Transcript A genotype of Pp

BASIC GENETICS
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Gregor Mendel
(1822-1884)
Austrian monk
Studied science and math
High school teacher and gardener
Experimented on pea plants
Gregor Mendel
Father of Genetics
We credit Mendel for forming the basis of genetics.
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A zygote inherits traits from
both parents.
Heredity is the passing of traits or
characteristics from parents to offspring.
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Sexual Reproduction
Haploid
egg
(gamete)
Haploid
sperm
(gamete)
1n
+
1n
Diploid
zygote
=
2n
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So how can we predict
what the offspring will
look like?
?
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Through Probability!!!
Probability:
is the likelihood that a specific event will happen.
helps understand past events and the possibility
of future events.
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To find the probability of an event…
 compare the number of times a certain outcome can
occur to the total number of possible outcomes.
 write it as a fraction.
number of times one outcome is likely to occur
Probability =
total number of all possible outcomes
What are the possible outcomes
when you flip a coin?
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What is the likelihood that a flipped
coin will land heads up?
Equal chance of 2 possible outcomes:
“heads”
or
“tails”
“Heads” is one possible outcome out
of a total of 2 possible outcomes.
So…
What is the probability of flipping a coin and
it landing heads up?
1
Probability =
2
What does this have to do with genetics?
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We use probability to predict possible
outcomes of genetic crosses.
Genetic crosses involve 2 independent events
Because:
Alleles contributed by one parent
Do not depend on
P
Pp
Pp
p
Alleles contributed by the other
pp
So…
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We combine both probabilities.
We multiply the separate probabilities
of the two events.
What is the probability of two heterozygous
purple individuals (Pp x Pp) producing a
white offspring (pp)?
1
2
“p” from one parent = 2
?
“p” from other parent = 1
Pp
1
2
1 X 1 = 1
2
2
4
The probability of these parents producing
a white offspring ?
One chance in four
1
4
?
p
?
pp
p
Pp
1
4
1
2
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Probability using Pedigrees
and Punnett Squares
Pedigrees and Punnett squares
are scientific tools used to…
predict possible outcomes from genetic crosses.
simplify analysis of genetic probabilities.
P
P
p
PP
Pp
p
Pp
pp
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Pedigree
A chart of a family's history
showing relationships and how a
trait or disease has been
inherited over many generations
Unknown Gender
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Pedigree Analysis
Squares represent
Males.
Vertical lines and
brackets show
birth relationships.
Circles represent
Females.
Horizontal lines show
mating relationships.
Half-shaded circle or
square represents a
carrier of the trait.
Shaded circles or squares
show a person
expressing the trait.
Unshaded circles or squares
show a person that does not
express the trait.
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Punnett Squares
P
P
p
PP
Pp
p
Pp
pp
A diagram that shows the possible
gene combinations that might
result from a genetic cross
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Punnett Squares
P
P
p
PP
Pp
p
Pp
pp
The letters in a Punnett square
stand for alleles (one of a number
of different forms of a gene).
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We call the actual genetic make-up
of an organism its genotype.
Genotype: PP
Genotype: pp
Genotype: Pp
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We call the appearance of an
organism its phenotype.
Genotype: PP
Phenotype: Purple
Genotype: pp
Phenotype: White
Genotype: Pp
Phenotype: Purple
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The genotype
(the genes)
is represented by letters
such as Pp.
P
P
p
PP
Pp
The phenotype
(like a photograph)
is represented by a
description such as purple.
p
Pp
pp
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Offspring can be:
Homozygous
for a trait
or
PP
An organism that has
two identical alleles for
a particular trait
Heterozygous
for a trait
Pp
An organism that has
two different alleles
for the same trait
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More Examples…
Homozygous Dominant
Heterozygous
Genotype: PP
Genotype: Pp
Phenotype: Purple flowers
Phenotype: Purple flowers
Homozygous Recessive
Genotype: pp
Phenotype: White flowers
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Dominant and Recessive
Alleles
When 2 different alleles for the same trait occur together,
one may be expressed while the other is not expressed.
Some traits are…
dominant
while others are
recessive
Homozygous Dominant
Homozygous Recessive
Genotype: PP
Genotype: pp
Phenotype: Purple flowers
Phenotype: White flowers
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Dominant Alleles
The dominant trait will be expressed if a
dominant allele is present.
 Dominant alleles “overpower” recessive
alleles.
If there is a dominant allele and a
recessive allele, the dominant trait will
be the one that is expressed.
Example: A genotype of Pp (Purple/white)
shows a phenotype of Purple.
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Recessive Alleles
The recessive trait is expressed only when
the dominant allele is not present.
The allele that is overshadowed
by a dominant allele
Expressed only if both alleles
are recessive
Example: A genotype of pp (white/white)
shows a phenotype of white.
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Dominant and Recessive
Alleles
Pea plants have purple and white alleles for
flower color.
The allele for purple flowers is dominant
and the allele for white flowers is recessive.
If the allele for purple flowers is present, the
plant will produce purple flowers.
