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Mendelelian
Genetics
1
Inheritance
Parents and offspring often
share observable traits
Grandparents and grandchildren
may share traits not seen in
parents
Why do traits disappear in one
generation and reappear in
another?
2
Gregor Mendel
(1822-1884)
Father of
Genetics
Responsible
for the Laws
governing
Inheritance of
Traits
3
1822 – Born in what is now the Czeck Republic
Grew up in an agricultural environment
Joined St.Augustinian Monastery
Higher studies at Vienna University
1854 - Science teacher
Carried out “hybridization”
experiments with the common
garden pea plants
His hypotheses became the laws of
inheritance in modern genetics
4
Mendel cultivated and
tested some 28,000 pea
plants
He found that the
plants' offspring
retained traits of the
parents
Combined
Plant breeding
Statistics
Careful recordkeeping
5
Gregor Johann Mendel
Austrian monk
Developed the laws
of inheritance
Presented his findings
in the Natural History
Society of Brunn –
1865
Paper entitled,
“Experiments in Plant
Hybridization” – 1866
German language.
6
 Mendel's work was not recognized until the
turn of the 20th century
 1900 - Carl Correns, Hugo deVries, and Erich
von Tschermak rediscover and confirm
 Called the “Father of Genetics“
7
Site of
Gregor
Mendel’s
experimental
garden in the
Czech
Republic
8
Mendel’s workplace
Fig. 2.5
9
Mendel’s Pea Plant
Experiments
10
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 crosspollinated
Bisexual.
Many traits known.
Above all, easy to grow
11
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
12
Mendel’s Experimental
Methods
Mendel hand-pollinated
flowers using a paintbrush
He could snip (cut) the
stamens to prevent
self-pollination
He traced traits through
the several generations
13
14
How Mendel Began?
Mendel
produced
pure
strains by
allowing the
plants to
selfpollinate
for several
generations
15
True Breeding Plants
Plants which
consistently have
offspring with same
trait as parent are
true breeding plants
16
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)
17
18
Monohybrid Cross
What happens when true breeding
plants with two distinct forms
of a trait are crossed?
Progeny show only one form of the
trait
The observed trait is called
DOMINANT
The masked trait is called
RECESSIVE
19
Test Cross
Is a plant showing the
dominant trait truebreeding or not?
Test by crossing with a
plant showing the
RECESSIVE trait!!
All tall offspring
indicate parent is
true-breeding
Mixed offspring
indicate parent is
hybrid
20
Crossing Hybrids to Each Other
Hybrid parents show the dominant trait (tall)
Offspring…
dominant trait (tall) and true-breeding (1/4
total)
dominant trait (tall) and NOT true-breeding (1/2
total)
recessive trait (short) and always true-breeding
(1/4 total)
Mendel concluded that among the hybrid parents
the short trait (recessive) was hidden but not
absent
21
22
Mendel’s Data…
1.
1.
1.
Crossed true-breeding plants differing at one
of seven traits
Crossed hybrid offspring to each other (all
show dominant trait)
Counted offspring of hybrid crosses
23
Mendel’s Experimental Results
24
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
25
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
26
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
27
Law of Segregation
Why do traits “disappear” in one generation only to
reappear in a subsequent generation?
Each plant possesses two distinct separable units
(alleles) for each trait inherited from each
parent
Gametes contain ONE allele for each trait
Only one version is observed in an individual
the unit (allele) does not disappear
it may be present but hidden
28
29
Alleles
Mendel’s units (or “elementen”) are called
ALLELES
Versions of the same gene or DNA sequence
Differ in DNA sequence at one or more sites
30
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
31
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
32
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)
33
More Terminology
 Genotype - gene combination
for a trait (e.g. RR, Rr, rr)
 Phenotype - the physical
feature resulting from a
genotype (what you see)
(e.g. red, white)
34
Genotypes
 Homozygous genotype – When the
two alleles are same (dominant or
2 recessive genes) e.g. TT or tt;
also called pure
 Heterozygous genotype – When
the 2 alleles are different- one
dominant & one recessive
allele
(e.g. Tt); also called
hybrid
35
36
Wildtype
Most common version in the general population
Wildtype phenotype (most common phenotype)
Mutant phenotype (phenotype different from the
wildtype)
Wildtype allele (most frequent allele association
with the common phenotype
Mutant allele (allele associated with the mutant
phenotype)
37
Law of segregation: monohybrid cross
Two heterozygous parents produce gametes
with T or t allele equally frequently.
Offspring genotypes 1/4 TT : 1/2 Tt : 1/4 tt
Offspring phenotypes
3/4 tall
: 1/4 short
38
Modes of Inheritance
Indicates the patterns with which the mutant
phenotype is associated
Autosomal recessive
Autosomal dominant
X-linked recessive
X-linked dominant
Y-linked
Mitochondrial
Most common
39
Autosomal dominant inheritance
Heterozygotes exhibit the
affected phenotype.
Males and females are
equally affected and may
transmit the trait.
Affected phenotype does
not skip generation.
