MENDEL’S GENETICS
WHO IS HE?
Gregor Mendel
Ordained priest
Father of modern
genetics
GREGOR MENDEL
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Raised on farm and understood the value of plant
breeding.
At 21, entered priesthood and studied plant breeding
in a monastery in the Czech Republic.
Loved to read especially about natural sciences and
was aware of Darwin’s findings.
Studied the inheritance of traits in pea plants.
WHAT DID HE DO?
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Research with pea plants
Developed ideas of dominance and trait
segregation
One of the first scientists to use the
scientific method and mathematical
models
MENDEL CHOSE PEAS WISELY
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• Pea plants are good for genetic research
– available in many varieties with distinct heritable
features with different variations
• flower color, seed color, seed shape, etc.
Mendel had strict control over which plants mated
with which
• each pea plant has male & female structures
• pea plants can self-fertilize
• Mendel could also cross-pollinate plants: moving pollen
from one plant to another
MENDEL CHOSE PEAS LUCKILY
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• most characters are controlled by a single
gene
• each gene has only 2 alleles, one of which is
completely dominant to the other
HIS STUDIES
•
Observed peas
to determine
what traits were
inherited
He noticed at
least 7
characteristics
that appeared to
be inherited
Characters
investigated
•
by Mendel
VOCABULARY
Character –heritable feature
 Trait – each variant for a character
 True-breeding – plants that self-pollinate all
offspring are the same variety
 Monohybrid cross – a cross that tracks the
inheritance of a single character
 P generation – (parental) true-breeding
 F1- (first filial) offspring of P generation
 F2 – (second filial) offspring from F1 cross
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VOCABULARY (CONTINUED)
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Allele- alternate version of a gene
Dominate allele – expressed in the heterozygote
Recessive allele – not expressed in the heterozygote
Homozygote – pair of identical alleles for a character
 Homozygous dominant- BB
 Homozygous recessive - bb
Heterozygote – two different alleles for a character (Bb)
Genotype – genetic makeup
Phenotype – appearance of an organism
Mendel crossed purebred plants with opposite
forms of a trait. He called these plants the
parental generation , or P generation. For
instance, purebred tall plants were crossed
with purebred short plants.
Parent Short
P generation
Parent Tall
P generation
Mendel observed that all of the offspring grew
to be tall plants. None resembled the short
parent. He called this generation of offspring
the first filial , or F1 generation, (The word
filial means “son” in Latin.)
Parent Short
Offspring Tall
P generation
F1 generation
Parent Tall
P generation
EXPERIMENT 1 (PART 2)
Mendel then crossed two of the offspring tall plants (F1)
produced from his first experiment.
F1 Tall
F1 Tall
¾ Tall & ¼ Short
F2 Generation
Mendel called this second generation of plants the second filial, F2,
generation. To his surprise, Mendel observed that this
generation had a mix of tall and short plants. This occurred even
though none of the F1 parents were short.
THE PUNNETT SQUARE
Mendelian Genetics
Classical Punett's Square is a way to determine ways traits can segregate
parental P0 cross
a
a
A
Aa
Aa
F1 cross
A
Aa
Aa
A
AA
Aa
A
a
a
Aa
aa
backcross: mating F1 generation to the parental (usually recessive) strain
A
a
a
Aa
aa
a
Aa
aa
in this backcross, F2 generation gives 50% (1:1) recessive offspring
test cross: mating to a homozygous recessive individual
LAW OF SEGREGATION
states that allele pairs separate or segregate during gamete formation, and
randomly unite at fertilization. There are four main concepts involved in
this idea. They are:
1. There are alternative forms for genes. This means that a gene can exist in
more than one form. For example, the gene that determines pod color can
either be (G) for green pod color or (g) for yellow pod color.
2. 2. For each characteristic or trait organisms inherit two alternative forms of
that gene, one from each parent. These alternative forms of a gene are
called alleles
LAW OF SEGREGATION
3. When gametes (sex cells) are produced, allele pairs separate or segregate
leaving them with a single allele for each trait. This means that sex cells
contain only half the compliment of genes. When gametes join during
fertilization the resulting offspring contain two sets of alleles, one allele
from each parent. For example, the sex cell for the green pod plant had a
single (G) allele and the sex cell for the yellow pod plant had a single (g)
allele. After fertilization the resulting F1 plants had two alleles (Gg).
4. When the two alleles of a pair are different, one is dominant and the other
is recessive.
CROSS-POLLINATION OF PUREBREAD
PLANTS
cross-pollination between
true breeding green and
yellow pods
- all F1 green
F1 Generation
Gg =heterozygous
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When Mendel performed cross-pollination between a
true-breeding yellow pod plant and a true-breeding
green pod plant, he noticed that all of the resulting
offspring, F1 generation, were green.
