Download Mendelian Genetics

Genetics
•
Mendel and the Gene Idea
1
Heredity
•
•
•
What genetic principles account for the transmission of
traits from parents to offspring?
One possible explanation of heredity is a “blending”
hypothesis - 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
An alternative to the blending model is the “particulate”
hypothesis of inheritance: the gene idea - Parents pass on
discrete heritable units, genes
2
Gregor Mendel
•
Documented a particulate mechanism of inheritance
through his experiments with garden peas
Gregor Mendel’s
monastery garden.
Fig. 2.2
3
Mendelian Genetics
•
Gregor Johann Mendel (1822-1884)
–
Augustinian monk, Czech Republic
–
Foundation of modern genetics
–
–
–
–
Studied segregation of traits in the garden pea (Pisum
sativum) beginning in 1854
Published his theory of inheritance in 1865.
“Experiments in Plant Hybridization”
Mendel was “rediscovered” in 1902
Ideas of inheritance in Mendel’s time were vague. One
general idea was that traits from parents came together
and blended in offspring. Thus, inherited information
was predicted to change in the offspring, an idea that
Mendel showed was wrong. “Characters,” or what we
now call alleles, were inherited unchanged. This
observation and the pattern of inheritance of these
characters gave us the first definition of a gene
4
Themes of Mendel’s work
•
•
•
•
Variation is widespread in nature
Observable variation is essential for following
genes
Variation is inherited according to genetic laws
and not solely by chance
Mendel’s laws apply to all sexually reproducing
organisms
5
Mendel’s Experimental, Quantitative Approach
•
•
•
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
Mendel chose to work with the garden pea (Pisum
sativum)
– Because they are available in many varieties, easy to
grow, easy to get large numbers
– Because he could strictly control which plants mated
with which
6
Crossing Pea Plants
1
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
5 Examined
First
generation
offspring
(F1)
offspring:
all purple
flowers
7
Mendel’s experimental design
•
•
Statistical analyses:
– Worked with large numbers of plants
– counted all offspring
– made predictions and tested them
Excellent experimentalist
– controlled growth conditions
– focused on traits that were easy to score
– chose to track only those characters that varied
in an “either-or” manner
8
Mendel’s Studied Discrete Traits
9
Genetic Vocabulary
•
•
•
•
Character: a heritable feature, such as flower color
Trait: a variant of a character, such as purple or
white flowers
Each trait carries two copies of a unit of
inheritance, one inherited from the mother and the
other from the father
Alternative forms of traits are called alleles
10
Antagonistic traits
Dominant
Recessive
11
Mendel’s experimental design
•
Mendel also made sure that he started his experiments with
varieties that were “true-breeding”
X
X
X
X
X
X
12
Genetic Vocabulary
•
•
•
•
•
•
Phenotype – observable characteristic of an
organism
Genotype – pair of alleles present in and individual
Homozygous – two alleles of trait are the same
(YY or yy)
Heterozygous – two alleles of trait are different
(Yy)
Capitalized traits = dominant phenotypes
Lowercase traits= recessive phenotypes
13
Genetic Vocabulary
•
•
Generations:
– P = parental generation
– F1 = 1st filial generation, progeny of the P generation
– F2 = 2nd filial generation, progeny of the F1 generation
(F3 and so on)
Crosses:
– Monohybrid cross = cross of two different true-breeding
strains (homozygotes) that differ in a single trait.
– Dihybrid cross = cross of two different true-breeding
strains (homozygotes) that differ in two traits.
14
Phenotype vs 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
1
15
Phenotype vs Genotype
Dominant & recessive alleles (Fig. 10.7):
16
Mendel’s Experimental Design
•
•
•
•
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
The hybrid offspring of the P generation are called
the F1 generation
When F1 individuals self-pollinate the F2
generation is produced
17
Mendel’s Observations
•
•
•
When Mendel crossed contrasting, true-breeding 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
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.
F2 Generation
18
Mendel’s Rationale
•
•
•
•
In the F1 plants, only the purple trait was affecting
flower color in these hybrids
Purple flower color was dominant, and white flower
color was recessive
Mendel developed a hypothesis to explain the 3:1
inheritance pattern that he observed among the F2
offspring
There are four related concepts that are integral to
this hypothesis
19
Heredity Concepts
1.
Alternative versions of genes account for variations in inherited
characters, which are now called alleles
Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers
2.
3.
4.
For each character an organism inherits two alleles, one from each
parent, A genetic locus is actually represented twice
If the two alleles at a locus differ, the dominant allele determines the
organism’s appearance
The law of segregation - the two alleles for a heritable character
separate (segregate) during gamete formation and end up in different
gametes
20
Law of Segregation
•
Mechanism of gene transmission
Gametogenesis:
alleles segregate
Fertilization:
alleles unite
21
Mendelian Genetics
• Classical Punett's Square is a way to determine ways traits
can segregate
Parental P0 cross
P
P
p
p
F1 cross
P
p
P
p
Determine the genotype and phenotype
23
Mendelian Genetics
• Classical Punett's Square is a way to determine ways traits
can segregate
Parental P0 cross
P
Pp
Pp
p
p
P
Pp
Pp
F1 cross
P
p
P
p
Determine the genotype and phenotype
24
Mendelian Genetics
• Classical Punett's Square is a way to determine ways traits can
segregate
Parental P0 cross
p
p
P
p
P
Pp
Pp
F1 cross
P
PP
Pp
P
Pp
Pp
p
Pp
pp
Determine the genotype and phenotype
25
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.
Union of the parental gametes
produces F1 hybrids having a Pp
combination. Because the purpleflower allele is dominant, all
these hybrids have purple flowers.
When the hybrid plants produce
gametes, the two alleles segregate,
half the gametes receiving the P
allele and the other half the p allele.
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.
P Generation

