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TRANSCRIPT
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Dihybrid Crosses
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Dihybrid Crosses
▪ the natural progression for Mendel was to study 2
characteristics at the same time.
▪ The study of 2 pairs of contrasting traits at the
same time = a dihybrid cross
▪ex. round yellow seeds X wrinkled green seeds
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Mendel’s Law
▪ The Law of Independent Assortment
▪ During gamete formation, segregating pairs of unit
factors assort independently of each other.
▪ the two traits are inherited totally independently of
each other
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Solving Dihybrid Crosses
▪ Follow the steps
▪ If you were to cross a homozygous yellow wrinkled
plant with a homozygous green round plant, what
would your phenotypic and genotypic ratios be?
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Step 1: Make a Key
If yellow is dominant over green and
round is dominant over wrinkled:
▪ Y= yellow
▪ y = green
▪ R = round
▪ r = wrinkled
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Step 2: Assign genotypes of the parents
▪ Homozygous yellow wrinkled = YYrr
▪ Homozygous green round = yyRR
▪ P = YYrr x yyRR
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Step 3: Determine the Gametes
▪ If Parents = YYrr X yyRR
YYrr can produce the combinations
Yr, Yr, Yr, Yr
yyRR can produce the combinations
yR, yR, yR, yR
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Step 4: Fill in the Punnett square
Yr Yr Yr Yr
yR YyRr YyRr YyRr YyRr
yR YyRr YyRr YyRr YyRr
yR YyRr YyRr YyRr YyRr
yR YyRr YyRr YyRr YyRr
Note: Keep the alleles for each gene together and
write the dominant allele first. Ex. YyRr NOT YRyr
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Step 5: Answer any questions
▪ Genotype probabilities: ▪ = 100% YyRr (all 16 possible combinations)
▪ Phenotype probabilities: ▪ = 100% yellow round (all 16 possible
combinations)
Key
Y= yellow
y = green
R = round
r = wrinkled
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Now try: F1 X F1
= YyRr X YyRr
▪ Remember to determine the gametes for your Punnett
square.
Gamete combinations:
YR, Yr, yR, yr and YR, Yr, yR, yr
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Fill in the Punnett Square
YR Yr yR yr
YR YYRR YYRr YyRR YyRr
Yr YYRr YYrr YyRr Yyrr
yR YyRR YyRr yyRR yyRr
yr YyRr Yyrr yyRr yyrr
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Results:
▪ Phenotypes:
▪ 9 Yellow Round▪ Dominant Dominant
▪ 3 Yellow Wrinkled▪ Dominant Recessive
▪ 3 Green Round▪ Recessive Dominant
▪ 1 Green Wrinkled▪ Recessive Recessive
▪ Genotyes:
▪ 1 YYRR
▪ 2 YYRr
▪ 1 YYrr
▪ 2 YyRR
▪ 4 YyRr
▪ 2 Yyrr
▪ 1 yyRR
▪ 2 yyRr
▪ 1 yyrr
Y= yellow
y = green
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R = round
r = wrinkled
Ratio = 9:3:3:1
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Try this:
I = Inflated Pod i = constricted pod
T = Tall t = dwarf
▪ The cross: a plant heterozygous for inflated pod and
heterozygous tall is crossed with a another heterozygous inflated
pod and heterozygous tall plant. What are the genotypic and
phenotypic ratios?
▪ The parents are:
▪ IiTt x IiTt
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Gamete Combinations
▪ If Parents are IiTt x IiTt
Gamete Combinations are
IT, It, iT, it and IT, It, iT, it
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Fill in the Punnett Square
IT It iT it
IT IITT IITt IiTT IiTt
It IITt Iitt IiTt Iitt
iT IiTT IiTt iiTT iiTt
it IiTt Iitt iiTt iitt
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Results:
▪ Phenotypes:
▪ 9 Inflated pod Tall
▪Dominant Dominant
▪ 3 Inflated pod Dwarf
▪Dominant Recessive
▪ 3 Constricted pod Tall
▪Recessive Dominant
▪ 1 Constricted pod Dwarf
▪Recessive Recessive
▪ Genotypes:
▪ 1 IITT
▪ 2 IITt
▪ 1 IItt
▪ 2 IiTT
▪ 4 IiTt
▪ 2 Iitt
▪ 1 iiTT
▪ 2 iiTt
▪ 1 iitt
Key
I= inflated pod
i = constricted podT = tall
t = dwarf
Ratio = 9:3:3:1