F2 GENERATION
- self-pollination of
green F1 plants
- ¾ in F2 green,
¼ yellow
- 3 : 1 ratio in pod
colour in F2
G = dominant = green
g = recessive = yellow
GG, gg =homozygous
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He then allowed all of the green F1
plants to self-pollinate. He referred
to these offspring as the F2
generation. Mendel noticed a 3:1
ratio in pod color. About 3/4 of the
F2 plants had green pods and
about 1/4 had yellow pods. From
these experiments Mendel
formulated what is now known as
Mendel's law of segregation.
SEED COLOUR
C = dominant = yellow
c = recessive = green
INHERITANCE OF PEA COLOUR
phenotype:
genotype:
ALL THE REST…
LAW OF INDEPENDANT ASSORTMENT
This law states that allele pairs separate independently during the formation of
gametes. Therefore, traits are transmitted to offspring independently of
one another.
Mendel performed dihybrid crosses in plants that were true-breeding for two
traits. For example, a plant that had green pod color and yellow seed color
was cross-pollinated with a plant that had yellow pod color and green
seeds. In this cross, the traits for green pod color (GG) and yellow seed
color (YY) are dominant. Yellow pod color (gg) and green seed color (yy) are
recessive. The resulting offspring (or F1 generation were all heterozygous
for green pod color and yellow seeds (GgYy).
LAW OF INDEPENDANT ASSORTMENT
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Mendel performed dihybrid
crosses in plants that were truebreeding for two traits. For
example, a plant that had green
pod color and yellow seed color
was cross-pollinated with a plant
that had yellow pod color and
green seeds. In this cross, the
traits for green pod color (GG) and
yellow seed color (YY) are
dominant. Yellow pod color (gg)
and green seed color (yy) are
recessive. The resulting offspring
(or F1 generation were all
heterozygous for green pod color
and yellow seeds (GgYy).
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After observing the results of the
dihybrid cross, Mendel allowed all
of the F1 plants to self-pollinate.
He referred to these offspring as
the F2 generation. Mendel noticed
a 9:3:3:1 ratio About 9 of the F2
plants had green pods and yellow
seeds, 3 had green pods and
green seeds, 3 had yellow pods
and yellow seeds and 1 had a
yellow pod and green seeds.
OK, LETS RECAP…
LAW OF INDEPENDENT ASSORTMENT
Allows for new gene combinations or genetic
recombination
 Can mathematically predict the possible
combinations
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 Number
of possible genotypes = 2n
where n = the number of genes or traits considered
 Example: considering 100 traits:
 2100
= 1.26765 x 1030
LET’S TRY SOME
Cross
TT X tt
Tt X Tt
TT X TT
tt X tt
TT X Tt
Tt X tt
Genotype
Phenotype
USING PUNNETT SQUARES TO PREDICT
THE INHERITANCE OF SICKLE CELL
ANEMIA
SICKLE CELL ANEMIA
Due to presence of recessive allele denoted
by HbS.
 Causes distortion of red blood cells into long,
thin sickles.
 Causes pain, tissue damage, and cells to
rupture prematurely.
 Carriers: have heterozygous genotypes and
do not manifest symptoms.
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PUNNETT SQUARES
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Used to predict
possible offspring
genotypes
Place alleles for each
parent on each side
HbS
HbA
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HbA/ HbA
x HbS/ HbS
HbA
Mother’s genotype
Father’s genotype
HbS
PREDICT INHERITANCE
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Given parents’ genotypes,
you can predict offspring’s
genotypes and phenotypes
HbS/ HbS = homozygous
recessive results in sickle
cell anemia
Both HbA/ HbS
(heterozygous) and HbA/
HbA (homozygous
dominant) are normal.
MENDEL’S LAWS
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Law of Independent
Assortment: Alleles of
one gene are passed
to offspring
independently of the
alleles of other genes.
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Applies to the
inheritance of two or
more genes
simultaneously.
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Can use the inheritance of traits in fruit flies to
illustrate the law of independent assortment.
Dihybrid cross
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A fertilization in which the parents differ in two
distinct traits or characteristics.
WHY AREN’T MEMBERS OF THE SAME
SPECIES IDENTICAL?
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Sources of Genetic Variation
 Law
of Independent Assortment
 Mutation
MUTATIONS
Definition = change in DNA sequence resulting
the appearance of a new allele
 Can occur at any time, but not always beneficial
 Some can be harmful and result in death
 Some can result in the organism having traits
that make them better suited to the
environment
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CO-DOMINANCE
When an organism has two different alleles for
a gene that does not follow the
dominant/recessive pattern
 The organism shows a trait that is a blend of
the traits represented by the two alleles
 Also called INCOMPLETE DOMINANCE
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For example:
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The gene for the color of some flowers has one
allele for red and one for white. When both
alleles are present, neither is dominant, and
the flower color is pink
INCOMPLETE AND CODOMINANCE
INCOMPLETE DOMINANCE
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Pattern of
inheritance in which
alleles from both
parents are blended
CODOMINANCE
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Pattern of
inheritance in which
both alleles of a gene
are expressed