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Chapter 5B
Gregor Mendel
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Famous pea plant study
Mendelian genetics
“Father of Genetics”
Father of Genetics
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Monk and teacher.
Experimented with purebred tall and
short peas.
Discovered some of the basic laws of
heredity.
Studied seven purebred traits in peas.
Called the stronger hereditary factor
dominant.
Called the weaker hereditary factor
recessive.
Presentation to the Science Society
in1866 went unnoticed.
He died in 1884 with his work still
unnoticed.
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His work rediscovered in 1900.
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Known as the “Father of
Genetics”.
Mendelian Genetics
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Began with 34 varieties of
pea seeds
Chose 7 sets of opposing
characteristics
Chart, page 113
Mendel’s Peas
Self-pollination
vs.
Cross-pollination
Mendel’s Observations
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He noticed that peas are easy to breed for
pure traits and he called the pure strains
purebreds.
He developed pure strains of peas for seven
different traits (i.e. tall or short, round or
wrinkled, yellow or green, etc.)
He crossed these pure strains to produce
hybrids.
He crossed thousands of plants and kept
careful records for eight years.
Mendel’s Peas
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In peas many traits appear in two forms (i.e. tall
or short, round or wrinkled, yellow or green.)
The flower is the reproductive organ and the
male and female are both in the same flower.
He crossed pure strains by putting the pollen
(male gamete) from one purebred pea plant on
the pistil (female sex organ) of another purebred
pea plant to form a hybrid or crossbred.
Mendel’s Results
Mendel crossed purebred tall plants with
purebred short plants and the first
generation plants were all tall.
When these tall offspring were crossed
the result was a ratio of 3 tall to 1 short.
Mendel’s Experiments
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He experimentally
crosses different
strains to develop
hybrids.
He then crossed
the hybrids and
analyzed the
results.
Dominant Traits RULE
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Strong Hereditary
traits cover weak
traits.
Mendal called
stronger traits
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DOMINANT
Mendal called
weaker traits
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recessive
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Dominant traits are
represented by
capital letters (T)
while recessive traits
are represented by
lower case letters (t).
try and follow the
diagram on the next
slide while keeping
the DOMINANT and
recessive letters in
mind. ( TT) (tt )
In the diagram above, the dominant allele is represented by
___and the recessive allele is represented by __ .
Mendelian Genetics
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P1: the original parent
generation
F1: first filial generation;
offspring of the P1 generation
F2: second filial generation;
offspring of the F1 generation
If we consider your parents to
be the P1 generation, which
generation are you?
1.
2.
3.
4.
P1
P2
F1
F2
The concept of unit
characteristics
Factors occur in pairs.
Genes occur in pairs because
diploid organisms have
1. daughter chromosomes.
2. homologous pairs of
chromosomes.
3. gametes.
The concept of
dominant &
recessive
Dominant Traits RULE
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Strong Hereditary
traits cover weak
traits.
Mendal called
stronger traits
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•
DOMINANT
Mendal called
weaker traits
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recessive
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Dominant traits are
represented by
capital letters (T)
while recessive traits
are represented by
lower case letters (t).
try and follow the
diagram on the next
slide while keeping
the DOMINANT and
recessive letters in
mind. ( TT) (tt )
Dominant trait
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A trait that is expressed
and masks the expression
of the other trait
Examples on page 113
Recessive trait
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A trait which, when in the
presence of a dominant
trait, is not expressed
Language of Genetics Problems
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Capital letters (T) =
dominant
Lowercase letters (t) =
recessive
A, a; B, b; R, r; etc.
The concept of
segregation
A cell forms gametes during
which process?
1.
2.
3.
4.
