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Dihybrid Cross: a type of cross that involves
two genes
Example: Mendel self-fertilized a pea plant
that was heterozygous for purple colour and
tall traits
Use a Punnett square to show the possible
genotypes and phenotypes of the offspring?
Step 1: determine which alleles are
dominant/recessive
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Step 2: determine the genotypes of the
parents
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Step 3: Before making a Punnett Square it is
useful to determine the possible gamates
that can be made by the parents
* Keep in mind the law of independent
assortment and use FOIL*
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Step 4: Prepare your Punnett square to determine the
F1 generation. You will need a grid that fits all possible
gamates – in this case _______ x _______ grid
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Step 5: Fill in the grid
 Stick to the ordering of traits from the parents (Colour,
Height)
 Put the alleles for the same trait back together
 Put dominant letter first
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Step 6: State the final answer (remember to re-read
the question)
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Mendelian ratio for a dihybrid cross (2 traits,
both heterozygous) is 9 : 3 : 3 : 1
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The genotypic or phenotypic ratio you find is
the percent chance or the probabilty (aka
frequency) that an individual born from those
parents will have those traits. It is not the
exact results, or birth order
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In humans, curly hair is dominant over straight
hair and the ability to curl your tongue is
dominant over not being able to
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A man who is homozygous for curly hair and
heterozygous for the ability to roll his tongue
has children with a woman with straight hair
who cannot roll her tongue
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What are the genotypic and phenotypic ratios of
their possible offspring?
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A male and female guinea pig are heterozygous for
fur colour and fur texture. Both dark fur (D) and rough
fur (R) are dominant traits
What are the parents phenotypes?
How many different gametes are formed and what
are they?
 What is the chance of having offspring that are
homozygous for both traits?
 What is the chance of having offspring with dark,
rough fur?
 What is the chance of having offspring with both
recessive traits?
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