1. Heredity and Genetics Part Two Dihybrid Crosses

2. 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.

3. Review of Monohybrid Crosses
Can you determine the genotype
of a white-eyed fly just by looking at it?
Yes- it’s rr
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

4. 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

5. Dihybrid Cross: a cross that shows the possible offspring for two traits
Coat Texture: R: Rough r: Smooth
Fur Color: B: Black b: White
In this example, we will cross a heterozygous individual with another heterozygous individual. Their genotypes will be:
BbRr x BbRr

6. Dihybrid Cross BbRr 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.

7. Dihybrid Cross BbRr x BbRr Possible gametes: BR Br bR br
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…

8. Dihybrid Crosses: a cross that shows the possible offspring for two traits
BbRr x BbRr BR bR br Br
Fur Color: B: Black b: White
Coat Texture: R: Rough r: Smooth
Then, find the possible genotypes of the offspring

9. Dihybrid Crosses: a cross that shows the possible offspring for two traits
BBRR
BbRR
BbRr
BBRr
BBrr
Bbrr
bbRR
bbRr
bbrr
BbRr x BbRr
Fur Color: B: Black b: White
Coat Texture: R: Rough r: Smooth

10. 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? BR bR br Br
BBRR
BbRR
BbRr
BBRr
BBrr
Bbrr
bbRR
bbRr
bbrr
Fur Color: B: Black b: White
Coat Texture: R: Rough r: Smooth

11. 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? BR bR br Br
BBRR
BbRR
BbRr
BBRr
BBrr
Bbrr
bbRR
bbRr
bbrr
Phenotypic Ratio
9:3:3:1
Fur Color: B: Black b: White
Coat Texture: R: Rough r: Smooth

12. 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.

13. YyRr X yyrr YR yR yr Yr
YyRr
yyRr
yyrr
Yyrr