1. Multiple Alleles

2. allele = (n) a form of a gene which codes for one possible outcome of a phenotype
For example, in Mendel's pea investigations, he found that there was a gene that determined the color of the pea pod.  One form of it (one allele) creates yellow pods, & the other form (allele) creates green pods.
Two possible phenotypes of one trait (pod color) are determined by two alleles (forms) of the one "color" gene.

3. Review of incomplete dominance
GENOTYPES BB = Homozygous Dominant Bb = Heterozygous
bb = Homozygous recessive
RESULTING PHENOTYPE BB = Black Fur Bb = Grey Fur bb = White Fur

4. Review of codominance GENOTYPES BB = Homozygous ? BW = Heterozygous
WW = Homozygous ?
RESULTING PHENOTYPE BB = Black Fur BW = Black AND White Fur WW = White Fur

5. There may be multiple alleles within the population, but individuals have only TWO of those alleles.
Why?
Because individuals have only two biological parents.  We inherit half of our genes (alleles) from mom, & the other half from dad, so we end up with two alleles for every trait in our phenotype.

6. Example: Human Blood Type
An excellent example of multiple allele inheritance is human blood type. Blood type exists as four possible phenotypes: A, B, AB, & O.

7. There are 3 alleles for the gene that determines blood type
There are 3 alleles for the gene that determines blood type. (Remember: You have just 2 of the 3 in your genotype from mom & 1 from dad).
The alleles are as follows:  
ALLELE CODES FOR IA Type "A" Blood IB Type "B" Blood
i Type "O" Blood

8. According to the symbols used in the table below, the allele for "O" (i) is recessive to the alleles for "A" & "B".
With three alleles we have a higher number of possible combinations in creating a genotype.  
GENOTYPES RESULTING PHENOTYPES IAIA Type A IAi Type A
IBIB Type B IBi Type B
IAIB Type AB
ii Type O

9. Sample Problem
A woman with Type O blood and a man who is Type AB are expecting a child.  What are the possible blood types of the kid?

10. Solve this using the symbols for blood type alleles & the good old punnett square. 
1. Figure out the genotypes of mom & dad using the given info.
2. "Woman with Type O" must be ii, because that is the one & only genotype for Type O.
3. "Man who is AB" must be IAIB, again because it is the one & only genotype for AB blood.
4. So our cross is: ii x IAIB.  Make a punnett square 
As you can see, our results are as follows: 50% of kids will be heterozygous with blood Type A 50% will be heterozygous with blood Type B

11. Multiple Alleles vs. Polygenic
Polygenic = is the inheritance pattern of a trait that is controlled by two or more genes. Each gene may have two or more alleles
The resulting phenotype usually shows a range of value for the trait. (ie. human height or human hair color)