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
Independent Assortment
Dihybrid cross Punnett square and the law of
independent assortment:
Example:
P1 GW GW GW GW
gw
X
gw
yellow, round green, wrinkled gw
GGWW ggww
gw
F1
All
yellow, round
GgWw
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Dihybrid cross con’t : Punnett square and the law of
independent assortment:
F1 F1
X GW Gw gW gw
All All GW
yellow, round yellow, round
Gw
GgWw GgWw
gW
gw
F2 9:3:3:1 Phenotypic ratio; Genotypic ratio as follows:
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
The Trihybrid Cross:
Trihybrid cross
Example:
Theoretical gene pairs represented by the symbols A, B, and C
P1 AABBCC X aabbcc
Gametes: ABC abc
F1 AaBbCc
Gametes:
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
2
The Forked-Line Method (branch diagram):
The Forked-Line Method (branch diagram):
*NOTE: We are assuming that independent assortment of these 3
gene pairs is a random process!
Mendel Rediscovered:
Why did Mendel’s work go unnoticed for so long?
Charles Darwin and Alfred Russel Wallace:
continuous variation – offspring were a blend of parents’ phenotypes
*1879 Walter Flemming
*early 20th century
*Hugo de Vries
*Karl Correns
*Erich Tcshermak
*1902 Walter Sutton and Theodor Boveri
Chromosomal Theory of Inheritance
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How do we account for genetic variation? Cross over:
*Independent assortment
*Crossing over
*Random fertilization
Independent Assortment:
Human Pedigrees
Pedigree
= Female
= Male
=Unknown
*Proband (p)
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