DNA has information about our cells and how every cell should act.
Since I am made entirely of cells, including things that make my mind, aren't decisions coded for by DNA by proxy of our cells.
Even asking this question is a decision that has already been made because that's what my DNA told my cells to do?
DNA (among other epigenetic factors) code for proteins, so ultimately it determine our cells, which in turn are responsible for our neurology, although how this translates to behavior is variable depending on factors beyond DNA@Remi.b's answer. De facto you're asking about the extent that our brain structure and neural activity determines our conscious stream. This boils down to "Do we have free will?". That question historically was only accessible to philosophers. It is now accessible (but not completely answerable) to biologists, more specifically neuroscientists. It's a great question, but is still open to a lot of subjectivity.
Anil Seth, professor of cognitive and computational neuroscience at the Sackler Centre at the University of Sussex, gives a very good answer to the question at 25:44 in this BBC Life Scientific interview.
We are determined to have it in both senses of the word... Yes, we do have free will because we can behave the way we wish to behave. We cannot decide what to will.
Paraphrasing the guts of the answer: it's very important for the brain to differentiate between internalized experience, and reflex.
Reflex, such as placing your hand on a hot stove, never reach the "conscious stream". These have an obvious correct choice; in this case, removing the hand from a hot stove is clearly the right choice, every time.
Then there is internalized experience. Complex problems have many potential losses and gains. To cope with this, the brain needs to internalize, reflect, and is taking note and making a plan; the sensation of free will. By taking note of that situation within it's context and the plan, we can perhaps change our actions the next time we encounter a similar event so that we can optimize our choices to attempt to cause the least damage to ourselves and/or maximize the benefit.
We have established that there is a need for new memories to form in order to solve complex problems. Long term memories are the result of new physical synaptic connections in the brain. This connection endures whether it’s being used or not.
Psychologists put these into two broad types are two kinds, implicit (akin to reflex; how to read, how to play your favorite song on an instrument etc.) and explicit (the things we are consciously aware of). Explicit memory comes in two forms: episodic and semantic (Tulving, 1972). (i) Episodic memory includes experience. For example, that time you planted some seeds and a potato grew. (ii) Semantic memory is abstract to your own experience. Something more like "Primordial potatoes originated in southern Peru".
To bring this back to the original question "Is every decision we make already pre-programmed because of DNA?" It's long winded. Our choices are built upon the results of our past choices in a given environment. We are able to make these choices, to an extent depending also on other variables, because of the products and machinery encoded in our DNA.
If this answer is a bit scary or makes your choices seem futile, you can simply sit around and do nothing instead of doing whatever you normally would do; it's your choice.
See a similar question answered from a different perspective over at Philosophy.SE.
Is every decision we make already pre-programmed because of DNA?
No! An obvious way to realize this is to compare identical twins. While two randomly chosen twins ressemble each other (in terms of appearance but also in terms of behaviour) more than two randomly sampled individuals in the population, it is still true that they are not exactly identical. So something else than DNA must affect behaviour. What is it?
Let's first define what a phenotypic trait is. Broadly speaking, the phenotype is the consequence of the genotype on the world. Loosely speaking, a phenotypic trait is any trait that an individual is made of! This includes the colour of the hair, the behaviour, the nature of emotions and the decisions. Decisions are nothing but the phenotype of a person.
For any trait, you may (or may not) see variation for this specific trait in a population. You can therefore calculate a phenotypic variance for a specific trait and then wonder what is causing this variance. Is it only DNA or is there something else in play? Let's call the phenotypic variance for a given trait $V_P$
Why would a population display any phenotypic variance? Why wouldn't we just look exactly asame? What explain these differences?
There are two main sources of variance that are underlying this phenotypic variance. The first one is the genetic variance and the second one is the environmental variance. We will call the genetic variance $V_G$ and the environment variance $V_E$.
To appreciate the role of genetic variance $V_G$, consider comparing difference in height among countries as an example. To appreciate the role of environmental variance $V_E$, consider comparing the body shape of people eating at McDonald and those that don't.
Is there something else than environmental and genetic variance that explains the phenotypic variance? The answer is yes. Therefore $V_P>V_G+V_E$. So what is this something that causes this inequality?
Gene effects and environmental effects covariance
Well, first there is a potential non-zero covariance between the environment and the genes. Let's call it $COV(E,G)$. For example, thinking of animals that compete for territory, individuals that have alleles (an allele is the variant of a gene) that make them bigger might be better competitor and have access to bigger territory and therefore to more food which in turn will make them bigger. In such case, there is a positive covariance (or positive correlation) between gene effects and environemtnal effects.
Developmental noise
The developmental process can be quite noisy. An obvious way to appreciate its noisiness is to acknowledge differences between right and left side of people's face. Such noise bring some variance in phenotypic trait in the popualtion. let's cal the contribution of this variance $V_{DN}$. Of course, the amount of developmental noise might depends upon the environment or the genes so that we should also consider covariances.
Epigenetics
Epigenetics also yield to some variance and epigenetic effects may well covariate with gene effects, environmental effects and the effect of developmental noise, so again we should consider all covariances. Let's call the variance due to epigenetic effects $V_{I}$.
Putting all these variances together
So, if $V_P=V_G+V_E$ is inaccurate, $V_P = V_G + V_E + V_I + V_{DN} + COV(V_G, V_E) + COV(V_G, V_I) + COV(V_G, V_{DN}) + COV(V_E, V_I) + COV(V_E, V_{DN}) + COV(V_I, V_{DN})$ would be a bit more correct.
So, genes do explain part of the variance in behaviour and appearances in human (or any other living being) populations. But it is by far, not the whole thing! The things one experiences through its life typically will greatly affect its future decisions.
If you are looking for definitions that are a bit more rigorous than those I have been using here, or for a more complete explanation, then you should definitely have a look at my answer at the post Why is a heritability coefficient not an index of “how genetic” something is?. It will also teach you the concept of heritability.
As Remi already explained, there can be phenotypic variations even if the genotype is the same. This is applicable in a broad sense. I'll focus on your query on decision making.
DNA does encode the developmental programmes in the body (including the brain). Because of conserved developmental programmes, we do have some very basic instincts and reflexes that are usually common between different people (as infants and to some extent, as grown ups too).
However, while the gross developmental programme of the brain is conserved (brain organization etc), memories and other learned preferences arise because of experience. At the cellular scale, neurons establish synapses with multiple other neurons. Experiences either strengthen or weaken the synapse. These experiences are based on how often (and in what way) these neural connections are used, which is in turn based on interactions with the external world. The DNA does have the information to allow the cell to carry out the processes that can strengthen or weaken a synapse but it does not have the prior information of which synapse to strengthen (this is somewhat grossly analogous to body-building, just that you are not always consciously aware of which neural circuits you are training).
Apart from that, there are several cellular processes that are inherently stochastic in nature. The effect of stochasticity dominates when the number of the participating entities in the system is small (such as a few neurons deciding which other neuron to interact with). In such conditions, deterministic models cannot explain the outcome. The stochasticity is also the source of the gene expression noise. When we say that stochastic effects dominate, it means that the system dynamics are governed by probabilities and not fixed deterministic rates.
So the bottomline is:
Decisions are not pre-coded in the DNA, but the ability to gather information and make decisions, is.
So it all depends on what information you gather.
that would be ridiculous. though the situations that we encounter may be quite similar to those for others of the past, we must decide anew what to do in each situation. it is not written in our dna what will happen.
