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. 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.

5. 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

6. Parent 1: BbRr Parent 2: BbRr
Possible gametes: BR Br bR br
Parent 2
Possible gametes: BR Br bR br
F O I L

7. 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…

8. 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

9. 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

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. 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.

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

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

15. 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

16. Sex Linked Traits Humans have 23 pairs of chromosomes.
22 of the pairs are called autosomes.
One pair of chromosomes is related to the sex of an individual , these chromosomes are called sex chromosomes
XX = Female
XY = Male

18. Sex Linked Traits
Genes that are located on the X chromosome are called sex-linked genes.
Traits determined by sex-linked genes are called sex-linked traits
 Ex: Color Blindness
(c = colorblind, C = normal)

19. Sex Linked Traits Color Blindness is recessive.
FEMALES: both X chromosomes must have the gene in order for the trait to be expressed.
MALES: only have one X chromosome. If that X chromosome has the colorblind allele, then the trait will be expressed.
Colorblindness affects 1 in 100 females; 1 in 10 males

20. X-linked Recessive Inheritance
Male
Female X Y X X
One copy of an altered gene on the X chromosome causes the disease in a male.
= Hemizygote
An altered copy on one of the X chromosome pair causes carrier status in a female.
= Heterozygote

21. Sex Linked Traits XCXC – Normal Female XCXc – Carrier Female
XcXc – Color Blind Female
XCY– Normal Male
XcY– Colorblind Male

22. Sex Linked Punnett Squares
1. A colorblind male marries a normal female. What are the offspring genotypes and phenotypes? (C = normal, c = colorblind)
Xc Y XC

23. Sex Linked Punnett Squares
1. A colorblind male marries a normal female. What are the offspring genotypes and phenotypes?
(C = normal, c = colorblind)
Xc Y
XC XC Xc XC Y
XC XC Xc XC Y
Genotypic Ratio: __________ Phenotypic Ratio: __________
What percentage of sons with colorblindness? _______ daughters? ______

24. 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

25. Rules for Blood Type A and B are codominant
A and B are dominant over O

26. 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)
ratio 2:2:0:0:0:0 A B AO BO
PHENOTYPES:
- type A (2); type B (2)
- ratio 2:2:0:0

27. 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 1:1:1:1:0:0 B O AB OO BO AO
PHENOTYPES:
type AB (1); type B (1)
type A (1); type O (1)
- ratio 1:1:1:1