Transcript Spring 2012 Mendelian Genetics

Mendelelian
Genetics
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1
A Tale of Two Families
Modes of inheritance are the rules
explaining the common patterns of
inheritance.
Huntington’s Disease and Cystic Fibrosis
demonstrate two important modes of
inheritance.
HD is autosomal dominant and CF is
autosomal recessive.
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Fig. 4.1
Autosomal Dominant – affects both sexes
and appears every generation
Fig. 4.2
Autosomal Recessive – affects both sexes and
skip generations through carriers
Gregor Mendel
(1822-1884)
Responsible
for the Laws
governing
Inheritance of
Traits
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Gregor Johann Mendel
Austrian monk
Studied the
inheritance of
traits in pea plants
Developed the laws
of inheritance
Mendel's work was
not recognized until
the turn of the
20th century
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Gregor Johann Mendel
Between 1856 and
1863, Mendel
cultivated and
tested some 28,000
pea plants
He found that the
plants' offspring
retained traits of
the parents
Called the “Father
of Genetics"
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Site of
Gregor
Mendel’s
experimental
garden in the
Czech
Republic
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Particulate Inheritance
Mendel stated that
physical traits are
inherited as
“particles”
Mendel did not know
that the “particles”
were actually
Chromosomes & DNA
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Genetic Terminology
 Trait - any characteristic that
can be passed from parent to
offspring
 Heredity - passing of traits
from parent to offspring
 Genetics - study of heredity
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Types of Genetic Crosses
 Monohybrid cross - cross
involving a single trait
e.g. flower color
 Dihybrid cross - cross involving
two traits
e.g. flower color & plant height
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Punnett Square
Used to help
solve genetics
problems
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Designer “Genes”
 Alleles - two forms of a gene
(dominant & recessive)
 Dominant - stronger of two genes
expressed in the hybrid;
represented by a capital letter (R)
 Recessive - gene that shows up less
often in a cross; represented by a
lowercase letter (r)
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More Terminology
 Genotype - gene combination
for a trait (e.g. RR, Rr, rr)
 Phenotype - the physical
feature resulting from a
genotype (e.g. red, white)
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Genotype & Phenotype in Flowers
Genotype of alleles:
R = red flower
r = yellow flower
All genes occur in pairs, so 2
alleles affect a characteristic
Possible combinations are:
Genotypes
RR
Rr
rr
Phenotypes
RED
RED
YELLOW
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Genotypes
 Homozygous genotype - gene
combination involving 2 dominant
or 2 recessive genes (e.g. RR or
rr); also called pure
 Heterozygous genotype - gene
combination of one dominant &
one recessive allele
(e.g. Rr);
also called hybrid
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17
Genes and Environment
Determine Characteristics
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Mendel’s Pea Plant
Experiments
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Why peas, Pisum sativum?
Can be grown in a
small area
Produce lots of
offspring
Produce pure plants
when allowed to
self-pollinate
several generations
Can be artificially
cross-pollinated
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Reproduction in Flowering Plants
Pollen contains sperm
Produced by the
stamen
Ovary contains eggs
Found inside the
flower
Pollen carries sperm to the
eggs for fertilization
Self-fertilization can
occur in the same flower
Cross-fertilization can
occur between flowers
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Mendel’s Experimental
Methods
Mendel hand-pollinated
flowers using a paintbrush
He could snip the
stamens to prevent
self-pollination
Covered each flower
with a cloth bag
He traced traits through
the several generations
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How Mendel Began
Mendel
produced
pure
strains by
allowing the
plants to
selfpollinate
for several
generations
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23
Eight Pea Plant Traits
Seed shape --- Round (R) or Wrinkled (r)
Seed Color ---- Yellow (Y) or Green (y)
Pod Shape --- Smooth (S) or wrinkled (s)
Pod Color --- Green (G) or Yellow (g)
Seed Coat Color ---Gray (G) or White (g)
Flower position---Axial (A) or Terminal (a)
Plant Height --- Tall (T) or Short (t)
Flower color --- Purple (P) or white (p)
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Mendel’s Experimental Results
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Did the observed ratio match
the theoretical ratio?
The theoretical or expected ratio of
plants producing round or wrinkled seeds
is 3 round :1 wrinkled
Mendel’s observed ratio was 2.96:1
The discrepancy is due to statistical
error
The larger the sample the more nearly
the results approximate to the
theoretical ratio
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Generation “Gap”
Parental P1 Generation = the parental
generation in a breeding experiment.
F1 generation = the first-generation
offspring in a breeding experiment. (1st
filial generation)
From breeding individuals from the P1
generation
F2 generation = the second-generation
offspring in a breeding experiment.
