Genetics

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Genetics
Dihybrid Crosses
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Pea Plants
Seed Color
Height
Tall TT, Tt Short tt
Yellow YY, Yy Green yy
Lets cross a homozygous tall (TT), homozygous
yellow seed (YY) plant with a short (tt), green
seed (yy) plant.
TTYY x ttyy
These are the genotypes of the two plants.
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Homozygous?
Seed Color
Homozygous means that both genes for a trait are
either DOMINANT or recessive.
Height
Tall TT, Tt Short tt
Yellow YY, Yy Green yy
Lets cross a homozygous tall (TT), homozygous
yellow seed (YY) plant with a short (tt), green
seed (yy) plant.
TTYY x ttyy
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Independent Assortment
Mendels principle of Independent Assortment
states that genes for different traits can
segregate independently during the formation of
gametes (eggs sperm in animals, eggs and pollen
in plants).
TTYY
First T
with first Y
T
Y
Gamete 1
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Independent Assortment
Mendels principle of Independent Assortment
states that genes for different traits can
segregate independently during the formation of
gametes (eggs sperm in animals, eggs and pollen
in plants).
TTYY
First T
with second Y
TY
T
Y
Gamete 2
Gamete 1
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Independent Assortment
Mendels principle of Independent Assortment
states that genes for different traits can
segregate independently during the formation of
gametes (eggs sperm in animals, eggs and pollen
in plants).
TTYY
Second T
with first Y
TY
TY
T
Y
Gamete 2
Gamete 3
Gamete 1
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Independent Assortment
Mendels principle of Independent Assortment
states that genes for different traits can
segregate independently during the formation of
gametes (eggs sperm in animals, eggs and pollen
in plants).
TTYY
Second T
with second Y
TY
TY
TY
Y
T
Gamete 2
Gamete 3
Gamete 4
Gamete 1
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Dihybrid Punnett Square
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Dihybrid Punnett Square
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Dihybrid Punnett Square
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Dihybrid Punnett Square
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Dihybrid Punnett Square
Genotype ratio TtYy - 16/16
Phenotype ratio Tall, yellow - 16/16
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Dihybrid Punnett Square
Lets cross two of the plants from the F1
generation.
We need to pair up the genes which can be given
to each gamete (egg and pollen).
x
T
Y
T
y
t
Y
y
t
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Dihybrid Punnett Square
Both the plants can give the same gene
combinations to their gametes, so the pairs along
the top and down the side are the same.
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Dihybrid Punnett Square
When you pair up the gametes from the two plants,
always put like letters together and within the
like letters, put the CAPITAL letter in front of
the lowercase letter.
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Dihybrid Punnett Square
Your Turn!!
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Dihybrid Punnett Square
F2 generation
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Dihybrid Punnett Square
Genotype and phenotype ratios?
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Genotype Ratio
TTYY - 1 TTYy - 2 TtYY - 2 TtYy - 4 TTyy - 1 Ttyy
- 2 ttYY - 1 ttYy - 2 ttyy - 1
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Phenotype Ratio
Tall, Yellow - 9
TTYY - 1 TTYy - 2 TtYY - 2 TtYy - 4 TTyy - 1 Ttyy
- 2 ttYY - 1 ttYy - 2 ttyy - 1
Tall, Green - 3
Short, Yellow - 3
Short, Green - 1
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Dihybrid Punnett SquareHomework
Question 3 on Rats Practice Problems and
questions 2 3 on Foxes and Watermelons Practice
Problems.