Appearance:
Purple flowers White flowers
Genetic makeup:
PP
pp
Gametes:
p
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
1/
2 P
p
F1 sperm
P
p
PP
Pp
F2 Generation
P
F1 eggs
p
pp
Pp
Random combination of the gametes
results in the 3:1 ratio that Mendel
observed in the F2 generation.
2
3
:1
26
Mendel’s Monohybrid Cross
White
(pp)
Purple
(Pp)
Gametes
p
Purple
(PP)
p
P
Gametes
Gametes
Purple
(Pp)
P
p
PP
Pp
P
Pp
Pp
Gametes
p
P
Pp
Pp
F1 generation
All purple
Pp
pp
F2 generation
¾ purple, ¼ white
27
Smooth and wrinkled
parental seed strains
crossed.
•
Punnett square
–
–
F1 genotypes: 4/4 Ss
F1 phenotypes: 4/4
smooth
28
Mendel Observed The Same Pattern In Characters
29
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
30
Test Cross
•
To determine whether an individual with a dominant phenotype is
homozygous for the dominant allele or heterozygous, Mendel crossed
the individual in question with an individual that had the recessive
phenotype:
Alternative 1 – Plant with
dominant phenotype is
homozygous
Dominant
Phenotype
PP
?
Alternative 2 – Plant with
dominant phenotype is
heterozygous
?
Dominant
Phenotype
Pp
Gametes
Recessive
phenotype
P
p
pp
Gametes
P
P
Recessive
phenotype
p
Gametes
p
p
pp
Gametes
p
31
Test Cross
•
To determine whether an individual with a dominant phenotype is
homozygous for the dominant allele or heterozygous, Mendel crossed
the individual in question with an individual that had the recessive
phenotype:
Alternative 1 – Plant with
dominant phenotype is
homozygous
Dominant
Phenotype
PP
?
Alternative 2 – Plant with
dominant phenotype is
heterozygous
?
Dominant
Phenotype
Pp
Gametes
Recessive
phenotype
pp
P
P
p
Pp
Pp
p
Pp
Pp
Gametes
Recessive
phenotype
Gametes
If all offspring are purple;
unknown plant is
homozygous.
pp
Gametes
P
p
p
Pp
pp
p
Pp
pp
If half of offspring are
white; unknown plant
is heterozygous.
32
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
Pp
33
The Law of Independent Assortment
•
Mendel derived the law of segregation by following a single trait
–
–
•
2 alleles at a single gene locus segregate when the gametes are
formed
The F1 offspring produced in this cross were monohybrids,
heterozygous for one character
Mendel identified his second law of inheritance by following two
characters at the same time
–
–
Mendel was interested in determining whether alleles at 2 different
gene loci segregate dependently or independently
Crossing two, true-breeding parents differing in two characters
produces dihybrids in the F1 generation, heterozygous for both
characters
34
Dihybrid Cross
•
•
With his monohybrid crosses, Mendel determined
that the 2 alleles at a single gene locus segregate
when the gametes are formed.
With his dihybrid crosses, Mendel was interested in
determining whether alleles at 2 different gene loci
segregate dependently or independently.
35
Dihybrid Cross
•
•
For example, in pea plants seed shape is
controlled by one gene locus where round (R) is
dominant to wrinkled (r) while seed color is
controlled by a different gene locus where yellow
(Y) is dominant to green (y).
Mendel crossed 2 pure-breeding plants: one with
round yellow seeds and the other with green
wrinkled seeds.
36
Dihybrid Cross
•
•
For example, in pea plants seed shape is
controlled by one gene locus where round (R) is
dominant to wrinkled (r) while seed color is
controlled by a different gene locus where yellow
(Y) is dominant to green (y).
Mendel crossed 2 pure-breeding plants: one with
round yellow seeds and the other with green
wrinkled seeds.
37
Dependent Segregation
•
If dependent segregation (assortment) occurs:
–
–
Alleles at the 2 gene loci segregate together, and are
transmitted as a unit.
Therefore, each plant would only produce gametes with the
same combinations of alleles present in the gametes inherited
from its parents:
Parents
Parental Gametes
r
y
R
Y
Rr
Yy
F1 Offspring
F1 Offspring’s Gametes
rr
yy
RR
YY
R
Y
r
y
What is the expected phenotypic ratio for the F2?
38
F2 With Dependent Assortment:
R
Y
r
y
R
Y
RR
YY
Rr
Yy
r
y
Rr
Yy
rr
yy
Ratio is 3 round, yellow : 1 wrinkled, green
39
Independent Segregation
•
Alleles at the 2 gene loci segregate (separate) independently, and are
NOT transmitted as a unit. Therefore, each plant would produce
gametes with allele combinations that were not present in the gametes
inherited from its parents:
Parents
Parental Gametes
F1 Offspring
F1 Offspring’s
Gametes
R
Y
R
y
RR
YY
rr
yy
R
Y
r
y
Rr
Yy
r
Y
r
y
What is the expected phenotypic ratio for the F2?
40
Mendelian Genetics
Dihybrid cross - parental generation differs in two traits
example-- cross round/yellow peas with wrinkled/green ones
Round/yellow is dominant
RY
Ry
rY
ry
RY
Ry
rY
ry
What are the expected phenotype ratios in the F2 generation?
round, yellow =
round, green =
wrinkled, yellow =
wrinkled, green =
41
F2 with independent assortment:
RY Ry
RY
Ry
rY
ry
rY
ry
RR
YY
RR
Yy
Rr
YY
Rr
Yy
RR
Yy
RR
yy
Rr
Yy
Rr
yy
Rr
YY
Rr
Yy
rr
YY
rr
Yy
Rr
Yy
Rr
yy
rr
Yy
rr
yy
Phenotypic ratio is 9 : 3 : 3 : 1
42
A Dihybrid Cross
•
How are two characters transmitted from parents to offspring?
–
–
•
As a package?
Independently?
A dihybrid cross
–
–
Illustrates the inheritance of two characters
Produces four phenotypes in the F2 generation
P 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.
YYRR
yyrr
Gametes
F1 Generation
YR

