Biology teaches us that what best identifies an individual as belonging to a specific species is their gene pool, i.e., the information contained in the molecules of their DNA. The genome, the information contained in the DNA of cells, determines the species to which each organism belongs and individually identifies each member of the species by its two special properties: its “individuality”, the property of being unrepeatable among different members of the species (except in cases of monozygotic twins); and its “continuity” throughout life (except for occasional mutations in some cells of the organism as a whole).

The uniqueness of the genetic information of each member of the species is a result of the mechanisms that generate diversity (mainly mutations and recombination through sexual reproduction mechanisms). Mutations in turn can be due to changes, losses or additions of nucleotide bases in the DNA, or other more extensive mutations that affect chromosome regions or even entire chromosomes. In the end, a genome with more than 3.1 billion nucleotide base pairs in its DNA — as is the case with human DNA — is essentially unrepeatable due to its size and capacity for variation. The diversity among individuals of the species not only affects the part of the genome that is expressed (exons of about 21,000 genes), which constitutes only 2%, but also the remaining 98%, which includes gene regions that are not translated into proteins (introns), non-coding regulatory regions, repetitive and non-repetitive non-coding sequences, relics of genes that are no longer expressed (pseudogenes), etc.

The best marker of diversity between individuals of the species is the DNA that differentiates them and is conserved from beginning to end throughout the life cycle in all the cells. Mutations can occur, of course, but their frequency is extremely low and their tolerance is subject to the control of highly effective molecular repair systems. In man, a mutation in a gene can have a frequency of the order of 1 in 100,000 to 1 in 1,000,000 per generation.

Another thing is the phenotype, such as fingerprints, eye color or any other physical trait determined by polygenic systems. Some phenotypes are determined by hundreds of genes whose initial manifestation involves environmental factors and local epigenetic phenomena, which have been active since their formation and may determine variations in expression without changes in the DNA information.

What determines cell differentiation and morphogenetic changes from the embryonic state to the end of life is the information in the genes and their capacity for expression, which can vary within a range more or less limited by epigenetic factors and the influence of the genome as a whole.

Accordingly, after fertilization, if the zygote formed comes from human gametes, this is a human being and it will develop like one, since the nature of the organism does not change throughout its life cycle. Regardless of what biology tells us, in the case of human beings, a spiritual component is indissolubly bound to bodily materiality, which by its very indissolubility is inherent to each individual being.

Confining ourselves to the biological aspects, against the strength of the concept of “genetic identity”, associated with the determining role of DNA information, some controversial ideas have emerged that seek to diminish its importance. In his book “The Master Builder (John Murray Press, 2023), biologist Alfonso Martínez Arias of the Pompeu Fabra University of Barcelona argues that “it is not our genes that define who we are, but our cells”. In other words, it is proposed to accord the cells the role of determining identity, rather than DNA.

The first thing to note is that there is misuse of the word “identity” in this. The Cambridge dictionary defines identity as the “the qualities, beliefs, etc., that make a particular person or group different from others”. From the most elementary biology, if we move up the levels of organization that characterize an individual — DNA, cells, tissues, organs, individuals, populations, species, etc. — the only one on which our “own traits” that determine the differences depend, is DNA, the genetic information. The differences at all higher levels of organization are due to the expression of the genes they possess, starting with the more than 200 types of specialized cells involved in the supramolecular organization of a mammal. Genetic identity is an inherent and specific property of each individual from conception to death.

One type of contentious claim in the attempt to dethrone DNA from its identificatory biological role is to say that “nothing in our DNA explains why the heart is on the left side of the body, how many fingers we have, or even how our cells manage to reproduce,” or that “the long lineage of our cells (which goes back to the first cell or zygote produced after fertilization) and their intricate interactions within our bodies, make us who we are today”.  These claims are not supported by the knowledge amassed by Developmental Genetics. It is genes, not cells, which provide the information, the construction plans to build the organism with all its different components and details, of which the cells are the most basic.  Morphogenesis for assembling the organism responds to the instructions of the genome, specified in stretches of information in the genes (especially the constitutive 2% of the exons of the genes). The materials for constructing the organism are incorporated following their instructions and are executed in an orderly and regular manner, the first outcome being cell differentiation. This depends on the physiological states of activity of the “structural” genes under the direction of the “regulatory genes”. We will come back to this later.

