Heredity and Heredity and GeneticsGenetics

Part TwoPart Two

Dihybrid CrossesDihybrid 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.

Draw a Punnett square for each cross, and determine the genotypic and phenotypic ratios.

• Can you determine the genotype of a white-eyed fly just by looking at it?

– Yes- it’s rr

• Can you determine the genotype of a red-eyed fly just by looking at it?

– No- it could be RR or Rr

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

Review of Monohybrid Crosses

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

Dihybrid Cross:a cross that shows the possible

offspring for two traitsFur Color:

B: Black b: White

Coat Texture:R: Roughr: Smooth

In this example, we will cross a heterozygous individual with

another heterozygous individual. Their genotypes will be:

BbRr x BbRr

Dihybrid CrossBbRr x BbRr

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

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

Dihybrid CrossBbRr x BbRr

Possible gametes:BRBrbRbr

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: Roughr:

Smooth

BbRr x BbRr B

RbR

br

bR

Br

BR

br

Br

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: Roughr:

Smooth

BbRr x BbRr

BR

bR

br

bR

Br

BR

br

BrBBRR

BbRR BbRr

BBRr BBrr BbRr Bbrr

BbRR BbRr bbRR bbRr

BbRr Bbrr bbRr bbrr

BBRr

BR bR

br

bR

Br

BR

br

Br

BBRR

BbRR

BbRr

BBRr BBrr BbRr Bbrr

BbRR

BbRr bbRR

bbRr

BbRr Bbrr bbRr bbrr

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?

Fur Color:B: Black b: White

Coat Texture:R: Roughr:

Smooth

BR bR

br

bR

Br

BR

br

Br

BBRR

BbRR

BbRr

BBRr BBrr BbRr Bbrr

BbRR

BbRr bbRR

bbRr

BbRr Bbrr bbRr bbrr

BBRr

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?

Fur Color:B: Black b: White

Coat Texture:R: Roughr:

Smooth

Phenotypic Ratio

9:3:3:1

More Practice…• On the back of your notes…In pea plants, yellow seeds (Y) are dominant over

green seeds (y), and rounded peas (R) are dominant over wrinkled peas (r).

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

YR yR

yr

yr

yr

yr

yrYr

YyRr yyRr yyrr

YyRr Yyrr yyRr yyrr

YyRr Yyrr yyRr yyrr

YyRr Yyrr yyRr yyrr

Yyrr