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11-1 The Work of Gregor Mendel
Gregor Mendel’s Peas
Gregor Mendel’s Peas
Genetics is the scientific study of heredity.
Gregor Mendel was an Austrian monk. His work
was important to the understanding of heredity.
Mendel carried out his work with ordinary garden
peas.
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11-1 The Work of Gregor Mendel
Gregor Mendel’s Peas
Mendel knew that flowers
reproduce asexually one parent
• the male part of each
flower produces
pollen, (containing
sperm).
• the female part of
the flower produces
egg cells.
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11-1 The Work of Gregor Mendel
Gregor Mendel’s Peas
During sexual reproduction, sperm and egg cells join
in a process called fertilization - producing a new
cell (ZYGOTE)
The sperm and egg cells (gametes) were produced
during MEIOSIS - each containing ONE set of
chromosomes (23 chromosomes fro humans)
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11-1 The Work of Gregor Mendel
Gregor Mendel’s Peas
Pea flowers are self-pollinating.
Sperm cells in pollen fertilize the egg cells in the
same flower.
The seeds that are produced by self-pollination
inherit all of their characteristics from the single
plant that bore them (true-breeding)
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11-1 The Work of Gregor Mendel
Gregor Mendel’s Peas
Mendel wanted to produce seeds by joining male
and female reproductive cells from two different
plants.
He cut away the pollen-bearing male parts of the
plant and dusted the plant’s flower with pollen from
another plant - cross pollination.
*Producing seeds that
had two different
parents.
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11-1 The Work of Gregor Mendel
Genes and Dominance
Each original pair of plants is the P (parental)
generation.
The offspring are called the F1, or “first filial,”
generation.
The offspring of crosses between parents with
different traits are called hybrids.
The F2 generation results from self-pollination of the
F1 generation.
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11-1 The Work of Gregor Mendel
Genes and Dominance
Mendel's conclusion - biological inheritance is
determined by factors that are passed from one
generation to the next (from parents to offspring).
Today, scientists call the factors that determine
traits genes.
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11-1 The Work of Gregor Mendel
Genes and Dominance
Genes and Dominance
A trait is a specific characteristic that varies from one
individual to another.
A Gene is a unit of hereditary information that determines a
specific trait. Alternative forms of a gene are called alleles.
Dominant alleles (uppercase letter) - fully expressed
Recessive alleles (lowercase letter) - no noticeable effect
on the organism’s appearance.
Allele combinations - every trait has TWO alleles
DD - homozygous dominant
Dd - heterozygous
Dd - homozygous recessive
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Genotype and phenotype
Phenotype, or physical characteristics.
EXAMPLES: Tall, yellow, round
Genotype, or genetic makeup.
EXAMPLES: TT, Tt, tt
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11-1 The Work of Gregor Mendel
Genes and Dominance
Mendel’s second conclusion is called the principle of
dominance.
*The principle of dominance states that some alleles are
dominant and others are recessive.
An organism with a dominant allele for a trait will always
exhibit that form of the trait.
An organism with the recessive allele for a trait will
exhibit that form only when the dominant allele for that trait is
not present.
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11-1 The Work of Gregor Mendel
Punnett squares
Punnett Squares
Punnett squares can be used
to predict and compare the
genetic variations that will
result from a cross.
Gametes produced by each F1
parent are shown along the top
and left side.
USE only ONE letter - usually
the first letter of the name of
the dominant trait
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11-1 The Work of Gregor Mendel
Monohybrid cross
MONOHYBRID CROSS - ONE TRAIT
F1 RESULTS
T (tall)
T (tall)
Phenotypic
ratio = 3:1
(always the
same for this
cross)
t (short)
Genotypic
ratio = 1:2:1
t (short)
(always the
same for this
cross)
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Segregation
Results: parental generation (P) producing the F1
generation = Expresses ALL dominant
Were all of the recessive traits gone??
NO!
WHY? - Because of the Law of Segregation
PROOF - Mendel crossed the F1 generation with itself (selfpollination) to produce the F2 (second filial) generation.
The traits controlled by recessive alleles reappeared in one
fourth of the F2 plants.
