What do you stingy when you articulate that HLA (human leukocytic antigen) molecules are co-dominantly expressed?
Answer:
The above answerer is actually a bit imprecise.
As you probably know, you have two alleles (variations) of every gene within your body (other than those on the X and Y chromosomes if you are a man). One version come from your mother, and the other from your father. In genetics, 'dominant' alleles are those that produce their phenotypes (effects) even when another allele is present - they override 'recessive' alleles. (The one and only way recessive alleles ever capture to display their phenotype is when the other allele is also a recessive one.)
Codominance is a variation of this, where on earth BOTH alleles are displayed - both are dominant in the sense that they are both displayed regardless of the other allele.
Thus, for the HLA system, not individual will you receive one allele from each parent as you would near any gene, but you also display the results of BOTH alleles. Your cells will own BOTH one of your father's HLA protein groups AND one of your mother's HLA protein groups.
(Although exactly which one of your father's two, and which one of your mother's two, you end up getting is hit or miss, and so your siblings will usually be somewhat different. This is why not all siblings are necessarily devout matches for organ transplants.)
Hope that help!
It means, iirc, that it take two dominant genes for the HLA to manifest itself. So one gene alone can not be expressed in the phenotype.