Download Mendel's genetics

Genetics by Mendel
ONE GENE WITH TWO ALLELES
CONTROLING TWO
CONTRASTING/ ALTERNATIVE
FORMS OF A SPECIFIC TRAIT IS
CALLED MENDELELIAN
GENETICS
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Introduction
 Genetics is the study of genes.
 Inheritance is how traits, or characteristics,
are passed on from generation to
generation.
 Chromosomes are made up of genes,
which are made up of DNA.
 Genetic material (genes,chromosomes,
DNA) is found inside the nucleus of a cell.
 Gregor Mendel is considered “The Father
of Genetics“
 Mendel's work was not recognized until
the turn of the 20th century
Mendelian Genetics
 Dominant traits- traits that are expressed.
 Recessive traits- traits that are covered up.
 Alleles- the different forms of a characteristic.
 Punnett Squares- show how crosses are
made.
 Probability- the chances/ percentages that
something will occur.
 Genotype- the types of genes (Alleles)
present.
 Phenotype- what it looks like.
 Homozygous- two of the same alleles.
 Heterozygous- two different alleles.
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
lower case letter (r)
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Gregor Mendel
 Austrian Monk.
 Experimented with “pea plants”.
 Used pea plants because:
 They were available
 They reproduced quickly
 They showed obvious differences in the traits
Understood that there was something that
carried traits from one generation to the next
- “FACTOR”.
Gregor Mendel
(1822-1884)
Father of
genetics
Responsible for
the Laws
governing
Inheritance of
Traits
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Mendel cont……
In the mid-1800s, the rules underlying
patterns of inheritance were uncovered in
a series of experiments performed by an
Austrian monk named Gregor Mendel.
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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Mendel's Plant Breeding Experiments
Gregor Mendel was one of the first to apply
an experimental approach to the question
of inheritance.
For seven years, Mendel bred pea plants
and recorded inheritance patterns in the
offspring.
Particulate Hypothesis of Inheritance
Parents pass on to their offspring separate
and distinct factors (today called genes)
that are responsible for inherited traits.
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 selfpollinate several
generations
Can be artificially crosspollinated
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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 fertilisation
Self-fertilisation can occur
in the same flower
Cross-fertilisation 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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Mendel was fortunate he chose the Garden Pea
•Mendel probably chose to work
with peas because they are
available in many varieties.
•The use of peas also gave Mendel
strict control over which plants
mated.
•Fortunately, the pea traits are
distinct and were clearly
contrasting.
To test the particulate hypothesis, Mendel crossed truebreeding plants that had two distinct and contrasting traits—for
example, purple or white flowers.
What is meant by “true breeding?”
Mendel cross-fertilized his plants by hand. Why is it important to control
which plants would serve as the parents?
For each monohybrid cross, Mendel cross-fertilized true-breeding plants that
were different in just one character—in this case, flower color. He then allowed
the hybrids (the F1 generation) to self-fertilize.
Typical breeding experiment
P generation (parental
generation)
F1 generation (first filial
generation, the word filial
from the Latin word for
"son") are the hybrid
offspring.
Allowing these F1
hybrids to self-pollinate
produces:
F2 generation (second
filial generation).
It is the analysis of this
that lead to an
understanding of genetic
crosses.
Eight Pea Plant Traits WITH THEIR
CONTRASTING ALLELES
 Seed shape --- Round (R) or Wrinkled (r)
 Seed Colour ---- Yellow (Y) or Green (y)
s)
 Pod Shape --- Smooth (S) or wrinkled (




Pod Colour --- Green (G) or Yellow (g)
Seed Coat Colour ---Gray (G) or White (g)
Flower position---Axial (A) or Terminal (a)
Plant Height --- Tall (T) or Short (t)
p
 Flower colour --- Purple (P) or white ( )
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Mendel studies seven characteristics in the garden pea
:
Statistics indicated
a pattern.
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 colour
Dihybrid cross - cross involving
two traits
e.g. flower colour & plant height
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Punnett Square
Used to help
solve genetics
problems
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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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Mendel’s Experimental Results
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Did the observed ratio during the experiments 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 (see seed colour)
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Generations
 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

F1
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 SUMMARY
 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: Heterozygous Round seeds
seeds

Rr
x
Rr
F2
R
r
R
RR
Rr
r
Rr
rr
x Round
Genotype: RR, Rr, rr
Phenotype: Round &
wrinkled
G.Ratio: 1:2:1
P.Ratio: 3:1
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F1 Monohybrid SUMMARY
 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 test
crosses:
Homozygous dominant x Hybrid
Homozygous recessive x Hybrid
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F2 Monohybrid Cross (1st)
 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
R
r
r
Rr
Rr
r
rr
rr
x
Rr
Genotype: Rr, rr
Phenotype: Round &
Wrinkled
G. Ratio: 1:1
P.Ratio: 1:1
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F2 Monohybrid SUMMARY
 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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Genetic traits in humans can be
tracked through family pedigrees
 The inheritance of many human
traits follows Mendel’s principles
and the rules of probability
Figure 9.8A
 Family pedigrees are used to determine patterns of
inheritance and individual genotypes
Dd
Joshua
Lambert
Dd
Abigail
Linnell
D_?
Abigail
Lambert
D_?
John
Eddy
dd
Jonathan
Lambert
Dd
Dd
dd
D_?
Hepzibah
Daggett
Dd
Elizabeth
Eddy
Dd
Dd
Dd
dd
Female Male
Deaf
Figure 9.8B
Hearing
Genetic Practice Problems
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Breed the P1 generation
tall (TT) x dwarf (tt) pea plants
t
t
T
T
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Solution:
tall (TT) vs. dwarf (tt) pea plants
t
t
T
Tt
Tt
produces the
F1 generation
T
Tt
Tt
All Tt = tall
(heterozygous tall)
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Breed the F1 generation
tall (Tt) vs. tall (Tt) pea plants
T
t
T
t
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Solution:
tall (Tt) x tall (Tt) pea plants
T
t
T
TT
Tt
t
Tt
tt
produces the
F2 generation
1/4 (25%) = TT
1/2 (50%) = Tt
1/4 (25%) = tt
1:2:1 genotype
3:1 phenotype
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