(2nd filial generation)
From breeding individuals from the F1
generation
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Following the Generations
Cross 2
Pure
Plants
TT x tt
Results
in all
Hybrids
Tt
Cross 2 Hybrids
get
3 Tall & 1 Short
TT, Tt, tt
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Monohybrid
Crosses
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P1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Wrinkled seeds
RR
x
rr
r
r
R
Rr
Rr
R
Rr
Rr
Genotype: Rr
Phenotype: Round
Genotypic
Ratio: All alike
Phenotypic
Ratio: All alike
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P1 Monohybrid Cross Review
 Homozygous dominant x Homozygous
recessive
 Offspring all Heterozygous
(hybrids)
 Offspring called F1 generation
 Genotypic & Phenotypic ratio is ALL
ALIKE
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F1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Round seeds
Rr
x
Rr
R
r
R
RR
Rr
r
Rr
rr
Genotype: RR, Rr, rr
Phenotype: Round &
wrinkled
G.Ratio: 1:2:1
P.Ratio: 3:1
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F1 Monohybrid Cross Review
 Heterozygous x heterozygous
 Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
 Offspring called F2 generation
 Genotypic ratio is 1:2:1
 Phenotypic Ratio is 3:1
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What Do the Peas Look Like?
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…And Now the Test Cross
Mendel then crossed a pure & a
hybrid from his F2 generation
This is known as an F2 or test
cross
There are two possible
testcrosses:
Homozygous dominant x Hybrid
Homozygous recessive x Hybrid
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F2 Monohybrid Cross
st
(1 )
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Round seeds
RR
x
Rr
R
r
R
RR
Rr
R
RR
Rr
Genotype: RR, Rr
Phenotype: Round
Genotypic
Ratio: 1:1
Phenotypic
Ratio: All alike
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F2 Monohybrid Cross (2nd)
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Wrinkled seeds x Round seeds
rr
x
Rr
R
r
r
Rr
Rr
r
rr
rr
Genotype: Rr, rr
Phenotype: Round &
Wrinkled
G. Ratio: 1:1
P.Ratio: 1:1
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F2 Monohybrid Cross Review
 Homozygous x heterozygous(hybrid)
 Offspring:
50% Homozygous RR or rr
50% Heterozygous Rr
 Phenotypic Ratio is 1:1
 Called Test Cross because the
offspring have SAME genotype as
parents
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Practice Your Crosses
Work the P1, F1, and both
F2 Crosses for each of the
other Seven Pea Plant
Traits
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41
Mendel’s Laws
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Results of Monohybrid Crosses
Inheritable factors or genes are
responsible for all heritable
characteristics
Phenotype is based on Genotype
Each trait is based on two genes,
one from the mother and the
other from the father
True-breeding individuals are
homozygous ( both alleles) are the
same
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Law of Dominance
In a cross of parents that are
pure for contrasting traits, only
one form of the trait will appear in
the next generation.
All the offspring will be
heterozygous and express only the
dominant trait.
RR x rr yields all Rr (round seeds)
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Law of Dominance
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Law of Segregation
During the formation of gametes
(eggs or sperm), the two alleles
responsible for a trait separate
from each other.
Alleles for a trait are then
"recombined" at fertilization,
producing the genotype for the
traits of the offspring.
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Applying the Law of Segregation
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Law of Independent
Assortment
Alleles for different traits are
distributed to sex cells (&
offspring) independently of one
another.
This law can be illustrated using
dihybrid crosses.
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Dihybrid Cross
A breeding experiment that tracks
the inheritance of two traits.
Mendel’s “Law of Independent
Assortment”
a. Each pair of alleles segregates
independently during gamete formation
b. Formula: 2n (n = # of heterozygotes)
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Question:
How many gametes will be produced for
the following allele arrangements?
Remember:
2n (n = # of heterozygotes)
1. RrYy
2. AaBbCCDd
3. MmNnOoPPQQRrssTtQq
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Answer:
1. RrYy: 2n = 22 = 4 gametes
RY
Ry
rY ry
2. AaBbCCDd: 2n = 23 = 8 gametes
ABCD ABCd AbCD AbCd
aBCD aBCd abCD abCD
3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64
gametes
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Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round
r wrinkled
Y yellow
y green
RrYy
x
RrYy
RY Ry rY ry
RY Ry rY ry
All possible gamete combinations
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Dihybrid Cross
RY
Ry
rY
ry
RY
Ry
rY
ry
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Dihybrid Cross
RY
RY RRYY
Ry RRYy
rY RrYY
ry
RrYy
Ry
rY
ry
RRYy
RrYY
RrYy
RRyy
RrYy
Rryy
RrYy
rrYY
rrYy
Rryy
rrYy
rryy
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Round/Yellow:
Round/green:
9
3
wrinkled/Yellow: 3
wrinkled/green:
1
9:3:3:1 phenotypic
ratio
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Dihybrid Cross
Round/Yellow: 9
Round/green:
3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
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Test Cross
A mating between an individual of unknown
genotype and a homozygous recessive
individual.