Mitosis
Meiosis
Cytokinesis
Fertilization
Monohybrid cross
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A genetic cross dealing with
only one set of
characteristics
Punnett square
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A diagram used to show the
possible gamete
combinations from a genetic
cross
PUNNETT SQUARE
CROSS T T X Tt
CONT’D
T
T
TT X Tt
T
t
T
T
T
t
T
T
T
t
Allele Types
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Homo - Same
Hetero - Opposite
Pheno – Physical
Geno - Genetic
Phenotype
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The physical expression of an
organism’s genes
Examples: tall, short, black
Genotype
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The genetic make-up of an
individual; the genes it has
Examples: Tt, AA, bb
Homozygous
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When both alleles in a cell
are the same
Examples: tt, TT, BB, bb
Heterozygous
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When both alleles in a
cell are NOT the same
Examples: Bb, Tt
Locus
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The site on a chromosome
where a particular gene is
located
Allele
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One of a pair of genes that has
the same position on
homologous chromosomes
Examples: T or t
Gene Expression
Are the following sentences true or
false?
- Homozygous organisms are true
breeding for a particular trait.
• False
- Plants with the same phenotype
always have the same genotype.
• False
Probability
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In Mendel’s model of segregation,
what was the ratio of tall plants to
short plants in the F2 generation?
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The ratio was 3 : 1.
Incomplete Dominance
Incomplete Dominance
• Both alleles are expressed,
but neither one is dominant.
• KEY: a blending of the traits
• Example:
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When red snapdragons are
crossed with white
snapdragons, the resulting
offspring are pink.
1st generation
w
C
w
C
r
C
r
w
CC
r
w
CC
r
C
r
w
CC
r
w
CC
2nd generation
r
C
r
C
w
C
Genotypic:
r
r
CC
r
w
CC
1:2:1
_________
Phenotypic:
w
C
1:2:1
CrCw CwCw _________
Human example
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Brachydactyly
Codominance
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Two alleles for a gene are
both expressed.
KEY: both alleles are
expressed with no blending.
Example:
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In horses, red hair + white
hair = roan (red and white
hairs).
Multiple Alleles
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One of several alleles can
be at a given locus
Example: human blood
types (A, B, AB, O)
Human Blood Types
Dominant Alleles
A
I
and
B
I
Recessive Allele
i
A
I
A
I
A
I
i
IB IB
IB i
A
B
I I
ii
type A blood
type A
type B
type B
type AB
type O
Suppose a woman who has
type AB blood marries a
man who is heterozygous
for blood type A.
What blood types might
their children have?
type AB x heterozygous type A
A
I
A
I
i
A
I
B
I
A
I
A
I
i
A
I
B
I
B
I
i
Dihybrid Crosses
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Genetic crosses dealing
with TWO characteristics
at the same time
Example: green/yellow
peas AND tall/short pea
plants
Mendel’s Concept of
Independent Assortment
• The segregation of one set
of alleles during gamete
formation is not affected by
another set.
Multiple Gene Interaction
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Sometimes two or more
genes working together
result in a single trait.
Examples: many human
traits such as hair color
and skin color
Sex-Linked Traits
Two Types of Chromosomes
• Autosomes
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non-sex-determining
chromosomes
humans = 22 pairs
Sex chromosomes
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XX = female
XY = male
Female = XX
Male = XY
X
X
X XX XX
Y XY
XY
Sex-Linked Traits
• Some genes are carried
on the sex chromosomes.
• The X and Y
chromosomes are not
homologous.
Sex-Linked Traits
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Males inherit more sex-linked
disorders because they only
have one gene for the trait.
If there is a defective gene on
X, there wouldn’t be a normal
gene on Y to counteract it.
Sex-Linked Traits
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Indicated with a superscript
above the X and Y
chromosomes
Example:
• XHXh
• XHY
Human Examples
• Red-green colorblindness
• Hemophilia
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Lack a blood chemical that
allows for blood clotting
H
X
H
X
normal female
XH Xh carrier female
A heterozygous female that
does not have the disease, but
she does carry the gene for the
trait
Xh Xh hemophiliac female
XH Y
normal male
Xh Y
hemophiliac male
carrier female x normal male
H
h
X
X
H
X
H
H
X X
H
h
X X
Y
H
X Y
h
X Y