Download Review: Dihybrid cross- A genetic cross between two individuals

4/10/2012
Review:
*Unit factors in pairs - genetic characteristics are controlled by
unit factors that exist in pairs in individual organisms
*Principle of Dominance and Recessiveness - one factor is
dominant over the recessive one
*Law of Segregation - During gamete formation the paired unit
factors segregate randomly so that each gamete receives one or
the other
*Law of Independent Assortment – During gamete formation,
segregating pairs of unit factors assort independently of each other
Dihybrid cross- A genetic cross between two individuals
involving two characters
Example:
P1
GW
X
yellow, round
green, wrinkled
GGWW
ggww
GW
GW
GW
gw
gw
gw
F1
gw
All
yellow, round
GgWw
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F1
GW
F1
Gw
gW
gw
GW
X
Gw
All
yellow, round
All
yellow, round
GgWw
GgWw
gW
gw
9:3:3:1 Phenotypic ratio; Genotypic ratio as follows:
F2
9/16 yellow, round
1/16 GGWW, 2/16 GGWw, 2/16 GgWW, 4/16 GgWw
3/16 yellow, wrinkled
1/16 GGww, 2/16 Ggww
3/16 green, round
1/16 ggWw, 2/16 ggWw
1/16 green, wrinkled
1/16 ggww
Trihybrid cross
Example:
Theoretical gene pairs represented by the symbols A, B, and C
P1
Gametes:
AABBCC
aabbcc
abc
ABC
F1
Gametes:
X
AaBbCc
ABC ABc AbC Abc
aBC
aBc
abC
abc
The Forked-Line Method (branch diagram):
Recall:
*The F1 that result from a monohybrid cross (AA x aa) all have the
genotype Aa and the phenotype represented by A
*The F2 that result from a cross between 2 individuals from the F1, have a
phenotypic ratio of 3:1
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The Forked-Line Method (branch diagram):
*NOTE: We are assuming that independent assortment of these 3 gene
pairs is a random process!
How do we account for genetic variation?
Cross over:
*Independent assortment
*Crossing over
*Random fertilization
Independent Assortment:
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Chi-Square Analysis:
p value (probability): consider as a percentage (i.e. 0.05 = 5%)
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