Example: bbC__ x bbcc
BB = brown eyes
Bb = brown eyes
bb = blue eyes
CC = curly hair
Cc = curly hair
cc = straight hair
bC
b___
bc
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Test Cross
Possible results:
bc
bC
b___
C
bbCc
bbCc
or
bc
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bC
b___
c
bbCc
bbcc
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Summary of Mendel’s laws
LAW
DOMINANCE
SEGREGATION
INDEPENDENT
ASSORTMENT
PARENT
CROSS
OFFSPRING
TT x tt
tall x short
100% Tt
tall
Tt x Tt
tall x tall
75% tall
25% short
RrGg x RrGg
round & green
x
round & green
9/16 round seeds & green
pods
3/16 round seeds & yellow
pods
3/16 wrinkled seeds & green
pods
1/16 wrinkled seeds & yellow
pods
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Single-gene Inheritance in Humans
1. A Mendelian trait is caused by a single
gene.
2. Mendelian human conditions are
considered more rare than multifactorial
ones
3. Mendelian inheritance in humans is more
complex than in model organisms such as
pea plants.
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Eye color
The inheritance
of eye color is an
example of a
single gene trait
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Modes of Inheritance
Modes of inheritance – rules explaining common
patterns of inheritance
Genes may be autosomal or on a sex chromosome
Alleles can be dominant or recessvie
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Criteria for an autosomal dominant trait:
1. Males & females can be
affected; male to male
transmission can occur
2. Both transmit the trait
with equal frequency
3. Successive generations
are affected.
4. Transmission stops
after a generation where
no one is affected.
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Criteria for an autosomal recessive trait
1. Males & females can
be affected
2. Both can transmit the
gene, unless it causes
death before
reproductive age
3. The trait can skip
generations
4. Parents of an
affected individual are
heterozygous or have
the trait.
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Modes of inheritance, cont.
1. Blood relatives that have children together have
a much higher risk of having a child with a rare
recessive disorder.
2. Marriage between relatives introduces
consanguinity, literally meaning “shared blood”
However genes are not passed in blood
3. Punnett squares apply Mendel’s first law to
predict recurrence risks of inherited disorders
or traits.
4. A Mendelian trait applies anew to each child.
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On the Meaning of Dominance & Recessiveness
At the biochemical level, recessive disorders
often result from alleles that cause a loss of
function or loss of a normal protein
Dominant disorders can result from production of
an abnormal protein that interferes with the
function of a normal protein or imparts a gain of
function.
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Recessive disorders
Recessive orders tend to be more severe
producing symptoms earlier than dominant
disorders
Disease causing recessive alleles remain in
populations because heterozygotes pass them to
future generations.
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Dominant disorders
If a dominant mutation that harms early in life
arises people who have the allele are too ill or
don’t survive long enough to reproduce
The allele becomes rare in the population unless it
arises anew by mutation
Dominant disorders that do not appear until
adulthood or drastically disrupt health, remain in
the population until after a person has
reproduced.
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Pedigree Analysis
Pedigree charts depict family relationships
and transmission of inherited traits.
Squares represent males and circles
represent females.
Horizontal lines indicate parents, vertical
lines show generations, and elevated lines
depict siblings.
Symbols for heterozygotes are half-shaded,
and for an individual with a particular
phenotype, completely shaded
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Pedigrees Then and Now
Pedigrees have been in use since antiquity.
They have been used throughout history to show
family relationships, particularly royal
bloodlines.
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Pedigrees Display Mendel’s Laws
Pedigrees can reveal mode of inheritance,
and can suggest molecular information,
carrier status, and input from other
genes and the environment.
Interpretation of pedigrees can be
inconclusive when more than one mode of
inheritance can explain the pattern seen.
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Solving Genetics Problems
 List genotypes and phenotypes for
the trait
 Determine the genotypes of the
parents
 Possible gametes
 Possible genotypes of offspring
 Repeat for successive generations
Independent Events
The probability of simultaneous independent
events
= the product of the probability of each event
Example:
If both parents are heterozygous (Bb)) what is
the probability that they will produce a BB
child?
Probability of a sperm with B allele = ½
Probability of a ova with B allele = ½
Probability of a BB child is ½ X ½ = ¼
Dependent Events
The probability of dependent events
= the sum of probability of each event
Example
•Parents are heterozygous for a trait, R.
•What is the chance that their child carries at least
one dominant R allele?
•Probability of child carrying RR = ¼
•Probability of child carrying Rr = ½
•Probability of child carrying R_ = ¼ + ½ = ¾