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Segregation
•The trait controlled by the recessive allele showed up in
some of the F2 plants.
•The reappearance of the trait controlled by the recessive
allele indicated that at some point the allele for shortness
had been separated, or segregated, from the allele for
tallness.
• Mendel suggested that the alleles for tallness and
shortness in the F1 plants segregated from each other
during the formation of the sex cells, or gametes - during
MEIOSIS
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11-1 The Work of Gregor Mendel
probability
Probabilities Predict Averages
• Probabilities predict the average outcome of a
large number of events.
• In genetics, the larger the number of offspring,
the closer the resulting numbers will get to
expected values.
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Independent assortment
Independent Assortment
To determine if the segregation of one pair of alleles
affects the segregation of another pair of alleles,
Mendel performed a two-factor cross - TWO traits
A Dihybrid Cross
The alleles for round (R) and yellow (Y) are dominant
over the alleles for wrinkled (r) and green (y).
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11-1 The Work of Gregor Mendel
Dihybrid cross
A Round Yellow plant (dominant) crossed with a
wrinkled green plant (recessive)
PARENTAL GENERATION CROSS - PRODUCE F1
RESULTS: F1 generation
*ALL ROUND and
YELLOW (Dominant)
F1 Cross to produce F2
Self pollinate (fertilize)
RrYy x RrYy
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11-1 The Work of Gregor Mendel
9:3:3:1
The Punnett square
predicts a 9 : 3 : 3 :1
ratio in the F2
generation
(phenotypic ratio)
* Always the same for
this cross
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11-1 The Work of Gregor Mendel
RESULTS Dihybrid
In Mendel’s experiment, the F2 generation produced
the following:
• some seeds that were round and yellow
• some seeds that were wrinkled and green
• some seeds that were round and green
• some seeds that were wrinkled and yellow
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11-1 The Work of Gregor Mendel
Independent assortment
This principle is known as independent assortment.
Genes that segregate independently do not
influence each other's inheritance.
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Summary - Mendel
A Summary of Mendel's Principles
Genes (traits) are passed from parents to their offspring.
Principle of Dominance - some forms of the gene may be
dominant and others may be recessive.
Genes are segregated from each other when gametes are
formed (Principle of segregation)
The alleles for different genes usually segregate
independently of one another - Principle of independent
assortment
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11-1 The Work of Gregor Mendel
Incomplete dominance
Incomplete Dominance
• When one allele is not
completely dominant
over another it is called
incomplete
dominance.
• The heterozygous
phenotype is between
the two homozygous
phenotypes. ALL are
represented with
upper case letters.
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11-1 The Work of Gregor Mendel
Codominance
ABO blood group - CODOMINANCE
• In codominance, both alleles contribute to the phenotype.
• Examples:
• A chicken with black and white feathers
• Blood type - AB
• There are three alleles for this gene, IA, IB, and i
(recessive)
• 1. Alleles IA and IB are codominant.
2. Individuals with alleles IAIA or IAi produce only
the A antigen, making them blood type A.
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ABO
•The Rh blood group is determined by a single gene
with two alleles—positive and negative.
•The positive (Rh+) allele is dominant, so individuals
who are Rh+/Rh+ or Rh+/Rh are said to be Rhpositive.
•Individuals with two Rh- alleles are said to be Rhnegative.
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exceptions
Polygenic, Multiple alleles, Sex-linked
• Polygenic - Characteristics controlled by more than one
gene; hair color, height, skin color.
• Genes that are controlled by more than two alleles are
said to have multiple alleles; eye color (blue, green, and
brown)
• Linked genes - genes located close together on the same
chromosome
• Sex-linked - any gene located on the sex chromosome
More than 100 sex-linked genetic disorders have now
been mapped (located) to the X chromosome.
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Sex-linked
Sex-linked disorders are more common in males than
females
WHY????
For a recessive allele to be expressed in females, there
must be two copies of the allele, one on each of the two X
chromosomes.
Males have just one X chromosome. Thus, all X-linked
alleles are expressed in males, even if they are recessive.
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colorblindness
Possible Inheritance of Colorblindness Allele
Carrier - carries the
allele that contributes
to the disorder, but
DOES NOT have the
disorder.
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