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Complete Dominance
2 Heterozygous
Parents
(Genotype Pp)
(Phenotype Purple)
would produce
P
P
PP
p
Pp
1/4 (25%) Homozygous
Offspring
(Genotype PP)
(Phenotype Purple)
p
Pp
pp
2/4 (50%) Heterozygous
Offspring
(Genotype Pp)
(Phenotype Purple)
1/4 (25%) Homozygous
Offspring
(Genotype pp)
(Phenotype White)25
Complete Dominance
2 Heterozygous
Parents
(Genotype Pp)
(Phenotype Purple)
would produce
P
P
PP
p
Pp
1/4 (25%) Homozygous
Offspring
(Genotype PP)
(Phenotype Purple)
p
Pp
pp
2/4 (50%) Heterozygous
Offspring
(Genotype Pp)
(Phenotype Purple)
1/4 (25%) Homozygous
Offspring
(Genotype pp)
(Phenotype White)26
Complete Dominance
2 Heterozygous
Parents
(Genotype Pp)
(Phenotype Purple)
would produce
P
P
PP
p
Pp
1/4 (25%) Homozygous
Offspring
(Genotype PP)
(Phenotype Purple)
p
Pp
pp
2/4 (50%) Heterozygous
Offspring
(Genotype Pp)
(Phenotype Purple)
1/4 (25%) Homozygous
Offspring
(Genotype pp)
(Phenotype White)27
Complete Dominance
2 Heterozygous
Parents
Genotype Pp
Phenotype Purple
would produce
P
P
PP
p
Pp
1/4 (25%) Homozygous
Offspring
(Genotype PP)
(Phenotype Purple)
p
Pp
pp
2/4 (50%) Heterozygous
Offspring
(Genotype Pp)
(Phenotype Purple)
1/4 (25%) Homozygous
Offspring
(Genotype pp)
(Phenotype White)28
Let’s try some examples with Mendel’s pea plants.
Pea pod color
Genotype
Phenotype
GG
Green
Gg
Green
gg
Yellow
This is complete dominance because yellow is hidden29
in a heterozygous genotype.
A cross of a homozygous green pea plant and a
heterozygous green pea plant would yield:
G
g
G
GG
Gg
G
GG
Gg
all offspring with a phenotype of green, but
½ (50%) heterozygous (Gg) and
½ (50%) homozygous (GG).
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A cross of a homozygous yellow pea plant and a
heterozygous green pea plant would yield:
G
g
g
Gg
gg
g
Gg
gg
½ (50%) offspring with a phenotype of green, genotype Gg
and ½ (50%) with a phenotype of yellow, genotype gg.
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Let’s add another trait.
Pea seed shape
Genotype
Phenotype
RR
Round
Rr
Round
rr
Wrinkled
This is complete dominance because “wrinkled” is
hidden in a heterozygous genotype.
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A cross of two pea plants, both with heterozygous
green seed pods and heterozygous round seeds
would yield:
GgRr
GgRr
GR
Gr
gR
gr
GR
GGRR
GGRr
GgRR
GgRr
Gr
GGRr
GGrr
GgRr
Ggrr
gR
GgRR
GgRr
ggRR
ggRr
gr
GgRr
Ggrr
ggRr
ggrr
There is only one chance in 16 that both recessive
traits will be expressed.
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Incomplete Dominance
Two
homozygous
parents with
different
phenotypes
Produce a
heterozygous
offspring with
a blended
phenotype
red + white
rr+ ww
= pink
= rw
BUT THE ALLELES REMAIN DISTINCT; ONLY THE
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PHENOTYPE APPEARS BLENDED.
Incomplete Dominance
r
r
w
rw
rw
w
rw
rw
Crossing homozygous parents to produce F1 generation
THE ALLELES REMAIN DISTINCT; ONLY THE
PHENOTYPE APPEARS BLENDED.
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Incomplete Dominance
r
w
r
rr
rw
w
rw
ww
Crossing heterozygous F1 generation to produce F2 generation
THE ALLELES REMAINED DISTINCT; THE
ORIGINAL PHENOTYPES REAPPEAR IN THE F2
GENERATION
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Codominance
Both alleles in the
heterozygote express
themselves fully.
Example: Blood types
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Codominance
A person
homozygous for
Type A blood
And a person
homozygous for
Type B blood
Will produce a child
that will demonstrate
both Type A and Type
B blood (Type AB)
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In summary:
Incomplete
Dominance
Traits are
blended
(red + white =
pink)
Codominance
Both traits are
expressed
(A + B = AB)
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Pleiotropy
A single gene affects
more than one trait.
For example, sickle-cell disease results from one
gene, but it has numerous effects on the body.
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Polygenic
Traits
A trait controlled by more than
one gene.
Eye color is an example of a polygenic trait.
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In summary:
Pleitrophy
Single gene
affects more
than one trait.
Polygenic Trait
Single trait
controlled by
more than one
gene.
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Carrier
An individual who carries a recessive trait…
is heterozygous for the trait.
does not express the trait.
can pass the trait to offspring.
For Example:
C
c
X X
This female is heterozygous for
colorblindness.
She has normal vision.
She can pass the trait on to her
son who will be colorblind.
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Without sexual reproduction,
we would have very little genetic variation.
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With sexual reproduction, there is great variety in
the appearance of offspring.
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