Comparison of autosomal dominant and
autosomal recessive inheritance
Autosomal Autosomal
dominant recessive
Males and females
affected?
Yes
Yes
Males and females
transmit the trait?
Yes
Yes
Trait skips generations?
No
Yes
At least one parent of
affected child must be
affected?
Yes
No
Law of independent assortment
Two genes on different chromosomes segregate
their alleles independently.
The inheritance of an allele of one gene does
not influence which allele is inherited at a
second gene.
Law of independent assortment
Independent assortment of two traits
In a dihybrid cross, parents with two
differing traits are crossed.
Which allele is dominant?
Heterozygous peas are round and yellow.
Therefore
round is dominant to wrinkled
yellow is dominant to green
Two traits segregating independently
Two traits segregating independently
315 round yellow peas
423
108 round green peas
416
101 wrinkled yellow peas
140
133
32 wrinkled green peas
3.18 : 1
2.97 :1
Probability
The likelihood that an event will occur.
•No chance of event probability = 0
(e.g. chance of rolling 8 on a six-sided die)
•Event always occurs probability = 1
(chance of rolling 1,2,3,4,5,or 6 on a six-sided die)
The probabilities of all the possible events add up to 1.
# on die
probability
1
1/6
2
1/6
3
1/6
4
1/6
5
1/6
6
1/6
The probability of an event
= # of chance of event
total possible events
Independent events
The probability of independent events is calculated
by multiplying the probability of each event.
In two rolls of a die, the chance of rolling the number 3 twice:
Probability of rolling 3 with the first die
= 1/6
Probability of rolling 3 with the second die = 1/6
Probability of rolling 3 twice = 1/6 x 1/6 or 1/36
Dependent events
The probability of dependent events is calculated
by adding the probability of each event.
In one roll of a die, what is the probability of rolling either
the number 5 or an even number?
Probability of rolling the number 5
= 1/6
Probability of rolling an even number = 3/6
Probability of rolling 5 or an even number = 1/6 + 3/6 or 4/6
Independent events
What is the chance of an
offspring having the
homozygous recessive
genotype when both parents
are doubly heterozygous?
Dependent events
Parents are heterozygous for a trait, R.
What is the chance that their child is carries
at least one dominant R allele?
Probability of child carrying RR = 1/4
Probability of child carrying Rr = 1/2
Probability of child carrying R_ = 1/4 + 1/2 = 3/4
Punnett Square
Used to help
solve genetics
problems
52
53
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
54
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.
55
RESULTS
9 Tall, Red flowered
3 Tall, white flowered
3 short, Red flowered
1 short, white flowered
Or: 9:3:3:1
56
Genes and Environment
Determine Characteristics
57
Monohybrid
Crosses
58
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
59
P1 Monohybrid Cross Review
 Homozygous dominant x Homozygous
recessive
 Offspring all Heterozygous
(hybrids)
 Offspring called F1 generation
 Genotypic & Phenotypic ratio is ALL
ALIKE
60
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
61
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
62
What Do the Peas Look Like?
63
…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
64
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
65
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
66
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
67
Practice Your Crosses
Work the P1, F1, and both
F2 Crosses for each of the
other Seven Pea Plant
Traits
68
Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round
r wrinkled
Y yellow
y green
RrYy
RY Ry rY ry
x
RrYy
RY Ry rY ry
All possible gamete combinations
69
Dihybrid Cross
RY
Ry
rY
ry
RY
Ry
rY
ry
70
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
Round/Yellow:
Round/green:
9
3
wrinkled/Yellow: 3
wrinkled/green:
1
9:3:3:1 phenotypic
ratio
71
Dihybrid Cross
Round/Yellow: 9
Round/green:
3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
72
Question:
How many gametes will be produced
for the following allele arrangements?
Remember: 2n (n = # of heterozygotes)
1. RrYy
2. AaBbCCDd
3. MmNnOoPPQQRrssTtQq
73
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
74
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
75
Summary of 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.
4. The two alleles for each trait separate
during gamete formation.
This now
called: Mendel's Law of Segregation
76
The elegance of mendel’s experiments
was partly due to the complete
consistency between his observation and
Hypotheses he developed.
However, after mendel’s work was
rediscovered, it became clear that
simple
medelian
model
did
not
adequately
predict
experimental
observations in all situations.
77
Pedigrees
symbolic representations of family relationships and
inheritance of a trait
Autosomal dominant inheritance of brachydactyly
Heterozygotes exhibit the phenotype.
Autosomal recessive inheritance of albinism
Heterozygotes carry the recessive allele
but exhibit the wildtype phenotype
Genetic predictions
Ellen’s brother Michael
has sickle cell anemia,
an autosomal recessive
disease.
What is the chance that
Ellen’s child has a sickle
cell anemia allele (a)?
Ellen Michael
?
Ellen and Michael’s parents
must be carriers.
A
a
A AA Aa
a
Aa aa
Ellen is not affected and
cannot carry aa genotype
chance Ellen is a carrier = 2/3
chance child inherits sickle
cell allele = 1/2
Overall chance child carries
sickle cell allele from Ellen =
2/3 x 1/2 = 1/3