1. Special Inheritance Patterns

2. Simple Dominance
So far, we have only studied simple dominant/recessive heredity
In simple dominance – dominant allele completely hides the recessive allele
TT Tt tt
(Tall) (Tall) (short)

3. Incomplete Dominance
The dominant allele does not completely cover the recessive allele
Results in a blended phenotype
Ex: Hair type:
HH Hh hh
(Curly) (Wavy) (Straight)

4. Incomplete Dominance in Carnations
RR Rr rr

5. Codominance Two alleles can be expressed at the same time
Ex: people with type AB blood
(the A and B alleles are expressed at the same time)
Roan coat color in horses is another example
(both red and white show up)

6. ABO Blood Typing

7. Multiple Alleles
There are more than just 2 forms of a gene (alleles) for a trait
Example: Human blood type has 3 alleles
Alleles: IA , IB, i

8. Human Blood Types ii = Type O IAIA or IAi = Type A IBIB or IBi
Genotypes ii
IAIA or IAi
IBIB or IBi
IBIA
Phenotypes
= Type O
= Type A
= Type B
= Type AB

9. Blood Type in Humans
IA i
IA IB
IB i
IA i
i i IB i

10. Polygenic Traits A trait can be controlled by multiple genes
A certain combination is needed to show a specific phenotype
Example traits: Skin color, eye color, hair color
Example genotypes and phenotypes:
AaBbCc = black hair
AAbbCC = light brown hair
Aabbcc = blonde hair

12. Sex-Linked Inheritance
When a trait is on either the X or the Y chromosome
Males are more affected
Females are less affected, but can be carriers
Types of sex-linked inheritance
X-linked recessive
X-linked dominant
Y-linked

13. Environment Environment can influence gene expression
Environmental Factors:
Hormones
Age
Temperature