I always thought it was the DNA alone until I realized that if we have mostly two different codes for the same characteristic, one from each parent, who decides the proportion of each own contribution to produce our features?
Identical twins are identical. Apart from the obvious, that means that whatever mechanism operates, it is set by the time of conception, and it is not environmental.
On a side note, I also regrettably realized that although it was the moment when we were conceived that our parents DNAs were glued together pairwise in order to form life, biologically speaking, our parents DNA are continuously engaging sexually within our own bodies by truly mixing their DNAs into a single strand. Yuck. Damn you, tree of knowledge!
I am not a geneticist but my understanding is that all things being equal, the cell produces proteins from both the mother’s DNA and the father’s DNA and the resulting chemical soup informs the behavior and type of the cell.
Yes, humans are diploid, meaning that we have two copies of each chromosome, except the X & Y in the case of males. In general, both variants of a gene (both alleles) are expressed.
One basic principle is that some alleles are dominant, and some are recessive. A dominant allele determines the phenotype when only a single copy is present. A recessive allele determines the phenotype only when present on both copies of the chromosome. A simple example is a recessive genetic disease in which the functional allele is dominant because a single working copy of the gene is sufficient for a normal phenotype.
Rather than just differences within the the protein itself, characteristics generally depend upon gene regulation - quantity and timing of expression. Regulation may be either cis-acting or trans-acting. For example, if the paternal genome encodes a particularly high level of expression of a certain gene, in some cases that affects only the paternal copy of that gene (cis-), in other cases it may affect both copies (trans-).
Bear in mind that identical twins also share an identical prenatal environment, and this is where they go through the most critical early stages of development. And that identical twins are never completely identical. But in any event, this doesn’t really have much bearing on understanding the interplay between the two copies of the genome.
No, the two copies of each chromosome maintain separate integrity in the cell for most of our existence. The DNA itself is not “mixed”. It is only in meiosis, the formation sperm and egg cells, that the two copies of each chromosome are cut and rejoined to form a mosaic composite chromosome.