel's
Law of Segregation
Law of Segregation
Vocabulary
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Phenotype - the physical appearance of the
organism.
Genotype - the genetic makeup of the organism,
usually shown in a code.
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T = tall
t = short
Homozygous - When the two alleles are the same
(TT/tt).
Heterozygous- When the two alleles are different
(Tt).
6 Mendelian Crosses are Possible
Cross
TT X tt
Tt X Tt
TT X TT
tt X tt
TT X Tt
Tt X tt
Genotype
all Tt
1TT:2Tt:1tt
all TT
all tt
1TT:1Tt
1Tt:1tt
Phenotype
all Dom
3 Dom: 1 Res
all Dom
all Res
all Dom
1 Dom: 1 Res
Test Cross
Cross of a suspected heterozygote with a
homozygous recessive.
 Ex: T_ X tt
If TT - all dominant
If Tt - 1 Dominant: 1 Recessive
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Dihybrid Cross
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Cross with two genetic traits.
Need 4 letters to code for the cross.
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Ex: TtRr
Each Gamete - Must get 1 letter for each trait.
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Ex. TR, Tr, etc.
Number of Kinds of Gametes
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Critical to calculating the results of higher
level crosses.
Look for the number of heterozygous traits.
Equation
The formula 2n can be used, where “n” = the
number of heterozygous traits.
Ex: TtRr, n=2
22 or 4 different kinds of gametes are
possible.
TR, tR, Tr, tr
Dihybrid Cross
TtRr X TtRr
Each parent can produce 4 types of gametes.
TR, Tr, tR, tr
Cross is a 4 X 4 with 16 possible offspring.
Results
9 Tall, Red flowered
 3 Tall, white flowered
 3 short, Red flowered
 1 short, white flowered
Or: 9:3:3:1
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Law of Independent Assortment
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The inheritance of 1st genetic trait is NOT
dependent on the inheritance of the 2nd trait.
Inheritance of height is independent of the
inheritance of flower color.
Comment
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Ratio of Tall to short is 3:1
Ratio of Red to white is 3:1
The cross is really a product of the ratio of
each trait multiplied together. (3:1) X (3:1)
Probability
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Genetics is a specific application of the rules
of probability.
Probability - the chance that an event will
occur out of the total number of possible
events.
Genetic Ratios
The monohybrid “ratios” are actually the
“probabilities” of the results of random fertilization.
Ex: 3:1
75% chance of the dominant
25% chance of the recessive
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Rule of Multiplication
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The probability that two alleles will come
together at fertilization, is equal to the product
of their separate probabilities.
Example: TtRr X TtRr
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The probability of getting a tall offspring is
¾.
The probability of getting a red offspring is
¾.
The probability of getting a tall red
offspring is ¾ x ¾ = 9/16
Comment
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Use the Product Rule to calculate the results
of complex crosses rather than work out the
Punnett Squares.
Ex: TtrrGG X TtRrgg
Solution
“T’s” = Tt X Tt = 3:1
“R’s” = rr X Rr = 1:1
“G’s” = GG x gg = 1:0
Product is:
(3:1) X (1:1) X (1:0 ) = 3:3:1:1
Variations on Mendel
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2.
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5.
Incomplete Dominance
Codominance
Multiple Alleles
Epistasis
Polygenic Inheritance
Incomplete Dominance
When the F1 hybrids show a phenotype somewhere between
the phenotypes of the two parents.
Ex. Red X White snapdragons
F1 = all pink
F2 = 1 red: 2 pink: 1 white
 No hidden Recessive.
 3 phenotypes and 3 genotypes (Hint! – often a “dose” effect)
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Red = CR CR
Pink = CRCW
White = CWCW
Another example
Codominance
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Both alleles are expressed equally in the phenotype.
Ex. MN blood group
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MM
MN
NN
No hidden Recessive.
3 phenotypes and 3 genotypes (but not a “dose” effect)
Multiple Alleles
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When there are more than 2 alleles for a trait.
Ex. ABO blood group
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IA - A type antigen
IB - B type antigen
i - no antigen
Multiple genotypes and phenotypes.
Very common event in many traits.
Alleles and Blood Types
Type
A
B
AB
O
Genotypes
IA IA or IAi
IB IB or IBi
I A IB
ii
Comment
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Rh blood factor is a separate factor from the
ABO blood group.
Rh+ = dominant
Rh- = recessive
A+ blood = dihybrid trait
Epistasis
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When 1 gene locus alters the expression of a
second locus.
 Ex:
 1st gene: C = color, c = albino
 2nd gene: B = Brown, b = black
Gerbils
In Gerbils
CcBb X CcBb
Brown X Brown
F1 = 9 brown (C_B_)
3 black (C_bb)
4 albino (cc__)
 Ratios often altered from the expected.
