In the previous post, where I dealt with selection and cultural transmission, I asserted that genetic and hereditary are not synonymous. I would like to explore this topic a little further.
It’s not uncommon to see these terms, hereditary and genetic, treated as synonyms in many different areas of knowledge, mainly in the life sciences. They are used in the same situations and treated as perfectly interchangeable. Technically speaking, however, this freedom does not exist, and using these terms as synonyms can lead to a series of misunderstandings, some of them relatively serious. In most cases, a hereditary trait or feature (from Latin “heres”, heir) is also genetic (from the Greek “γέννα”, family), but there are situations in which the trait or feature may be genetic without being hereditary, and others in which it can be hereditary without being genetic.
The causal (aetiological) origin a given feature or trait of a biological entity is usually defined as being hereditary/genetic or as being dependent on the environment in which the organism is inserted. Before we go deeper into our discussion about the differences between hereditary and genetic, it’s worth noting the methodological fallacy in the previous reasoning (choosing between hereditary/genetic causes or environmental causes). This is a false dichotomy: considering only one of the extremes in a continuum of intermediate possibilities. This is the very problem in questions such as “is intelligence determined by genetics or environment?”, it considers genes and environment as two mutually exclusive factors, and clearly they are not. By the way, who has any familiarity with the concept of heritability knows that this silly question doesn’t make any sense at all (I will return to it in a future post). Thus, we have to be careful with false dichotomy fallacies: one can correctly assign a particular trait or feature to a combination of causes.
When we examine a trait strongly linked to a family, statistically related to a lineage of several generations, we tend to define the trait as hereditary or genetic, using either term, treating them as perfectly interchangeable synonyms. Because the feature in question is transmitted along that lineage, we deduce that the feature is related to the genetic makeup of those organisms, being directly dependent on a gene or on group of genes. And I believe that, doing this, we make a mistake. Genetic and hereditary are not synonymous. What is the difference?
The adjective genetic refers to a trait or a feature determined, directly or indirectly, by the organism’s genes, and phenotypically perceptible because of the occurrence of those gene’s protein products or the activity of these protein products in a given environment. Once the genes come (usually, but not always…) from parents, saying that the number of fingers is a genetic trait (determined by genes) means that such a trait is passed from parents to progeny, since children’s genes come from their parents. But this doesn’t allow us to conclude that everything that is transmitted from parents to offspring is genetic. In my opinion, the adjective hereditary will fulfil this role. Hereditary is what is passed from one generation to another.
Although the vast majority of hereditary characteristics are genetic, and vice versa, when confronting the definitions of genetic and hereditary we easily perceive the non overlapping areas. For example, the language we speak is clearly transmitted hereditarily: I learnt to speak my language by listening to my ancestors, and my descendants will learn it by listening to me. But it would be absurd to say that the language we speak is genetic! I would like to remind you that I’m talking about the language as type: Portuguese, English, Russian, French etc. I’m not talking about the linguistic capacity, the neurological ability to speak found in all humans, a capacity that depends on a complex neural network and, because of this, is both hereditary and genetic.
Dawkins, on “The selfish gene”, writes:
Cultural transmission is analogous to genetic transmission in that, although basically conservative, it can give rise to a form of evolution. Geoffrey Chaucer could not hold a conversation with a modern Englishman, even though they are linked to each other by an unbroken chain of some twenty generations of Englishmen, each of whom could speak to his immediate neighbours in the chain as a son speaks to his father. Language seems to ‘evolve’ by non-genetic means, and at a rate which is orders of magnitude faster than genetic evolution […] We need a name for the new replicator, a noun that conveys the idea of a unit of cultural transmission, or a unit of imitation. “Mimeme” comes from a suitable Greek root, but I want a monosyllable that sounds a bit like ‘gene’. I hope my classicist friends will forgive me if I abbreviate mimeme to meme. Examples of memes are tunes, ideas, catch-phrases, clothes fashions, ways of making pots or of building arches. Just as genes propagate themselves in the gene pool by leaping from body to body via sperms or eggs, so memes propagate themselves in the meme pool by leaping from brain to brain via a process which, in the broad sense, can be called imitation.
A trait statistically linked to a family, a trait strongly related to a certain lineage can be hereditary without being genetic, that is, it can be passed from parents to their children by a non-genetic pathway strong enough to create a nearly infallible pattern of heredity. Cultural characteristics, such as those I mentioned in the last post, are hereditary, but are not genetic. Imagine the following situation: a particular family has had, for generations, a strong fondness for Beethoven. A student of genetics, observing that family’s heredogram or family tree, with several squares and circles painted in black (indicating family members that enjoy Beethoven), can unwittingly conclude that the fondness for Beethoven is genetic and assume the existence of a “gene for liking Beethoven”, when the explanation is much simpler: the father passed on his taste in music for his son, who passed to his own son, and so on.
I think that keeping in mind that there are non-genetic hereditary traits and features is particularly important for understanding a series of human behaviours, especially certain pathological behaviours. The common notion that genetic and hereditary are synonyms can seriously impair our knowledge of the aetiology of mental health disorders, making us believe that there are genetic causes where there are only hereditarily transmitted dysfunctional behaviours. Let’s suppose that neurosis is hereditary and non-genetic: neurotic parents will raise neurotic kids, which will by their turn generate neurotic grandchildren. And we don’t need to assume the existence of a “neurosis gene”.
On the other hand, a feature can be genetic without being hereditary. The examples that quickly come to mind are cancers or malignant neoplasms. They are genetic disorders, since they are due to the activity of mutant genes; however, the vast majority of cases are not hereditary, we acquire that mutations throughout our lives, we don’t receive them from our parents. Curiously, most people associate cancers to heredity (the association is correct in the cases of retinoblastomas, breast cancers and in a few others types of neoplasms), and, partly because of that, they continue to foolishly smoking, sunbathing, having sex without a condom, eating barbecue, that is, a series of activities related to agents that are well known tumour initiators or tumour promoters.
In an organism, the majority of hereditary traits are also genetics. However, there are traits that are genetic without being hereditary, as well as traits that are hereditary without being genetic.