The human brain and intelligence it facilitates is without a doubt the most incredible and complex manifestation to be found in the universe. Exactly how the human cognitive facility was manifest and how it works, remain somewhat of an enigma. We know that memories are somehow stored through the formation of synaptic connections between neurons, but scientists have yet decipher how we use memories in the processes of perceptual contemplation, reasoning and rational induction.
One great quandary with respect to human intelligence, is whether it is a trait of genetic inheritance or developmental facet of the human human brain. The answer to this question quite simply is both, but it is an answer requiring both qualitative and quantitative specification. First of all, a discrete definition of what constitutes intelligence in the first place would be helpful, although elusive with respect to consensus of opinion, even within disciplines of neuroscience.
If you consider the human brain from a purely anatomical perspective, forgoing any instances of anomalous polymorphism, every brain exhibits the very same anatomical structures but does not exhibit the same degree of intelligence. This clearly establishes, that genetics, in so far as structural congruence has nothing at all to do with intelligence, accept that the genetic blueprint defines what structures are included in the human mental edifice.
If however, the chemical metabolism going on within the brain is given focus and consideration, it becomes readily obvious that all brains do not function with chemical analog; a degree of hormonal diversity being conspicuous. In this case, the disparity is accentuated and made obvious in cases where polymorphism dramatically influences hormonal balance or imbalance in the brain. Dr. Oliver Sacks book "Awakenings" (later made into the movie starring Robin Williams and Robert DeNiro) is a true account of how the drug L-dopa was used to alter brain metabolism and cause people suffering with sleeping sickness to essentially regain cognitive conscious function. L-dopa is a precursor of dopamine, a neurotransmitter in the brain which is responsible for many differing functions and has differing effects in different regions of the brain.
Even today, there are many hypotheses and great debate in neuroscience communities over the effects and entropy of dopamine in the brain. But the fact that surpluses and deficiencies of the chemical can be directly linked to genetic predisposition, clearly indicate a genetic effect, if not directly on intelligence then on the neural mechanisms which facilitate it. For instance, Parkinson's disease is a condition relating to an insufficiency of dopamine and has be demonstrated in many cases to have a hereditary component. But Parkinson's, or any other neurotransmitter disorder, while certainly effecting the qualitative expression of intelligence, do not necessarily have anything to do with intelligence from a quantitative perspective.
All humans are born with a similar quantity of brain cells or neurons, but quantity in this case is not the basis for intelligence. Almost all neuroscientists will tell you that intelligence rather than related to brain cell count, is the sum of synaptic connections between them. Are any of these synaptic connection the result of genetic programing? Absolutely, but the synapses we are born with are in large part associated with autonomic and motor development rather than cognitive ability. So, if we are talking about Intelligence with respect to intangible cognitive function, thought, heredity has little to do with it. All animals experience perceptive and instinctive cognitive processes to some degree or another. All animals seem to possess at least a fundamental instinctive ability to learn and create new synaptic connections, but in this respect, humans are substantially set apart from other animals and cognitive intelligence is the predominant facet of existentialism.
If you were to pin down the basic element of human cognitive intelligence, it would have to be language and the diversity of the human lexicon with respect to it. It stands, that the more words we learn and word meanings we understand, the greater, at very least, our intellectual potential. In rare cases where human individuals are deprived of language development prior to adolescence, adult mental retardation, in some cases severe, is almost always the result (Example Genie: http://www.3quarksdaily.com/3quarksdaily/2009/01/the-illogic-of-us-foreign-language-education.html ). But linguistic skill is not inherited, it must be learned. It is the words and word meanings learned and cemented into synaptic patterns in our brain which establish our cognitive intelligence potential. While no clinical study has or ever will be performed to confirm the effects of such linguistic deprivation, the available evidence if incomplete remains overwhelming and compelling .
Beyond linguistic development, another parametric of intelligence is how often and how well humans can recall and rearrange remembered information. This parametric may be genetically influenced to some degree, in that it is one which involves the physical chemistry going on in our brains. Unfortunately, no direct linkage of intelligence capability between parental intelligence and that of offspring has ever been established with any degree of certainty, although there is a general tendency for smarter parents to produce smarter offspring, simply due to developmental environment influence. Furthermore, if intelligence was hereditary, identical twins separated at birth and raised in different environments would be presumed to reflect the same status of intelligence by adulthood, but intrinsic evidence clearly counters this notion.
Intelligence itself is manifest in different ways by different individuals. For instance, one person may become an accomplished composer of music, displaying extraordinary cognitive ability while totally incoherent and dysfunctional to a degree in other aspects of presumably intelligent exercise. On the other hand, an individual who can consider and solve complex multilevel mathematical algorithms in their head may be unable to carry a tune. Consider the writer who has focus on spelling proficiency and grammatical correctness, but would have difficulty in expressing themselves with respect to novelty of concept or elaborate dissertation thereon. In contrast now, appraise the dyslexic who would have incredible difficulty writing a single sentence displaying spelling proficiency or grammatical correctness, and yet sports an IQ of borderline genius and can profoundly elaborate in exquisite detail. Who is the more intelligent? Can any objective assessment in this case even be attempted let alone rendered?
On the surface of it, it might appear simple to assess weather intelligence is a component of genetic inheritance or solely a learned phenomenon. Perhaps as geneticists begin to further unravel the intricacies of the DNA molecule and neuroscience improves our understanding of the brain, we will be able to find a more conclusive answer to this quandary. It would appear to be a certainty, based on available evidence at this point, that heredity has an absolute influence on the composition of the platform upon which human intelligence is expressed, but that the expression of that intelligence is much more of a developmental issue. In other words, our genetic inheritance constructs the vehicle for our intelligence, but it is up to each individual to learn how to drive it and ultimately develop our intellectual proficiency to its fullest potential.