Transcript Dihybrid Crosses

Dihybrid Crosses
When is it used?
• A dihybrid cross is exploring the possibilities of
TWO traits. For example, what will the
offspring look like when a yellow, wrinkled pea
plant is crossed with a green, smooth pea
plant. You are looking at two traits- pea color
and pea texture.
Steps to Set up a Dihybrid Punnett
Square
• Write out allelic combination
• USE THE F.O.I.L. method to set up Punnett Square
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F=first allele combination
O= outer allele combination
I= inner allele combination
L= last allele combination
• Conduct the test cross
• Evaluate the genotypes and phenotypes
Practice Example
• In summer squash, white fruit color (W) is
dominant over yellow fruit color (w) and diskshaped fruit (D) is dominant over sphereshaped fruit (d).. If a squash plant truebreeding for white, disk-shaped fruit is
crossed with a plant true-breeding for yellow,
sphere-shaped fruit, what will the phenotypic
and genotypic ratios be for:
• a. the F1 generation? b. the F2 generation?
1. Write down the cross in terms of the
parental (P1) genotypes and phenotypes:
WWDD (white, disk-shaped fruit) X wwdd
(yellow, sphere-shaped fruit)
F= WD
F=wd
O= WD
O=wd
I=WD
I= wd
L=WD
L= wd
All F1 progeny will be heterozygous for both characters
(WwDd) and will have white, disk-shaped fruit .
WD
WD
WD
WD
wd
WwDd
WwDd
WwDd
WwDd
wd
WwDd
WwDd
WwDd
WwDd
wd
WwDd
WwDd
WwDd
WwDd
wd
WwDd
WwDd
WwDd
WwDd
F2 generation will have two heterozygous parents crossed
Determine the F2 gametes and write out all
possible allele combinations
• WwDd X
F=WD
O=Wd
I=wD
L=wd
WwDd
F= WD
O=Wd
I=wD
L= wd
Conduct the test cross
WD Wd wD
WD WWDD WWDd WwDD
Wd WWDd WWdd WwDd
wD WwDD WwDd wwDD
wd WwDd Wwdd wwDd
wd
WwDd
Wwdd
wwDd
wwdd
Evaluate your results
• Genotypic ratios:
1/16 will be homozygous dominant for both traits (WWDD)
2/16 will be homozygous dominant for color and heterozygous for
shape (WWDd)
2/16 will be heterozygous for color and homozygous dominant for
shape (WwDD)
1/16 will be homozygous dominant for color and homozygous
recessive for shape (WWdd)
4/16 will be heterozygous for both traits (WwDd)
2/16 will be heteozygous for color and homozygous recessive for
shape (Wwdd)
1/16 will be homozygous recessive for color and homozygous
dominant for shape (wwDD)
2/16 will be homozygous recessive for color and heterozygous for
shape (wwDd)
1/16 will be homozygous recessive for both traits (wwdd)
• Phenotypic ratios:
9/16 will have white, disk-shaped fruit
3/16 will have white, sphere-shaped fruit
3/16 will have yellow, disk-shaped fruit
1/16 will have yellow, sphere-shaped fruit
• This is a 9:3:3:1 phenotypic ratio.
Practice this in your notes
• President Obama and Michelle Obama want to
have another child and want to figure out their
chances of having certain traits in their offspring.
They wonder about the following traits.
• E= round eyes D= dark skin
T= tall W= widows peak
• e= oval eyes d= light skin t= short w= no widows peak
Problem 1
Barack Obama
• 1-Homozygous Dominant eyes w/ heterozygous
for height
Michelle Obama
• 1-Heterozygous for eyes w/ homozygous for
height