Hypothesis of
dependent
assortment
yr
YyRr
Hypothesis of
independent
assortment
Sperm
Sperm
1⁄ YR 1⁄ yr
2
2
Eggs
1⁄
YR
F2 Generation 2
YYRR YyRr
(predicted
offspring)
1 ⁄ yr
2
YyRr yyrr
3⁄
4
CONCLUSION The results support the hypothesis
ofindependent assortment. The alleles for seed color
and seed shape sort into gametes independently of
each other.
1⁄
4
1⁄
4
1⁄
4
Yr
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
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
43
Dihybrid cross
•
F2 generation ratio:
9:3:3:1
44
Law of Independent Assortment
•
•
Mendel’s dihybrid crosses showed a 9:3:3:1
phenotypic ratio for the F2 generation.
Based on these data, he proposed the Law of
Independent Assortment, which states that when
gametes form, each pair of hereditary factors
(alleles) segregates independently of the other
pairs.
45
Laws Of Probability Govern Mendelian Inheritance
•
Mendel’s laws of segregation and independent
assortment reflect the rules of probability
– The multiplication rule

–
States that the probability that two or more
independent events will occur together is the product
of their individual probabilities
The rule of addition

States that the probability that any one of two or more
exclusive events will occur is calculated by adding
together their individual probabilities
46
Laws Of Probability - Multiplication Rule
•
•
•
•
•
•
The probability of two or more independent events
occurring together is the product of the probabilities that
each event will occur by itself
Following the self-hybridization of a heterozygous purple
pea plants (Pp), what is the probability that a given offspring
will be homozygous for the production of white flowers
(pp)?
Probability that a pollen seed will carry p: ½
Probability that an egg will carry p: ½
Probability that the offspring will be pp:
1/2 X 1/2 = 1/4
47
Laws Of Probability - Addition Rule
•
•
•
•
•
•
•
The probability of either of two mutually exclusive events
occurring is the sum of their individual probabilities
Following the self-hybridization of a heterozygous purple
pea plant (Pp), what is the probability that a given offspring
will be purple?
Probability of maternal P uniting with paternal P: 1/4
Probability of maternal p uniting with paternal P: 1/4
Probability of maternal P uniting with paternal p: 1/4
Probability that the offspring will be purple:
1/4 + 1/4 + 1/4 = 3/4
48
Probability In A Monohybrid Cross
•
Can be determined using these rules

Rr
Rr
Segregation of
alleles into eggs
Segregation of
alleles into sperm
Sperm
1⁄
R
2
1⁄
Eggs
1⁄
r
2
r
r
R
R
2
r
2
R
R
1⁄
1⁄
1⁄
4
R
1⁄
4
r
4
r
1⁄
4
49
Mendel’s conclusions
•
•
•
•
Genes are distinct entities that remain
unchanged during crosses
Each plant has two alleles of a gene
Alleles segregated into gametes in equal
proportions, each gamete got only one allele
During gamete fusion, the number of alleles
was restored to two
50
Summary of Mendel’s Principles
•
Mendel’s Principle of Uniformity in F1:
–
–
•
Mendel’s Law of Segregation:
–
–
•
F1 offspring of a monohybrid cross of true-breeding strains resemble only
one of the parents.
