1. More Patterns Than Mendel Thought
Chapter 11.4
More Patterns Than Mendel Thought
AP Biology
Fall 2010

2. Codominance in ABO Blood Types
Codominance: a pair of non-identical alleles affecting two phenotypes are both expressed at the same time in heterozygotes
In codominance, both alleles are expressed in heterozygotes

3. Codominance in ABO Blood Types
Blood type is determined by markers produced by three genes
The occurrence of 3+ alleles for a single gene locus available to individuals in the population is called a multiple allele system
IA and IB are each dominant to i, but are codominant to each other
Therefore, some persons can express both genes and have AB blood

4. Incomplete Dominance
Incomplete dominance: a dominant allele cannot completely mask the expression of another
Phenotype will be intermediate between the two traits

5. Incomplete Dominance Example:
True breeding red flowered snapdragon crossed with a white flowered snapdragon
Produce pink flowers because there is not enough red pigment (produced by the dominant allele) to completely mask the effects of the white alleles

6. Epistasis One gene pair can influence other gene pairs
Their combined activities producing some effect on phenotype

7. Epistasis Example: Labrador retrievers
One gene pair codes for the quantity of melanin produced while another codes for melanin deposition
Another gene locus determines whether melanin will be produced at all- absence of melanin produces an albino (recessive)
Two interacting gene pairs can give rise to a phenotype that neither produces by itself

8. Epistasis Example: Comb shape in chickens
Is of at least four types, depending on the interaction of two gene pairs (R and P)

9. Single Genes With A Wide Reach
Sometimes the expression of alleles at one location can have effects on two or more traits
Termed pleiotrophy
Cystic fibrosis, sickle cell anemia
Marfan syndrome is caused by autosomal dominant mutation of the gene for fibrillin, a protein found throughout the body in connective tissue
Marfan syndrome is characterized by these effects:
Lanky skeleton, leaky heart valves, weakened blood vessels, deformed air sacs in lungs, pain, and lens displacement in the eyes