Heredity and Genetics�Part Two��Dihybrid Crosses

Review of Monohybrid Crosses

• Remember, monohybrid crosses involve only ONE trait
• Practice… In fruit flies, red eyes are dominant over white eyes.
• Cross a white-eyed fly with a homozygous dominant red-eyed fly.
• Cross two heterozygous red-eyed flies.

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Draw a Punnett square for each cross,

and determine the genotypic and phenotypic ratios.

Review of Monohybrid Crosses

• Can you determine the genotype

of a white-eyed fly just by looking at it?

• Yes- it’s rr

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• Can you determine the genotype

of a red-eyed fly just by looking at it?

• No- it could be RR or Rr

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• How could you determine the genotype of the red-eyed fly?
• Perform a test cross

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Review of Monohybrid Crosses

• What two organisms would you cross in the test cross?
• The unknown red-eyed fly with a white eyed fly
• If the red-eyed fly was RR, what would the offspring of the test cross look like?
• All of the offspring would have red eyes
• If the red-eyed fly was Rr, what would the offspring of the test cross look like?
• Half of the offspring would have red eyes, half would have white eyes

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Dihybrid Cross:�a cross that shows the possible offspring for two traits

Fur Color:� B: Black � b: White

Coat Texture:� R: Rough� r: Smooth

In this example, we will cross a heterozygous individual with another heterozygous individual. Their genotypes will be:

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BbRr x BbRr

Dihybrid Cross

BbRr x BbRr

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First, you must find ALL possible gametes that can be made from each parent.

Remember, each gamete must have one B and one R.

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Dihybrid Cross

BbRr x BbRr

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Possible gametes:

BR

Br

bR

br

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Next, arrange all possible gametes for one parent along the top of your Punnett Square, and all possible gametes for the other parent down the side of your Punnett Square…

Dihybrid Crosses:�a cross that shows the possible offspring for two traits

Fur Color:� B: Black � b: White

Coat Texture:� R: Rough� r: Smooth

BbRr x BbRr

Then, find the possible genotypes of the offspring

Dihybrid Crosses:�a cross that shows the possible offspring for two traits

Fur Color:� B: Black � b: White

Coat Texture:� R: Rough� r: Smooth

BbRr x BbRr

How many of the offspring would have a black, rough coat?

How many of the offspring would have a black, smooth coat?

How many of the offspring would have a white, rough coat?

How many of the offspring would have a white, smooth coat?

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Fur Color:� B: Black � b: White

Coat Texture:� R: Rough� r: Smooth

How many of the offspring would have black, rough coat?

How many of the offspring would have a black, smooth coat?

How many of the offspring would have a white, rough coat?

How many of the offspring would have a white, smooth coat?

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Fur Color:� B: Black � b: White

Coat Texture:� R: Rough� r: Smooth

Phenotypic Ratio

9:3:3:1

More Practice…

• Try this

In pea plants, yellow seeds (Y) are dominant over green seeds (y), and rounded peas (R) are dominant over wrinkled peas (r).

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Cross a plant that is heterozygous for both traits with a plant that is homozygous recessive for both traits. Draw a Punnett square to show all possible offspring, and determine the genotypic and phenotypic ratios.

YyRr X yyrr