1. Chapter 11: Introduction to Genetics
Section 11-3: Other Patterns of Inheritance

2. Beyond Dominant and Recessive Alleles
There are exceptions to every rule
In most organisms, genetics is more complicated than Mendelian rules because most genes have more than two alleles, and many important traits are controlled by more than one gene
Mendel’s principles alone cannot predict these traits

3. Incomplete Dominance
A cross between two four o’clock plants shows a common exception
The F1 generation produced by a cross between red-flowered (RR) and white-flowered (WW) plants consists of pink-colored flowers (RW)

4. Incomplete Dominance In this case, neither allele is dominant
Called incomplete dominance - the heterozygous phenotype lies somewhere between the two homozygous phenotypes

5. Codominance
Codominance occurs when the phenotypes o both alleles are clearly expressed
In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers
Heterozygous chickens have a color described as “erminette,” speckled with black and white feathers

6. Multiple Alleles
A gene with more than two alleles is said to have multiple alleles
Many genes have multiple alleles, including the human genes for blood type.

7. Multiple Alleles Blood Type Possible Genotypes Type A IA IA or IA i
Type A, Type B, Type AB, Type O
Three alleles: IA, IB, i
Blood Type
Possible Genotypes
Type A
IA IA or IA i
Type B
IB IB or IB i
Type AB
IA IB (also shows codominance)
Type O ii

8. Polygenic Traits
Traits controlled by two or more genes are said to be polygenic traits
Often show a wide range of phenotypes
Human traits like skin color, facial appearance, and height are polygenic

9. Genes and Environment
The characteristics of any organism are not determined solely by the genes that organism inherits – the expression of genes also depends on the environment
Let’s look at the Western white butterfly

10. Genes and Environment
Environment in which the butterflies develop influences the expression of their genes for wing coloration
Studies revealed that those hatching in springtime had greater levels of pigment in their wings than those hatching in the summer, causing them to have different phenotypes
It is related to flight – the body temperature of the butterfly needs to be in a certain range to fly effectively
More pigment means that the butterflies can reach the optimal body temperature faster (spring)
Less pigment means the butterflies will not overheat (summer)