Why? Smooth seeds (allele S) are completely dominant to wrinkled seeds
(alleles).
Recessive characters masked in the F1 progeny of two true-breeding
strains, reappear in a specific proportion of the F2 progeny.
Two members of a gene pair segregate (separate) from each other during
the formation of gametes.
Mendel’s Law of Independent Assortment:
–
–
Alleles for different traits assort independently of one another.
Genes on different chromosomes behave independently in gamete
production.
51
Exceptions To Mendel’s Original Principles
•
•
•
•
•
Incomplete dominance
Codominance
Multiple alleles
Polygenic traits
Epistasis
•
•
•
•
Pleiotropy
Environmental effects on
gene expression
Linkage
Sex linkage
52
Incomplete dominance
•
•
Neither allele is dominant and heterozygous individuals have an
intermediate phenotype
For example, in Japanese “Four o’clock”, plants with one red allele and
one white allele have pink flowers:
P Generation
Red
CRCR
White
CW CW

Gametes CR
CW
Pink
CRCW
F1 Generation
Gametes
Eggs
F2 Generation
1⁄
2
CR
1⁄
2
Cw
1⁄
2
1⁄
2
CR
1⁄
2
CR
CR 1⁄2 CR
Sperm
CR CR CR CW
CR CW CW CW
53
Incomplete Dominance
Gametes
CR
CW
CRCR
CRCW
CRCW
CWCW
CRCR
CR
Gametes
CW
F1 generation
All CRCW
F2 generation
CWCW
1:2:1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
54
Codominance
•
•
Neither allele is dominant and both alleles are
expressed in heterozygous individuals
Example ABO blood types
55
Polygenic Traits
•
•
Most traits are not controlled by a single gene locus, but by
the combined interaction of many gene loci. These are
called polygenic traits.
Polygenic traits often show continuous variation, rather then
a few discrete forms:
56
Epistasis
•
•
•
•
Type of polygenic inheritance where the alleles at one gene
locus can hide or prevent the expression of alleles at a
second gene locus.
Labrador retrievers one gene locus affects coat color by
controlling how densely the pigment eumelanin is deposited
in the fur.
A dominant allele (B) produces a black coat while the
recessive allele (b) produces a brown coat
However, a second gene locus controls whether any
eumelanin at all is deposited in the fur. Dogs that are
homozygous recessive at this locus (ee) will have yellow fur
no matter which alleles are at the first locus:
57
Epistasis
ee
No dark pigment in fur
eebb
Yellow fur
eeB_
Yellow fur
E_
Dark pigment in fur
E_bb
Brown fur
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
E_B_
Black fur
58
Pleiotropy
•
•
This is when a single gene locus affects more than
one trait.
For example, in Labrador retrievers the gene locus
that controls how dark the pigment in the hair will
be also affects the color of the nose, lips, and eye
rims.
59
Environmental Effects on Gene Expression
•
•
The phenotype of an organism depends not only on which
genes it has (genotype), but also on the environment under
which it develops.
Although scientists agree that phenotype depends on a complex interaction between
genotype and environment, there is a lot of debate and controversy about the relative
importance of these 2 factors, particularly for complex human traits.
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