Re: Are there codominant genetic traits in humans other than blood groups?

The following is from the Open Door website which discusses sickle cell 
anemia. This disorder is caused by a defect in hemoglobin the protein 
which carries oxygen in the blood.  Heterozygotes for the condition are 
more resistant to malaria and that is why this condition is thought to 
have some selective benefit in evolutionary terms. 
Hope this helps
Gabriel Vargas, md/phd

GENETICS 
CO-DOMINANCE
Not all genes have dominant and recessive alleles. Some have alleles that 
are both expressed together in the heterozygote individuals.
Co-dominant alleles have three phenotypes, one for each genotype.
In humans there are a number of conditions that are co-dominant. 


Sickle cell anaemia is a genetic disease that affects the hemoglobin of 
the red blood cells. Hemoglobin is normally a ball-shaped molecule but the 
sickle cell allele makes it form long strands. The red blood cell carrying 
these molecules distorts into characteristic long shapes.
The shape of the hemoglobin molecule is controlled by two alleles
Normal Hemoglobin allele and the Sickle Cell Hemoglobin allele
There are three phenotypes:


Normal individuals have two normal hemoglobin alleles

Sickle cell anemia, a severe form where all the red blood cells are 
affected. Sickle cell anemia patients have two sickle cell alleles in 
their genotype

Sickle cell trait, a mild condition where 50% of the red blood cells are 
affected. Sickle cell trait individuals are heterozygotes, having one of 
each allele.


Reference: http://www.s
aburchill.com/IBbiology/chapters03/004.html

Medical encyclopedia from NIH on Sickle Cell
http://ww
w.nlm.nih.gov/medlineplus/ency/article/000527.htm