Transcript Mitosis & Meiosis Processes Review and Dihybrid Crosses

Mitosis & Meiosis Processes
Review and Dihybrid Crosses using
Punnett Square Prediction
 Mitosis and meiosis have several key differences.
 The chromosome number is reduced by half in meiosis, but not
in mitosis.
 Mitosis produces daughter cells that are genetically identical to
the parent and to each other.
 Meiosis produces cells that differ from the parent and each
other.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
 Mitosis produces two identical daughter cells, but meiosis
produces 4 very different cells.
Fig. 13.8
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 13.8
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Di-Hybrid Crosses
 When two traits are involved in a cross, it is called a
di-hybrid cross.
Di-Hybrid Crosses
 If you want to make a cross of two traits on the same
chromosome; you would now need a 4 X 4 square with 16
different possible offspring.
 Here is an example of this larger punnett square.
•For the following crosses:
use these symbols.
•R = Red fur
r = white fur
X = Long fur
x = short fur
•Each parent will always pass a trait for fur
color, and a trait for fur length on to the
offspring.
Separating the Gametes
One Parent’s Genotype = RrXx
Use FOIL to separate the gametes.
1st gamete = RX (first two)
2nd gamete = Rx (outer two)
3rd gamete = rX (inner two)
4th gamete = rx (last two)
Example Problem
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Cross a male, heterozygous for both traits.
Genotype = RrXx
with a female that is also heterozygous for both traits.
Genotype = RrXx
Gametes for a
Di-Hybrid Cross
Father = RrXx
Possible Gametes are:
RX
Rx rX
rx
Mother = RrXx
Possible Gametes are:
RX
Rx rX
rx
 Punnett Square for two traits
RX
RX
Rx
rX
rx
Rx
rX
rx
• Punnett Square for two traits
RX
Rx
rX
rx
RX
RRXX
RRXx
RrXX
RrXx
Rx
RRXx
RRxx
RrXx
Rrxx
rX
rx
RrXX
RrXx
rrXX
rrXx
RrXx
Rrxx
rrXx
rrxx
Genotypic Ratios
RRXX= 1/16 RRXx= 2/16 RRxx=1/16
RrXX= 2/16 RrXx= 4/16 Rrxx=2/16
rrXX= 1/16 rrXx= 2/16
rrxx= 1/16
• Punnett Square for two traits
RX
Rx
rX
rx
RX
RRXX
RRXx
RrXX
RrXx
Rx
RRXx
RRxx
RrXx
Rrxx
rX
rx
RrXX
RrXx
rrXX
rrXx
RrXx
Rrxx
rrXx
rrxx
Phenotypic Ratios
• Red / Long = 9/16
• Red / short = 3/16
• white / Long = 3/16
• white / short = 1/16
Another Di-Hybrid Cross
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Cross a male homozygous dominant for both traits.
Genotype = RRXX
with a female who is homozygous recessive for both traits.
Genotype = rrxx
How many different gametes are possible for each parent?
Separating the Gametes
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Father = RRXX
Possible gametes = RX
Mother = rrxx
Possible gametes = rx
RX
rx RrXx
•100 % of the offspring will be
heterozygous for both traits and
100% will have Red, Long Fur
Another Di-Hybrid Cross
• Cross a male homozygous
dominant for fur color and
heterozygous for fur length.
• Genotype = RRXx
• With a female who is
homozygous recessive for fur
color and heterozygous for fur
length.
• Genotype = rrXx
• How many different gametes are
possible for each parent?
Separating the Gametes
Father = RRXx
Possible gametes = RX Rx
Mother = rrXx
Possible gametes = rX rx
RX
Rx
rX RrXX RrXx
rx RrXx Rrxx
Genotypic and Phenotypic
Ratios
• Genotypic Ratio
•1/4 RrXX
•1/2 RrXx
•1/4 Rrxx
• Phenotypic Ratio
•3/4 Red and Long Fur
fur
1/4 Red, short