1. Exceptions to the Rule of Dominance & Recessiveness
Incomplete Dominance, Codominance, & Sex-Linked Traits

2. Incomplete Dominance and Codominance

3. Incomplete Dominance Example: Snapdragons (flower)
red (RR) x white (rr)
RR = red flower
rr= white flower R r

4. Codominance Both alleles are dominant and expressed
Ex. Erminette chicken feathers
Both black and white feathers

5. Dalmatians

6. Codominance Example: blood type 1. type A = AA or AO
2. type B = BB or BO
3. type AB = AB
4. type O = OO

7. Codominance Problem Example: homozygous B male: Type B (BB)
x heterozygous female Type A (AO) B A O AB BO
1/2 = AB Blood
1/2 = B Blood

8. Another Codominance Problem
Example: male Type O x female type AB O A B AO BO
1/2 = Type A
1/2 = Type B

9. Codominance
Question: If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents?
boy - type O (OO) X girl - type AB (AB)

10. Let’s review:
Determine if the picture shows codominance or incomplete dominance.

17. What number do you see?

18. Sex-linked Traits Traits (genes) located on the sex chromosomes
Sex chromosomes are X and Y
XX genotype for females
XY genotype for males
Many sex-linked traits carried on X chromosome

19. Sex-linked Trait Colorblind: Father is normal/Mom is a carrier C=Normal c=ColorblindXC y Xc
25% chance of having a female carrier
25% chance of having a male Colorblind male

20. Sex-linked Traits Example: Colorblindness and Hemophilia
Sex Chromosomes
XX chromosome - female
Xy chromosome - male

21. Sex-linked Trait Hemophilia: Father has hemophilia/Mom is a carrier H=Normal h=HemophiliaXH Xh y

22. Sex-linked Trait Problem
Example: Eye color in fruit flies
(red-eyed male) x (white-eyed female) XRY x XrXr
Remember: the Y chromosome in males does not carry traits.
RR = red eyed
Rr = red eyed
rr = white eyed
Xy = male
XX = female Xr XR y

23. Sex-linked Trait Solution:Xr XR y
50% red eyed female
50% white eyed male
XR Xr
Xr y

24. Female Carriers

25. Polygenic Traits
Traits that involve multiple genes for a single trait. -Skin Color -Eye Color

26. Polygenic traits – traits controlled by many genes.
Ex. The wide range of skin color in humans.