1. Dihybrid Crosses Incomplete Dominance Codominance Multiple Alleles
In a nutshell!

2. Review: Dominant/Recessive
One allele is dominant over the other (capable of masking the recessive allele)
Example: Purple flowers are dominant over white
PP = purple
Pp = purple
pp = white

3. Review Problem: Dominant/Recessive
In pea plants, purple flowers (P) are dominant over white flowers (p) show the cross between two heterozygous plants.
P p
GENOTYPES:
- PP (1); Pp (2); pp (1)
- ratio 1:2:1 P p PP Pp
PHENOTYPES: Pp pp
- purple (3); white (1)
- ratio 3:1

4. 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.

5. r r R r Rr Rr R RR Rr R R Rr Rr r Rr rr
Can you determine the genotype
of a white-eyed fly just by looking at it? Why?
Yes- it’s rr
There is only one genotype for white eyes
of a red-eyed fly just by looking at it? Why?
No- it could be RR or Rr
There are two genotypes for red eyes
How could you determine the genotype of the red-eyed fly?
Perform a test cross

6. 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

7. 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.

8. Step 1: Determine the parents’ genotypes
Step 2: You must find ALL possible gametes that can be made from each parent.
Remember, each gamete must have one B and one R.
Parent 1: BbRr Parent 2: BbRr

9. Parent 1: BbRr Parent 2: BbRr
Possible gametes: BR Br bR br
Parent 2
Possible gametes: BR Br bR br

10. Step 3: 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…

11. BR bR Br br BbRr x BbRr Fur Color: B: Black b: White
Coat Texture: R: Rough r: Smooth
Step 4: Find the possible genotypes of the offspring

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

13. 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

14. 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

15. On a blank sheet of paper…
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 phenotypic ratio.

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

17. Incomplete Dominance
A third (new) phenotype appears in the heterozygous condition.
Flower Color in 4 O’clocks
RR = red
rr = white
Rr = pink

18. Problem: Incomplete Dominance
Show the cross between a pink and a white flower.
GENOTYPES:
R r
- Rr (2); rr (2); RR (0)
- ratio 2:2:0 r rr Rr
PHENOTYPES:
- pink (2); white (2) Red (0)
- ratio 2:2:0

19. Codominance NS = some of each SS = sickle cells NN = normal cells sick
The heterozygous condition, both alleles are expressed equally
Sickle Cell Anemia in Humans
NS = some of each
SS = sickle cells
NN = normal cells
sick

20. Problem: Codominance
Show the cross between an individual with sickle-cell anemia and another who is a carrier but not sick.
N S
GENOTYPES:
- NS (2) SS (2)
- ratio 2:2:0 S NS SS
PHENOTYPES:
- carrier (2); sick (2)
- ratio 2:2:0

21. Multiple Alleles There are more than two alleles for a trait
Blood type in humans
Blood Types?
Type A, Type B, Type AB, Type O
Blood Alleles?
A, B, O (in book – IA, IB, I)

22. Rules for Blood Type A and B are codominant
AA = Type A
BB = Type B
AB = Type AB
A and B are dominant over O
AO = type A
BO = type B
OO = type O

23. Problem: Multiple Alleles
Show the cross between a mother who has type O blood and a father who has type AB blood.
O O
GENOTYPES:
- AO (2) BO (2) A B AO BO
PHENOTYPES:
- type A (2); type B (2)
- ratio 2:2:0:0

24. Problem: Multiple Alleles
Show the cross between a mother who is heterozygous for type B blood and a father who is heterozygous for type A blood.
GENOTYPES:
A O
AB (1); BO (1);
AO (1); OO (1)
- ratio 1:1:1:1 B O AB OO BO AO
PHENOTYPES:
type AB (1); type B (1)
type A (1); type O (1)
- ratio 1:1:1:1