1. Incomplete Dominance, Codominance, Sex-linked and Polygenic Inheritance

2. Mendel’s Dominance Mendel’s rule of dominance was complete dominance
Homozygous dominant organisms have the dominant phenotype
Heterozygous organisms have the dominant phenotype
Example: Both PP and Pp plants have the dominant PURPLE phenotype (P=purple and p=white flowers)

3. Incomplete Dominance R F1 generation Rr r All Rr = pink
Offspring have an appearance somewhat in between the phenotypes of the two parents and neither allele is dominant.
“Mixed”
Blended r R
All Rr = pink
(heterozygous pink)
F1 generation Rr

4. Incomplete Dominance
Incomplete dominance occurs when two or more alleles influence the phenotype
Neither allele is completely dominant over the other

5. Incomplete Dominance

6. Codominance
BOTH alleles are expressed equally in heterozygous individuals.
Use two capital letters for both alleles.
Neither allele is dominant over the other.
Usually phenotypes are stripes or spots.

7. YOU tell me which type of dominance…
Codominance!

8. Type of Dominance?
Incomplete Dominance!

9. Type of Dominance?
Incomplete Dominance!

10. Codominance Example: blood type
Determined by whether or not you have A or B proteins
IA = A protein
IB = B protein
I = no protein

11. Codominance

12. Codominance Problem Example:homozygous male Type B (IBIB)
x heterozygous female Type A (IAi) IA IB i
IAIB
IBi
1/2 = IAIB
1/2 = IBi

13. Another Codominance Problem
Example: male Type O (ii) x female type AB (IAIB) i IA IB
IAi
IBi
1/2 = IAi
1/2 = IBi

14. 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 (ii) X girl - type AB (IAIB)

15. Codominance Answer: IB IA i IAIB ii Parents: genotypes = IAi and IBi
phenotypes = A and B

16. 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

17. Sex – Linked Traits Example: Colorblindness
If the mother carries the colorblindness gene on her X chromosome, her son could get it.
As long as one X chromosome
is ok, a female will not
express the trait XC X y
XcX
X y
X X
X y

18. Sex-linked Traits Example: Colorblindness Sex Chromosomes
XX chromosome - female
Xy chromosome - male
Color-
blindness

19. Polygenic Inheritance
Inheritance pattern controlled by two or more genes
Genes can be on the same chromosome or on different chromosomes
Genotypes written as AA, Aa, or aa except you will have three or more genes each with a dominant and recessive trait
Skin color is influenced by the additive effects of melanin by three to six genes. Eye color is also a result of melanin added from multiple genes.

20. Polygenic Inheritance of Skin Color
Gametes
ABC
ABc
AbC
Abc
aBC
aBc
abC
abc
AABBCC
AABBCc
AABbCc
AaBBCC
AAbbcc
aaBBCc
aaBbcc
The table shows typical ranges of skin color combinations. Other polygenic influenced conditions include Spina bifida and Cleft palate.