 One trait may act as a recessive because it is
“hidden” by the second trait.
Epistasis in Mice
Problem
Wife is type A
 Husband is type AB
 Child is type O
Question - Is this possible?
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Bombay Effect
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Epistatic Gene on ABO group.
Alters the expected ABO outcome.
H = dominant, normal ABO
h = recessive, no A,B, reads as type O blood.
When ABO blood type inheritance patterns
are altered from expected.
Genotypes
Wife: type A (IA IA , Hh)
 Husband: type AB (IAIB, Hh)
 Child: type O (IA IA , hh)
Therefore, the child is the offspring of the wife
and her husband.
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Polygenic Inheritance
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Factors that are expressed as continuous
variation.
Lack clear boundaries between the phenotype
classes.
Ex: skin color, height
Genetic Basis
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Several genes govern the inheritance of the
trait.
Ex: Skin color is likely controlled by at least
4 genes. Each dominant gives a darker skin.
Result
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Mendelian ratios fail.
Traits tend to "run" in families.
Offspring often intermediate between the
parental types.
Trait shows a “bell-curve” or continuous
variation.
Genetic Studies in Humans
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Often done by Pedigree charts.
Why?
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Can’t do controlled breeding studies in humans.
Small number of offspring.
Long life span.
Pedigree Chart Symbols
Male
Female
Person with trait
Sample Pedigree
Dominant Trait
Recessive Trait
Human Recessive Disorders
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Several thousand known:
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Albinism
Sickle Cell Anemia
Tay-Sachs Disease
Cystic Fibrosis
PKU
Galactosemia
Sickle-cell Disease
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Most common inherited disease among
African-Americans.
Single amino acid substitution results in
malformed hemoglobin.
Reduced O2 carrying capacity.
Codominant inheritance.
Tay-Sachs
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Eastern European Jews.
Brain cells unable to metabolize type of lipid,
accumulation of causes brain damage.
Death in infancy or early childhood.
Cystic Fibrosis
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Most common lethal genetic disease in the
U.S.
Most frequent in Caucasian populations (1/20
a carrier).
Produces defective chloride channels in
membranes.
Recessive Pattern
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Usually rare.
Skips generations.
Occurrence increases with consaguineous
matings.
Often an enzyme defect.
Human Dominant Disorders
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Less common then recessives.
Ex:
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Huntington’s disease
Achondroplasia
Familial Hypercholsterolemia
Inheritance Pattern
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Each affected individual had one affected
parent.
Doesn’t skip generations.
Homozygous cases show worse phenotype
symptoms.
May have post-maturity onset of symptoms.
Genetic Screening
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Risk assessment for an individual inheriting a
trait.
Uses probability to calculate the risk.
General Formal
R=FXMXD
R = risk
F = probability that the female carries the
gene.
M = probability that the male carries the gene.
D = Disease risk under best conditions.
Example
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Wife has an albino parent.
Husband has no albinism in his pedigree.
Risk for an albino child?
Risk Calculation
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Wife = probability is 1.0 that she has the
allele.
Husband = with no family record,
probability is near 0.
Disease = this is a recessive trait, so risk is
Aa X Aa = .25
R = 1 X 0 X .25
R=0
Risk Calculation
Assume husband is a carrier, then the risk is:
R = 1 X 1 X .25
R = .25
There is a .25 chance that every child will be
albino.
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Common Mistake
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If risk is .25, then as long as we don’t have 4
kids, we won’t get any with the trait.
Risk is .25 for each child. It is not dependent
on what happens to other children.
Carrier Recognition
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Fetal Testing
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Amniocentesis
Chorionic villi sampling
Newborn Screening
Fetal Testing
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Biochemical Tests
Chromosome Analysis
Amniocentesis
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Administered between 11 - 14 weeks.
Extract amnionic fluid = cells and fluid.
Biochemical tests and karyotype.
Requires culture time for cells.
Chorionic Villi Sampling
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Administered between 8 - 10 weeks.
Extract tissue from chorion (placenta).
Slightly greater risk but no culture time
required.
Newborn Screening
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Blood tests for recessive conditions that can
have the phenotypes treated to avoid damage.
Genotypes are NOT changed.
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Ex. PKU
Required by law in all states.
Tests 1- 6 conditions.
Required of “home” births too.
Multifactorial Diseases
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Where Genetic and Environment Factors
interact to cause the Disease.
Ex. Heart Disease
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Genetic
Diet
Exercise
Bacterial Infection
Summary
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Know the Mendelian crosses and their
patterns.
Be able to work simple genetic problems
(practice).
Watch genetic vocabulary.
Be able to read pedigree charts.
Be able to recognize and work with some of
the “common” human trait examples.