Genetic similarity between similar people

A question over the years has been how much of what we have or look like is by nature, encoded in DNA, and how much is nurtured, what we learn at home, school, or the society in which we live. nature vs. Breeding. Nature vs. Nurture say in English. There are characteristics eminently determined by DNA, such as having arms and eyes. There is another eminently determined by upbringing, such as language. We speak the language that we were taught as children. For years there have been a number of studies in which cohorts of identical twins have been formed. That is, people who are completely identical in DNA, because they arose from a single fertilized egg that split in two and then gave rise to two individuals that have exactly the same genetic material. Cohorts consist of those twins who grew up together, compared to those who for some reason were separated at birth. It is considered that those variables with similar concordance between twins who were separated and those who grew up together are likely to be largely determined by DNA, by heredity. An example is blood pressure levels.

Genetics

A recent work by a group of researchers in Spain published in Cell Reports (doi.org/10.1016/j.celrep.2022.11125) presents a very revealing and interesting analysis of the weight of DNA in determining a person’s facial characteristics. But in this case the starting point is the resemblance between people. The study was possible thanks to the previous work of a Canadian photographer named Francoise Brunelle, who has traveled the world bringing together couples of people who, without being relatives or acquaintances on many occasions, are very similar to each other. They are double. The project is entitled “I’m not a Look-Alike” (www.francoisbrunelle.com). This collection of black and white photographs was started by Brunelle in 1999, inspired by her own resemblance to British actor Rowan Atkinson (Mr. Bean). She to date has a collection of over 250 doubles pairs in 32 cities. The photographs are so impressive that, of course, they lend themselves to a number of paranormal theories, for those who like to explain the world with supernatural bases. However, the work of Esteller and collaborators does not leave them much choice in this regard.

Many of the couples photographed by Brunelle were invited to the study in which they filled out a questionnaire with questions related to various anthropometric aspects and habits, and a saliva sample was taken for DNA, DNA methylation and microbiota analysis. The photos were subjected to careful analysis with three different facial recognition algorithms. MatConvNet, Microsoft and Custom-Net. Of 32 couples analyzed, 25 correlated with two of the three systems and 16, half of the original sample, correlated with the three algorithms. These 16 couples behaved before these algorithms similar to identical twins. These couples were called supersimilar and were the main object of the study.

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4,327,108 DNA variations known as SNPs (Single Nucleotide Polymorphisms), DNA methylation in 850,000 CpG islands and the microbiota of the mouth were studied in these individuals. The SNPs are the individual variations that explain the great versatility that exists between human beings. They are single base changes in DNA that may or may not be in genes and thus may or may not change the sequence of a protein. They are very polymorphic, which means that they occur very frequently (at least 1% of the population) and there are millions of these specific changes in the DNA. The combination of SNPs can predispose to diseases or confer protection against diseases or for certain types of responses to drugs, etc. And it is these variations that are behind many differences between one human and another. DNA methylation is the way in which the environment can modulate whether or not a gene is expressed. If the DNA of a gene is methylated, this modifies its transcription and, therefore, its expression. The microbiota are the bacteria that live with us, in this case in the mouth and of which we now know that by various mechanisms they have effects on the hemostasis of the organism.

The result of the work shows that the concordance in the SNPs between people who are very similar is very high. That is, within the normal variation that exists in the DNA, couples with a strong facial similarity between them also have a significant similarity in the DNA. We already assumed that this happens when it comes to relatives who share DNA ancestry. There are many brothers and cousins ​​who resemble each other. The surprising thing about the study is that this occurs in the DNA of pairs of subjects who look very similar, but have no family relationship, nor ancestry proven by the DNA itself. The work also showed that DNA methylation and the oral microbiota do not correlate between pairs of similar individuals, which suggests that these two variables do not seem to be related to the facial resemblance between them, but that the weight of the DNA is greater. Finally, it is interesting that some body parameters and even habits were also significantly correlated among doubles pairs than with the rest of the population. Height, weight, smoking, and level of education are the most prominent examples.

The work has the limitation that the number of individuals studied is small and that the various regions of the world are not equally represented, since the highest percentage of couples was from Europe. However, it shows a correlation in thousands of SNPs that is considerable. If you meet a person who you are a lot like, you may share more than you think.

Dr. Gerardo Gamba

National Institute of Medical Sciences and Nutrition Salvador Zubirán and

Biomedical Research Institute, UNAM

Genetic similarity between similar people