## Is intelligence, criminality or sexuality coded in our genes?

As early as 1904, the English psychologist Charles Spearman noticed that the level of the ability to react to certain sensory perception and the overall intelligence of a person are closely related. He saw the connection between the ability to orientate in space, the perception of colours, directions or solving mathematical problems. He verified his observations for the first time on 24 pupils who attended a small village school. Spearman tested how well students performed on various intelligence-related tasks and concluded that individuals who did well in one area usually scored higher in other areas. Based on this, he defined general intelligence, which, according to him, represents the central factor of our cognitive abilities.

Later studies of twins and adopted children pointed to the possible heritability of some factors that determine our intelligence. Behavioural geneticists have used genome-wide studies to identify specific genes. They identified genes in several candidate areas in which the individuals differed. In this way, one of the first genetic markers associated with intelligence was also identified - the IGF2R gene, which encodes a receptor for mannose-6-phosphate and participates in the transport of phosphorylated lysosomal enzymes from the Golgi network to lysosomes. This gene is located on chromosome 6 and one of its variants is more common in children with higher IQ. Despite this, it cannot be said with certainty that it is an intelligence gene, because the role of this receptor in the cognitive and analytical processes taking place in the human brain is not known. Later, another study identified six more genes related to intelligence, and upon closer examination, each of these genes was shown to have very little effect on its own. In the case of intelligence, it is therefore a polygenically determined phenotype that is largely influenced by the environment (for example, place of residence, physical activity, family income, parents' occupation and education).

The possibility of the influence of genes on criminal behaviour was first publicly presented in 1991 in the state of Georgia, USA. The central character was, paradoxically, one of the thieves who wanted to justify his act by saying that he comes from a long family line of criminals, and thus his behaviour is conditioned by the genetic mutation he inherited. His appeal was rejected, but the link between genes and crime began to be studied. Since the transmission of this type of trait initially appeared to be female-dependent, scientists turned their attention to the X chromosome. One of the first candidates was the MAOA gene identified during an extensive analysis of a Dutch family with several generations of violent criminals. The MAOA gene encodes the catabolic enzyme monoamine oxidase, which removes amino groups (-NH2) from biogenic amines. It is part of the mitochondrial membrane and is also found in neurons forming presynaptic connections. Disruption of the function of this enzyme also interferes with the activity of the brain. The scientists managed to identify the specific region in which the gene is located and also the point mutation that occurred in the monitored individuals. Such a mutation created a premature stop codon in the gene, which led to the creation of a shorter and therefore non-functional enzyme (to clarify the term stop codon, you can return to Chapter 3 - Meet DNA, the bearer of genetic information). Scientists also confirmed the connection between a mutation in the MAOA gene and aggressive behavior in rodent animal models. The introduction of the MAOA genes increased the aggressiveness of the monitored individuals, which was evaluated based on the number of wounds caused by bites from aggressive individuals. Although it was possible to demonstrate a link between genes and the potential for criminal behavior, as with intelligence, the effect of the mutation alone is clearly not sufficient to be considered the cause of aggressive behavior.

Homosexuality is also a common phenomenon that we observe not only in humans, but is also known from the animal kingdom. The genetic basis of this sexual behavior has been the subject of research for a long time and is still poorly understood. Like intelligence or aggressive behaviour, sexuality is conditioned by polymorphic genes. In the 1990s, Dean Hamer discovered a gene that he believed was responsible for homosexual behaviour in men. He attributed heredity to the X chromosome, as the kinship of homosexual men on the mother's side of the family was more often observed. By comparing the results with heterosexual individuals, he was able to identify a deviation in the section called Xq28. However, it is only a variant in this area, not a "homosexuality gene". Sexuality is related not only to genes, but also to hormonal influences, the course of pregnancy and cultural conditions. The proportion in which these factors contribute to an individual's phenotype is individual. In general, the influence of genes on behaviour can only be estimated from a probabilistic point of view, and we cannot talk about their determining influence.

The main focus of contemporary behavioural genetics is the analysis of variations in behaviour with an emphasis on mental state or cognition. Many medical and military agencies are making huge investments to understand the origins of behavioural differences, since many of the most pressing health problems in modern cultures are linked to pathological behaviour—for example, obesity, certain cardiovascular diseases, cancer, drug use, and psychopathology. Therefore, the goal of behavioural genetics is to identify the genetic sources of individual differences in behaviour and susceptibility to specific environmental influences.