1. Complex Inheritance Patterns
Incomplete Dominance, Co-Dominance, & Multiple Alleles
March 23, 2009

2. Exceptions to the Law of Dominance
Sometimes the pairing of different alleles in the heterozygous offspring does not result in a simple dominant/recessive result.
Rather, the heterozygous offspring have a trait that is not exactly like the trait of either purebred parent.
Examples:
Incomplete dominance
Codominance

3. Objectives
At the conclusion of this lesson students should be able to:
Identify and distinguish between in-complete dominance and codominance inheritance patterns.
Determine the probability of a specified incomplete dominant or codominant inheritance using a Punnett square.
Interpret the inheritance of a trait which is controlled by multiple alleles.

4. Incomplete Dominance
Incomplete dominance occurs when the heterozygous phenotype is an intermediate between the two homozygous parent phenotypes.
“Incomplete” means “in between”
Ex: flower color in snapdragons

5. Incomplete Dominance X
Figure 11.4 The color of snapdragon flowers is a result of incomplete dominance. When a plant with white flowers is crossed with a plant with red flowers, the offspring have pink flowers. Red, pink, and white offspring will result from self fertilization of a plant with pink flowers.
PROBLEM:
A red snapdragon is crossed with a pink snapdragon. What percentage of the offspring will be pink?

6. Incomplete Dominance Practice Problem:
In radishes, when a plant homozygous for red radishes is crossed with a plant homozygous for white radishes , plants bearing purple radishes are produced.
What would be the genotype and phenotype ratios of a cross between a purple radish plant and a white radish plant?
A red and a white?
A red and a purple?

7. Codominance
In codominance, both alleles are shown in the heterozygous phenotype.
“co-” means “together”
Ex: sickle cell disease

8. Sickle Cell Disease
Sickle cell disease is caused by the allele that controls the formation of the protein hemoglobin.
Hemoglobin is the part of the red blood cell that carries oxygen.
The allele for normal hemoglobin (A) results in red blood cells that are disc- shaped.
The sickle cell allele (S) changes the hemoglobin and results in red blood cells that are sickle -shaped.
People who are heterozygous for these two alleles (AS) have both normal and sickle-shaped red blood cells.
PROBLEM:
What is the risk that two people heterozygous for the sickle-shaped allele will have a child with sickle cell disease?

9. Blood Type – A Codominant Trait
Red blood cells have two different carbohydrates (called antigens) that coat their surface.
Allele for antigen “A” results in type A blood.
Antigen “B” results in type B blood.
Both antigens results in type AB blood.
If neither antigen is present, then type O blood results.
QUESTION?
Which alleles are codominant?
How can you account for type O blood?

10. Allele Combinations for Blood Type
For human blood type there are nine possible genotypes and four possible phenotypes.
Human blood type is a trait that is an example of multiple alleles.
If three or more alleles are found in the population, these genes are said to have multiple alleles.
Possible gametes from female parent
Possible gametes from male parent IA IB i
IAIA
IAIB
IAi
IBIB
IBi ii

11. A Codominant Problem
In a cross between a parent with type A blood and a parent with type B blood what are the possible genotypes and phenotypes of the offspring?
The type A parent can either be IA IA or IA i
The type B parent can either be IB IB or IB i
Complete the Punnett squares and determine the possible genotypes and phenotypes.

12. A Codominant Problem
Here are the possible Punnett Squares with their results: IA IB
IA IB IA i IB
IA IB
IB i IA IB
IA IB i
IA i IA i IB
IA IB
IB i
IA i
i i
IA IA x IB IB IA ix IB IB IA IA x IB i IA i x IB I
100% type AB % type AB % type AB % type AB
50% type A % type A % type B % type B
25% type O

13. Codominance Practice O AB Practice Problems:
A man with blood type AB marries a woman with blood type O. Is it possible for them to have children with the same blood type as either of the parents? Why or why not?
In some cattle the roan coloration (red and white together) results in heterozygous offspring. What would the genotypes and phenotypes of the parents be? If two roan cattle bred, what is the chance that their calf will also be roan?

14. Recognizing Complex Inheritance
An inheritance pattern that involves ...
...a single gene that affects...
...multiple traits is...
PLEIOTROPY
Melanin production in cats
Sickle cell disease
Examples:
...a single trait...
...in which neither allele dominates the other and both are expressed is...
CODOMINANCE
ABO bood group
...in which both alleles appear to "blend" in some instances is...
INCOMPLETE DOMINANCE
Flower color in snapdragons
Example:
...in which one allele dominates the expression of another allele is...
SIMPLE DOMINANCE
Huntington's disease
albinism
Tay Sachs disease
alkaptonuria
achondroplasia
cystic fibrosis
polydactyly
freckles
dimples
...multiple genes that affect ...
... two or more traits...
...that do not interact is...
LINKED GENES
Muscular dystrophy
Hemophilia
Colorblindness
...that interact so that one controls the expression of the other is...
EPISTASIS
Coat color in Labrador retrievers and other mammals
Rooster combs
...a single trait is a...
POLYGENIC TRAIT
Hair color
Skin color
Eye color
Height

15. References
Biggs, Alton, et. al. Biology. New York: The McGraw Hill Companies, Inc., Schmutz, Shelia. "Mendelian Inheritance and Beyond." 13 Feb Department of Animal and Poultry Science: The Unviersity of Saskatchewan. 13 Apr 2008 < "What is Sickle Cell Anemia?." National Heart Lung and Blood Institute Disease and Conditions Index. Nov U.S. Department of Health and Human Services: National Institutes of Health. 13 Apr 2008 < "Why people choose bloodless medicine." About.com: Heatlh Topics A-Z Apr 2008 <