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Patterns of Inheritance
(Mendelian Genetics)
Gregor Mendel
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1890’s
Central European Monk
Conducted research on pea plants
Used garden peas
Easy to grow
Can produce many generations quickly
Cross-pollination
Mendel’s experimental design
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Statistical analyses:
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Worked with large numbers of plants
counted all offspring
made predictions and tested them
Excellent experimentalist
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controlled growth conditions
focused on traits that were easy to score
chose to track only those characters that varied in
an “either-or” manner
GENE:
 the
unit of
heredity
 a section of
DNA sequence
encoding a
single protein
Genetic Vocabulary
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Character: a heritable feature, such as
flower color
Trait: a variant of a character, such as
purple or white flowers
Each trait carries two copies of a unit of
inheritance, one inherited from the mother
and the other from the father
Alternative forms of traits are called alleles
ALLELE:
Forms
of
genes
 Ex:
Purple
flowers vs.
white flowers
Mendel’s experimental design
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Mendel also made sure that he started his experiments with
varieties that were “true-breeding”
X
X
X
X
X
X
Genetic Vocabulary
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Generations:
 P = parental generation
 F1 = 1st filial generation, progeny of the P generation
 F2 = 2nd filial generation, progeny of the F1 generation
(F3 and so on)
Crosses:
 Monohybrid cross = cross of two different true-breeding
strains (homozygotes) that differ in a single trait.
 Dihybrid cross = cross of two different true-breeding
strains (homozygotes) that differ in two traits.
GENOTYPE
 the
specific
allelic
combination for
a certain gene
or set of genes
(PP, Pp, pp)
PHENOTYPE
 the
physical
appearance
of a
particular
trait (Purple,
White)
Phenotype vs Genotype
Phenotype
Genotype
Purple
PP
(homozygous)
Purple
3
1
Pp
(heterozygous)
2
Pp
(heterozygous)
Purple
1
Figure 14.6
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
1
Mendel’s Experimental Design
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Mendel mated 2 contrasting, true-breeding
varieties, a process called hybridization
True-breeding parents are called the P
generation
The hybrid offspring of the P generation are
called the F1 generation
When F1 individuals self-pollinate the F2
generation is produced
Mendel’s Observations
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When Mendel crossed true-breeding white and purple flowered pea plants all of
the offspring were purple
When Mendel crossed the F1 plants, many of the plants had purple flowers, but
some had white flowers
A ratio of about three to one, purple to white flowers, in the F2 generation
EXPERIMENT True-breeding purple-flowered pea plants
and white-flowered pea plants were crossed (symbolized
by ). The resulting F1 hybrids were allowed to selfpollinate or were cross- pollinated with other F1 hybrids.
Flower color was then observed in the F2 generation.
P Generation
(true-breeding
parents)

Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had
purple flowers
RESULTS Both purple-flowered plants and whiteflowered plants appeared in the F2 generation. In
Mendel’s experiment, 705 plants had purple flowers,
and 224 had white flowers, a ratio of about 3 purple :
1 white.
F2 Generation
Law of Segregation
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States that 2 alleles coding for the same
trait separate during gamete formation
 HOMOZYGOUS:
Having the
same alleles for a gene
(PP, pp)
 HETEROZYGOUS:
having
different alleles for a gene
(Pp)
DOMINANCE
An organism with a
dominant allele for
a particular form of
a trait will ALWAYS
have that form
(P = purple)
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RECESSIVE
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An organism with a
recessive allele for
a particular trait will
have that form only
when the dominant
allele is not
present
Practice
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•
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use 2 letters to represent genotype
A capital letter represents the dominant form of a
gene (allele) and a lowercase letter represents the
recessive form of the gene (allele).
Example below: P=dominant purple and p=
recessive white
The phenotype for this
flower is violet while
its genotype (if
homozygous) is PP.
The phenotype for this
flower is white while its
genotype is pp (must
have 2 of the recessive
copies of the allele).
Punnett Squares
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standard way of
working out what the
possible offspring of 2
parents will be
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helpful tool to show
allelic combinations and
predict offspring ratios
Before we go further lets review how to set up a
Punnett Square…
We begin by constructing a grid of two
perpendicular lines.
Next, put the genotype of one parent across
the top and the other along the left side.
For this example lets consider a genotype of BB crossed with
bb.
B
b
B
• Notice only one
letter goes above
each box
• It does not matter
which parent’s
genotype goes on
either side.
b
Fill in the boxes by copying the column & row
letters down and across into the empty spaces
B
B
b
Bb
Bb
b
Bb
Bb
W
w
W WW Ww
Usually write the
capital letter first
Lets say:
w Ww ww
W- dominant white
w- recessive violet
Parents in this cross are heterozygous (Ww).
Note: Make sure I can tell your capital letters from
lowercase letters.
What percentage of the offspring will have violet
flowers?
ANSWER: 25% (homozygous recessive)
Red hair (R) is dominant over blond hair (r). Make a
cross between a heterozygous red head and a
blond.
R
r
r
r
Rr
rr
Rr
rr
What percentage of the offspring will have red hair?
50%
Let’s try some more…
In pea plants, tall pea plants (T) are dominant
over short pea plants (t). Construct a Punnett
Square for a heterozygous tall pea plant and a
short pea plant.
t
t
T
t
Tt
tt
Tt
tt
What are the
percentage of
phenotypes?
50% tall
50% short
Black eyes (R) is dominant over red eyes (r)
in rats. Make a cross between a homozygous
rat with black eyes and a rat with red eyes.
r
r
R
Rr
Rr
R
Rr
Rr
What is the possibility of
a red eye off springs?
0%
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