First, it should be said that it makes no sense to say that “[a]n organism is the work of cells. Genes merely provide materials for their work.” It is quite the opposite: the organism, with all its cells, tissues and organs, are the final materials resulting from the instructions contained in the genes. In order for there to be a building, there must first be instructions and an orderly program of action. The cells are the materials that are incorporated in an orderly manner according to the information in the genes they contain and that mark the functional differences between them.

We can partly agree with the statement that “the real wonder is how the same genome can build such different structures as eyes and lungs in the same organism”, but that is precisely the outcome of the existence of a plan, a genetic program of development. The activation of some genes and silencing of others in a pluripotent embryonic cell, which is physically similar to another, occurs according to the time and position occupied by the cell in the growing embryo as a whole. This is one of the reasons we say that an embryo is not a mass of cells that are all the same, but an organism in full development.

Some genes send signals to the cells in their environment, with the cells closest to the embryonic environment receiving stronger signals. Depending on proximity and position, these determine changes in the epigenetic state and therefore gene expression in the cells, establishing intercellular expression gradients, which ultimately determine the differences in their specialization. In this way, a specific pathway towards the differentiation of tissues and organs is established in each part of the developing embryo, so that at the end of what seemed like a uniform cell agglomerate, nerve tissue, epithelial tissue, etc. and, later, an eye or a lung arises.

Regardless of their functional differences, all the cells in the body — whether they belong to an eye, lung, kidney or any other organ — retain the same genetic information constituted at the time of fertilization, differing only in the genes that are active in each of them. Cell differentiation, i.e., cell specialization, is the product of the differential activation or silencing of genes and not the other way around.

We mentioned two types of genes previously: “regulatory” and “structural” genes. The former are hierarchically the most important. These are the genes that direct the steps that lead to cell differentiation, and the ones that determine where and when the “structural genes” — to which the functional differences of the cells (proteomic differences) are due — should be expressed during the development that occurs immediately after fertilization. The best parallel to explain this is that of an orchestra that has to play a symphony. The regulatory genes correspond to the conductor, who directs the different musicians (represented by the structural genes) with his baton, joining in in an orderly manner only when instructed to do so. This was one of the favorite similes of the French geneticist and physician Jérôme Lejeune (1926-1994) to explain how the “symphony of life” develops in its early stages.

What Developmental Genetics has taught us is precisely how a program that is executed in an orderly manner in space and time is implemented, so that some cells are differentiated from others according to their position in the embryo, at whose hierarchical apex is the DNA, the genes.

In “The Master Builder“, the author argues that the existence of people with two different genomes (chimeras) challenges the idea that it is DNA that defines identity, in favor of the cells. However, the likelihood of survival of embryos originating from the fusion of two zygotes or two early embryos is extremely rare, and depends on the developmental instructions not being modified. These embryos can be viable as long as there are no interactions between genes or changes in critical genes that are incompatible with life. The fact that two genetic identities coexist by chance does not change the hierarchy of DNA as a development coordination centre, and the very non-viability of most of them demonstrates the need for a balance of genomic information to strengthen the regular construction of the organism. In this aspect, it is no different to the development of an organism from a zygote originating from normal fertilization.

From a bioethical perspective, nothing changes as regards the respect due to embryonic life from fertilization, when the genetic identity that constitutes the new being is established. The single-celled zygote is the first physical reality of a new being.

It is difficult not to relate the attempt to point to cells and not to DNA as determinants of identity, with the intention of lowering the moral status of the embryo. It surfaces when synthetic embryos or pseudoembryos appear, built by assembling cells from embryos produced by in vitro fertilization, and when the voices of researchers who claim interest in using them in research get louder. This type of attempt is very similar to what happened with the false concept of “pre-embryo”, used in order to mask the authentic nature of human embryos less than 14 days old. It also reminds us of the stage about 25 years ago when the nature of human embryos resulting from in vitro fertilization was denied in order to use them as a source of stem cells for experimentation, which is today at a standstill after the emergence of more ethical alternatives.

Nicolás Jouve

Professor Emeritus of Genetics at the University of Alcalá

Former member of the Spanish Bioethics Committee

Bioethics Observatory – Catholic University of Valencia

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